FEDERAL EMERGENCY MANAGEMENT AGENCY

Manufactured Home

Installation

En Flood Hazard Areas

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FEMA 85 / September 1985
MANUFACTURED HOME

INSTALLATION

IN

FLOOD HAZARD AREAS

FEDERAL EMERGENCY MANAGEMENT AGENCY

Disclaimer

The statements, recommendations, and procedures contained in this manual are those of the National

Conference of States on Building Codes and Standards, Inc., and do not necessarily reflect the views

\ of the U.S. Government in general or the Federal Emergency Management Agency in particular.
TABLE OF CONTENTS

INTRODUCTION ...............................

CHAPTERI ...................................

Overview .................................... Manufactured Home Characteristics............

Conventional Manufactured Home

Installation Techniques.................

Effects of Flooding on Manufactured Homes

Regulatory Requirements...............

CHAPTER 11 .............

Flood and Wind Hazards.

Hazards From Floods..

Wind Hazards........

CHAPTER III ...........................

Elevation and Anchoring Techniques ......

Elevation on Fill .....................

Elevated Foundations................

Anchoring ............................ Evacuation ...........................

CHAPTER IV ...........................

Design of Elevated Foundations ..........

A. Flood Forces and Their Application . .

Hydrostatic Forces..............

Lateral Forces .................

Buoyancy ..................... Hydrodynamic Forces ...........

Impact Forces .......... ;

Scour ........................ Design Loads..................

B. Evaluation of Elevated Foundations..

Design Forces and Loads ........

Vertical Support Members........

Piers ....................... Posts and Piles ..............

C. Bracing Support and Connections for

Elevated Foundations ...........

Horizontal Support Beams .......

Cross Bracing .................

End Bracing ...................

Connections ................... Horizontal Beam ...............

Chassis I-Beams ...............

Cross Bracing .................

End Bracing ...................

Longitudinal Support Beams......

........................ 3

3 ..

....................... ......... 5

....................... ......... 8

....... . . . . . . . . . . . . . . . . . . 1 1

...................... 15

...................... 15

...................15

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28 

31 

33 

33 

34 

34 

34 

35 

36 

39 

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41 ...

45 ..

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48 ...

48 ...

53 ..

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...... ~. . . ..

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. . . . . . . . . . . . . . . . . .

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...... . . . . . . . . . . . . . . . .-'!*'

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D. Additional Design Considerations . Jacking ..................... Utility Service ................ Mechanical Systems .......... Access and Egress ...........

................... 68 ........ 68 

................... 6V

69 

70 

CHAPTER V......................................................

Economics ...................................................... Conventional Installations ....................................... Earth Fill .....................................................

Piers ........................................................ Posts and Piles . ......................................... Jacking ........................................ Miscellaneous ......................................... Summary of Total Costs .........................................

APPENDIX A

APPENDIX B

APPENDIX C

APPENDIX D

APPENDIX E

APPENDIX F

Bibliography .......................... Federal and State Contacts.............

Sources of Flood Information ............

Calculational Procedures for Elevated

Foundation Design....................

Buoyancy and Drag Forces.............

Design Worksheet .....................

77 

81 

o5. 

...I....... I...... 89 ................. 9 ................. 

... 71

... 71

... 71

... 71 ... 73 .. 

..

.73

74

... 74

. . 74

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. . . . . . . . . . . . . . . I

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LIST OF ILLUSTRATIONS

Page Figures

Flood Damage ............. 1

Flood Damage ............. 1

Flood Damage ............. 1

NFIP Regulations ...... .... 22 

Manufactured Home ........ 3

Chassis/Floor System ....... 3

I-Beams ................. 4

Double Section ............ 4

Double Section ............ 4

HUD Regulations ........... 4

HUD Label Plate ........... 5

Manufactured Home

Owner's Manual ........... 5

ANSI Standard ............. 5

Manufactured Home

Development .............. 5

Low Profile Siting .......... 6

Conventional Installation ..... 6

Tie-Down Systems .......... 6

Tie-Down System ........... 7

Movement from

Foundation ................ 7

Inundation Damage ......... 7

Elevation Using Fill ......... 7

Elevation Using Wood

Foundation .............. 8

Flood Damage ............. 8

Watershed ................ 8

Floodplain .............. 8

Flash Flood Damage ......... 9

Flash Flood Damage ........ 9

Shallow Flooding Damage .. .9

Failed Ground Anchor ...... 10

Deep Flooding Damage .... 10

Coastal Area ............. 11

Elevation in Coastal Zone . .11

Wind Damage ............ 11

Special Flood Hazard

Areas ........ ...... 12

Flood Hazard Boundary

Map .............. 12

Flood Insurance Rate

Map ................. 13

1.33

1.34

1.35

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

3.1

3.2

3.3

3.4

3.5

3.6

3.7

3.8

3.9

3.10

3.11

3.12

3.13

3.14

3.15

3.16

3.17

3.18

3.19

3.20

3.21

3.22

3.23

3.24

Page

Flood Boundary and

Floodway Map ............ 13

State and Local Regulations. 13

Building Codes ........... 14

Hydrostatic Forces ......... 15

Hydrodynamic Forces ...... 15

Damage from Hydrodynamic

Forces ............... 15

Damage from Debris ....... 16

Wind Speed Map .......... 17

Lateral Wind Force ........ 17

Wind Damage ............ 18

Wind Uplift Force ......... 18

Wind Design Loads ........ 18

Earth Fill ................. 19

Earth Fill and Conventional

Installation ............... 19

Earth Fill and Elevated

Foundation ............... 20

Earth Fill Site Plan ........ 20

Earth Fill ................. 20

Fill Areas .............. 20

Elevated Foundations ...... 21

Conventional Installation

Components .............. 21

Pier Designs ............. 22

Large Pier Design ......... 22

Filling of Piers ............ 22

Cast-in-Place Pier ......... 23

Post Foundation .......... 23

Post Set on Soil ........... 24

Post Set on Bearing Pad . . .24

Post Backfilled with

Concrete ................. 24

Post Anchoring ........

Post Anchoring ........

Piles .................

Pile with Concrete Collar

Horizontal Members ....

Bracing .................

Knee Bracing...........

Diagonal Bracing.........

.25

.25

.25

.26

.26

.27

.27

.27

Figures

1.1

1.2

1.3

1.4

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

1.10

1.11

1.12

1.13

1.14

1.15

1.16

1.17

1.18

1.19

1.20

1.21

1.22

1.23

1.24

1.25

1.26

1.27

1.28

1.29

1.30

1.31

1.32
Page

Diagonal Bracing with Steel

Rods .................... 27

Perimeter Enclosure ....... 28

Perimeter Walls ........... 28

Over-the-Top Tie .......... 28

Frame Tie ............... 28

Tie Components .......... 29

Tie Components .......... 29

Failed Anchors ............ 29

Screw Auger ............. 29

Concrete Deadman Anchor. .30

Anchors in Concrete Slab. . ;30

Anchoring on Pier

Foundation ............... 30

Anchoring on Post or Pile

Foundation ............... 31

Frame Ties ............... 31

State and Local Evacuation

Plans ................... 31

Roof Live Load ........... 33

Wind Load ............... 33

State and Local Regulations.34

Horizontal and Vertical

Forces .................. 34

Lateral Forces ............ 34

Buoyancy Forces .......... 35

Buoyancy Force ........... 36

Drag Forces .............. 36

Drag Force ............... 37

Overturning .............. 38

Parallel to Flow ........... 38

Perpendicular to Flow ...... 38

Impact Forces ............ 39

Scour ................... 39

Addition of Scour Depth .... 40

Addition of Scour Depth .... 41

Home Elevated Above

Floodwaters .............. 44

Home Not Elevated Above

Floodwaters .............. 44

Design Loads ............. 44

Flood Loads .............. 44

Magnitude of Loads ........ 45

Forces Increase with

Elevation ................ 46

Roof Dominated Loads ..... 46

Wind Dominated Loads ..... 46

Vertical Member Load ...... 48

Pier Design A ............ 49

Figure

4.27

4.28

4.29

4.30

4.31

4.32

4.33

4.34

4.35

4.36

4.37

4.38

4.39

4.40

4.41

4.42

4.43

4.44

4.45

4.46

4.47

4.48

4.49

4.50

4.51

4.52

4.53

4.54

4.55

4.56

4.57

IS Page

Pier Design B ............ 49

Pier Design C ............ 50

Pier Design D ............ 51

Pier Design E ............ 51

Pier Design F ............. 52

Typical Ground Anchor

Detail ................... 52

Longitudinal Spacing of

Ground Anchors .......... 52

Elevated Pier Design ....... 53

Posts and Piles ........... 53

Manufactured Home Design

Conditions ............ 58... 

Vertical Member Load ...... 58

Vertical Member Selection . .58

Elevated Foundation

Loading ... ............. 59

Horizontal Support

Beam ................... 59

Longitudinal Support

Beams ......... ........ 61

Cross Bracing ............ 61

Cross Bracing Locations .... 62

Cross Bracing Single

Section ................... 62

Cross Bracing Double

Section .................. 62

Elevated Foundation

Connections .............. 63

Bolts at Horizontal Beam

and Vertical Member ....... 63

Minimum Bolt Spacing

Distances for Square

Vertical Members .......... 65

Minimum Bolt Spacing

Distances for Round

Vertical Members .......... 65

Chassis I-Beam Lag

Screws .................. 65

Cross Bracing Vertical

Member Connection .... 66... 

Two Bolt Connection ....... 67

Four Bolt Connection ...... 67

Longitudinal Support

Beam Loading ............68

Longitudinal Support

Beam Connection .......... 68

Utility Service Protection .... 69

Overhead Utility Service .... 69

Figures

3.25

3.26

3.27

3.28

3.29

3.30

3.31

3.32

3.33

3.34

3.35

3.36

3.37

3.38

3.39

4.1

4.2

4.3

4.4

4.5

4.6

4.7

4.8

4.9

4.10

4.11

4.12

4.13

4.14

4.15

4.16

4.17

4.18

4.19

4.20

4.21

4.22

4.23

4.24

4.25

4.26
Page Figures

Separate Utility Service

Tank Foundation .......... 69

Connected Utility Service

Tank Foundation .......... 70

Single Section General

Comparison of Material

Costs .................. 75

5.2

F.1

F.2

Double Section General

Comparison of Material

Costs ................... 75

Design Worksheet ........ 103

Design Plan ............. 105

LIST OF TABLES

Lateral Forces ...

Buoyancy Forces.

Drag Forces.....

Impact Forces ...

Depth of Scour .

Building Loads...

Building Loads...

Building Loads...

Building Loads...

35 ......... 

36 ......... 

37 ......... 

39 ......... ........... 40

41 ......... 

41 ......... 

42 ......... 

42 ......... 

Maximum Vertical Member

Load ...... 47

Minimum Acceptable Pier

Designs ...... 50

Ground

Vertical

Vertical

Anchor Spacing ...

Member Capacity. .

Member Capacity. .

.52

.54

.55

Vertical Member Capacity ... 56

Vertical Member Capacity . . 57

Minimum Sawn Lumber

Horizontal Beam .......... 60

Minimum Sawn Timber

Cross Bracing ............ 61

Minimum Sawn Timber

Cross Bracing ............ 62

Minimum Sawn Timber

Bracing at Each End.

4.22

4.23

4.24

4.25

4.26

4.27

5.1

5.2

5.3

5.4

5.5

5.6

5.7

5.8

E.1

E.2

- E.3

.62

Number and Diameter of

Bolts at Connection of

Horizontal Beams to Square

Vertical Members .......... 63

Number and Diameter of

Bolts at Connection of

Horizontal Beams to Round

Vertical Members .......... 64

Minimum Bolt Spacing

Distances Square Vertical

Members ................ 64

Minimum Bolt Spacing

Distances Round Vertical

Members ................ 64

Minimum Nominal Lag

Screw Size ............... 65

Bolt Capacity . ............ < 66

Diagonal Loading per Foot

of Manufactured Home ..... 67

Conventional Installation

Costs for Single Section ... . 72

Conventional Installation

Costs for Double Section ... 72

Earth Fill Costs ........... 72

Pier Costs ............... 73

Cost per Foot of

Manufactured Home ....... 73

Post and Pile Costs ........ 74

Cost per Foot of

Manufactured Home ....... 74

Jacking Costs ............ 74

Vertical Tension ........... 99

Minimum Spacing Between

Ground Anchors ......... 100

Minimum Spacing Between

Ground Anchors ......... 101

Figures

4.58

4.59

5.1

4.1

4.2

4.3

4.4

4.5

4.6

4.7

4.8

4.9

4.10

4.11

4.12

4.13

4.14

4.15

4.16

4.17

4.18

4.19

4.20

4.21

Page
INTRODUCTION

More than 10 million Americans live in manufactured homes. The 1980 Census revealed that

almost four million occupied manufactured homes

comprised over five percent of the nation's housing stock, and that two-thirds of the manufactured

homes had been built and occupied since 1970.

The increasing popularity of manufactured homes,

coupled with their high vulnerability to the potential effects of flood and wind forces in flood hazard

areas, has resulted in the need to apply special

siting techniques for protecting manufactured

homes in these areas.

This manual provides technical guidance on

how to reduce the risk of flood damages to

manufactured homes. As used in the context of

this manual, the term "manufactured home" also

includes those homes previously defined as

"mobile homes." The information presented

addresses techniques for elevating the manufactured home above anticipated flood levels and for

adequately anchoring against flood and wind

forces.

Application of these techniques will aid in reducing property loss and enhancing the safety of the

manufactured home environment. Figures 1.1,1.2,

and 1.3 highlight the importance of considering

potential damage to manufactured homes in flood

hazard areas.

Figure 1.2 Flood Damage

Figure 1.3 shows damage resulting from a lack

of elevation and anchoring combined with structural damage from other floating manufactured

homes.

Figure 1.3 Flood Damage

Figure 1.1 Flood Damage

Figure 1.1 shows complete devastation due to

high velocity floodwaters and floating debris of a

manufactured home installed without regard for

site considerations and elevation.

Figure 1.2 shows results of flotation and damage

caused by inadequate anchorage and elevation.

The following is a brief description of the contents of the chapters of this manual, which are

presented in a manner intended to parallel the

decision-making process in siting a manufactured

home in a flood hazard area.

CHAPTER I - OVERVIEW

A general background on manufactured homes,

typical siting practices, discussion of flooding and

its effects on manufactured homes, and an overview of the regulatory and building code requirements for manufactured homes.

1
CHAPTER If - FLOOD AND WIND

HAZARDS

A presentation of flood and wind characteristics

and their effects on manufactured homes.

CHAPTER III - ELEVATION AND

ANCHORING TECHNIQUES

An overview of the various elevation and anchoring techniques that are applicable to the installation of manufactured homes in flood hazard areas.

CHAPTER IV - DESIGN OF ELEVATED

FOUNDATIONS

Design criteria and calculations to be used in

evaluating and selecting various elevating and anchoring techniques. Included are data, tables, mathematical formulas for design computations, and example problems.

CHAPTER V - COST ANALYSIS

Information on the cost of elevating and anchoring a manufactured home.

Using this approach, the manual provides

necessary information for evaluating flood damage potential, determining the technical feasibility of

design options, and selecting an installation

technique that is both cost effective and technically sound. The ultimate goal is to reduce or eliminate

damage potential for manufactured homes sited

in flood hazard areas.

To provide assistance for users of the manual,

a design worksheet is included.

Figure 1.4 shows the regulations of the National

Flood Insurance Program (NFIP), which is discussed in Chapter I. The NFIP provides performance standards for new manufactured home

siting in flood hazard areas.

The technical criteria contained in this manual

can be used to comply with the NFIP performance standards. Other federal regulations and programs affecting manufactured housing installation, such

as those of the U. S. Department of Housing and

Urban Development (HUD) for manufactured

homes on permanent foundations, are not addressed in this manual or by the NFIP.

Figure 1.4 NFIP Regulations

2
CHAPTER I

OVERVIEW

MANUFACTURED HOME

CHARACTERISTICS

A manufactured home is a structure, transportable in one or more sections, that is built on a permanent chassis and is designed to be used with

or without a permanent foundation when connected to required utilities.

Figure 1.1 shows a sectional view of a manufactured home.

The chassis consists of the undercarriage,

wheel assembly, and towing hitch assembly. The

floor joists are structural members of the chassis.

Two longitudinal l-beams complete the

chassis/floor system, as shown in Figure 1.2.

CONSTRUCTION

ONE PIECE GALVANIZED ROOF

ALUMINUM SIDING

Figure 1.2 Chassis/Floor System

FEATURES

R44 BLANKET INSULATION

%;w -~LISTED RAFTERS 24'CC.

OVERHEAD TIE DOWN STRAPS

VAPOR BARRIER

G Y P MGYPSUM CEILING BOARD

-STUDS 16'O.C. EMERGENCY EXIT J 

WINDOW IN EACH BEDROO

SEAMLESS

VINYL FLOOR COVERING

INSULATED ALUMI"'IM EXT. DOORS__d

ALL COPPER WIRING WITH

200 AMP SERVICE

ROOF &SIDEWALL STRESS STRAPS

STEEL I BEAM FRAME CONSTRUCTII

Figure 1.1 Manufactured Home

3

4 EXTERIOR WALLS

-PRE-FINISHED PANELING

-FLOOR JOIST 15"O.C.

- ABS DRAIN LINES

'"--POLYBUTYLENE WATER LINES

'-ALUMINUM HEATING & COOLING DUCT
Some manufactured homes are designed for

removal of the steel chassis when the manufactured home is placed on a permanent basement

type foundation.

The longitudinal I-beams are the main support- ing members for a manufactured home placed on

an elevated foundation, as shown in Figure 1.3.

The standard I-beam spacings are 75 112-, 82-, or

99-inches.

wood/shingle exterior. The manufactured home

mechanical and electrical system is completely built into the manufactured home during construction. Installation of the manufactured home requires placement on a foundation system and connection to the required utilities.

A double section, as shown in Figures 1.4 and

1.5, refers to a manufactured home that is generally twice as wide as the normal 12 or 14 foot wide

single unit.

Figure 1.3 I-Beams

The floor decking material, which is attached to

the floor joists, is usually wood particle board or

plywood. The manufactured home walls are constructed using wood studs with the exterior of the

j TL V.

Figure 1.4 Double Section

home generally of aluminum or wood siding. The

roof/ceiling system is constructed using prefabricated roof trusses and a metal or

Figure 1.5 Double Section

The design and construction of the chassis/floor

system is comparable to the single section unit.

Transported in two sections, the units are joined

at the site on a foundation similar to those used

for single section units.

The design and construction of manufactured

homes are controlled through regulations of the

U. S. Department of Housing and Urban Development (HUD), shown in Figure 1.6.

DEPARTMENT OF

HOUSING

AND URBAN af DEVELOPMENT

Of fie, of the Assistant

Secretor for Housing

Production and Meortage

Credit-Federal Housing

Ceonmissioner (Federal Housing ^ > ~~~Admininstrations)

I >j MOBILE HOME

CONSTRUCTION

Figure 1.6 HUD Regulations

4

F 2*"

ANID SAFETY

STANDARDS
The Manufactured Home Construction and

Safety Standards (MHCSS) (24 CFR, Part 3280)

establish performance requirements for the design

of manufactured homes. A manufactured home

designed and constructed in accordance with

these standards will have a two- by four-inch

aluminum plate label, as shown in Figure 1.7,

located on the lower left side when viewing the tail

light end of the manufactured home.

Figure 1.7 HUD Label Plate

CONVENTIONAL MANUFACTURED

HOME INSTALLATION TECHNIQUES

Siting and installation requirements for manufactured homes are generally a state or local

regulatory responsibility and are not within the

scope of the MHCSS. The MHCSS do, however,

require that installation instructions be incorporated into the owner's manual supplied with

each new manufactured home. This is to assure

that the purchaser has the correct procedures

available for installing the manufactured home. A

typical owner's manual is shown in Figure 1.8 and

will be available for all manufactured homes constructed under the HUD Regulations.

In addition, the American National Standards Institute's (ANSI) Standard A225.1-1982, "Manufactured Home Installations,' shown in Figure 1.9,

provides installation criteria applicable to many

areas of the country.

Figure 1.9 ANSI Standard

It is important to recognize that the installation

procedures included in both the owner's manual

and ANSI Standard A225.1-1982 are not considered to be adequate in cases where flood forces

are anticipated.

The Manufactured Housing Institute estimates

over half of the manufactured homes in use are

sited on individually owned lots with the remainder

in manufactured home rental communities. In a

manufactured home community, shown in Figure

1.10, a rental fee is usually charged to cover the

o6st of land, required utility hookups, access

roads, parking, and community facilities.

Figure 1.8 Manufactured Home Owner's

Manual

Figure 1.10 Manufactured Home Development

5
Modern developments provide paved sidewalks

and streets, underground utilities, off-street parking and street lighting, park areas, playgrounds,

and other recreational facilities. In some manufactured home communities the individual lot, in addition to the home, may be owned by the manufactured home owner.

A manufactured home is normally placed on a

prepared site that has been stabilized and improved to provide adequate support for the

manufactured home and anchoring system. The

type of site and area improvements vary widely

across the country and include simple ground stabilization, application of gravel, concrete runner or slab, concrete block piers, or more elaborate

foundation construction. Because installation must

allow for the wheels and axles upon which the

manufactured home is transported and also for

utility connections, the manufactured home is

generally placed 24- to 36-inches above the

ground. Since the steel frame and joists will

generally be 14- to 20-inches in height, the finished

floor of the manufactured home may be as high

as 56 inches above grade.

For aesthetic reasons and improved access, it

is sometimes desirable to employ a "low profile" siting in which the site area is excavated below

grade as shown in Figure 1.11.

Figure 1.11 Low Profile Siting

This technique requires special provisions for

adequate drainage and ventilation.

A typical manufactured home installation, as

shown in Figure 1.12, involves supporting the

manufactured home by piers (concrete masonry blocks resting on a small concrete footing) placed typically every eight to ten feet of length beneath

the two chassis l-beams.

3- - -O - -a O- -I - - -n- - 0--... - - --E

I Axias .3 l.. Arn I0 s---a---- - ----- E----- - - -

I I

~ lAxis I~x X _/Center Line If IBseam

1 fl_ _- fl fl 11 - - -E]- - -Cl- - -

ri n n l n , ni ri

4-Psrmster Blacks

Frame sBlocks

w-Center Beam

Ulocks

rFrame Blocks

o-Perlmster Blacks

P Front Door I

Figure 1.12 Conventional Installation

Ground anchors, which are located under the

manufactured home, are secured to the manufactured home with frame ties connected to the

chassis I-beams, and where acceptable from a

design standpoint, over- the-top ties. Figures 1.13

and 1.14 show typical manufactured home installation techniques.

Figure 1.13 Tie-Down Systems

6

-19- MYId"' 
Figure 1.14 Tie-Down System

It should be emphasized that conventional installation described above would rarely be adequate for resisting the flood forces that can be ex- - a

pected in areas prone to flooding. A manufactured

home that is elevated a few feet above the ground

and anchored to resist wind forces will still be

vulnerable to the additional forces produced by

flooding. Movement of the home from the foundation supports, inundation, or both can result in

major damage, as shown in Figure 1.15.

Figure 1.16 Inundation Damage

Depending upon the characteristics of the flooding

anticipated, the home may be elevated using

piers, posts, pilings, or fill. The use of an elevated

foundation will differ from a normal siting techniofque in terms of the desired height above grade

and the structural capacity of the elevating system

,to withstand anticipated flood and wind loads.

tFigures 1.1a7 and 1.1 8 show elevating techniques.

Figure 1.15 Movement From Foundation

Even relatively minor inundation can cause

significant damage to a manufactured home as_

shown in Figure 1.16. For example, a water level

of only two feet above the floor can potentially

cause damage equivalent to approximately 80 percent of the value of the home.

In flood hazard areas, additional elevation and

flood and wind load considerations may

necessitate a siting technique that varies substantially from conventional installation methods. Figure 1.17 Elevation Using Fill

7
These include inundation with water, uplift and

movement of the manufactured home from its

foundation, and debris impact on the foundation

and manufactured home.

Riverine Flooding-A region's watershed,

shown in Figure 1.20, is the natural drainage basin

that conveys water runoff. Water that is not absorbed by the soil and vegetation becomes surface water runoff, seeking the natural drainage

lines according to local topography.

Figure 1.18 Elevation Using Wood Foundation

Elevating alternatives and related anchoring techniques are presented in detail in Chapter IlIl.

EFFECTS OF FLOODING ON

MANUFACTURED HOMES

Manufactured homes in flood hazard areas may be affected by a variety of conditions that vary in

intensity from site to site. Flood forces and

parameters are presented in detail in chapter two

of the manual. The following provides an introductory overview of the types of flooding and their effects on manufactured homes which are installed using the conventional set-up techniques

discussed above.

There are two general flooding types-riverine and coastal. Each environment exhibits

physiographic features that determine the

magnitude, duration, and frequency of floods.

Flood characteristics, especially the significant

parameters of depth and velocity, produce conditions that can adversely affect manufactured

homes, as shown in Figure 1.19.

Figure 1.19 Flood Damage

Figure 1.20 Watershed

Theselines merge to form a hierarchical system of streams that includes swales for drainage channels, creeks, and rivers, each of successively larger capacity.

The primary element of a stream is the channel, which carries the normal flow of water. The

area of flat or gently sloping land adjacent to the

channel, as shown in Figure 1.21, is the floodplain.

Figure 1.21 Floodplain

8
Flooding usually involves a build-up of water in

the channel followed by overflow that inundates

the floodplain. Generally, the rise in water surface

elevation is slow in large streams and more rapid

in small streams.

Several types of riverine flooding occur, each

posing specific threats to manufactured homes.

Flash flooding involves an extremely fast rise

in water surface elevation and abnormally high

water velocity, often creating a "wall" of water and

debris moving down the channel and floodplain.

Flash floods usually result from some combination of intense precipitation, steep slopes, a small

drainage basin, and a high proportion of impervious ground surfaces. They often occur in small

streams that are normally shallow or dry and can

cause extensive damage as shown in Figure 1.22.

Figure 1.22 Flash Flood Damage

The high water velocity characteristic of flash

floods can significantly affect manufactured homes

in terms of erosion or scour around the foundation, movement of the manufactured home off the

foundation, and impact on the manufactured home

by debris carried in the floodwaters. Although inundation is always a concern, damage sustained

from a flash flood event will generally depend upon

the strength of the foundation and anchoring

system to resist movement of the manufactured

home. Even in situations where floodwaters do not

reach the floor of the manufactured home, the

foundation and anchoring systems will have to

resist high water velocity and debris impact forces.

If these systems are not properly designed and

installed, failure resulting in foundation collapse

and manufactured home inundation will most likely

occur, as shown in Figure 1.23.

When greater flood depths are experienced, inundation of the manufactured home is likely to occur, and the forces on the foundation and anchoring systems are subsequently intensified.

- Shallow flooding includes unconfined flows over

broad, relatively low areas; intermittent flows in

Figure 1.23 Flash Flood Damage

arid or semi-arid regions that have not developed

a system of well-defined channels; minor stream

bank flows that remain unconfined; overland runoff

in dense urban areas; flows where heavy debris

deposits cause constantly shifting channels; and

"ponding" in topographic depressions. Flow direction is extremely difficult to predict. Generally,

shallow flooding areas are considered as those

areas exhibiting depths of three feet or less. A conventional manufactured home installation may provide adequate elevation during a shallow flooding

occurrence. A cautionary statement is appropriate,

however, calling attention to the substantial

damage that can be caused by a relatively small

amount of water within the manufactured home.

The aforementioned flood loads applied to the

foundation and anchoring systems can also be applicable to shallow flooding circumstances. Even

very shallow floodwater can have high velocity sufficient to cause erosion, scour, and subsequent

failure of the foundation system, as shown in

Figure 1.24.

Figure 1.24 Shallow Flooding Damage

9
Floodwaters need not be deep or fast to affect

the integrity of an anchoring system. Soil saturation occurring during periods of prolonged flooding

can reduce the holding capacity of ground anchors

causing them to pull out as shown in Figure 1.25.

Figure 1.26 Deep Flooding Damage

Figure 1.25 Failed Ground Anchor

Deep flooding accentuates the forces acting

upon the foundation and anchoring systems of a

manufactured home. In addition, increased flood

depths affect a non-elevated manufactured home

by directly transferring forces to the walls and floor.

The initial imbalance of water levels between the

exterior and interior of the manufactured home will

likely result in buoyancy. This uplift force can completely dislodge an inadequately anchored

manufactured home from its foundation. This

typically occurs in fast rising waters or when the

rate-of-rise of floodwaters is very fast.

In situations where the rate-of-rise of floodwaters

is slow, allowing the manufactured home to fill with

water and therefore counteracting the buoyancy

forces, or where adequate anchoring is employed

to resist dislodging the manufactured home from

its foundation, deep flooding can substantially

damage a non-elevated manufactured home by direct inundation. Although the manufactured

home may remain in place, as shown in Figure

1.26, it will likely be a total loss due to the damage caused by water inundation.

In addition to having high erosion and scour

potential, deep velocity flooding also emphasizes

the need to consider debris impact potential.

Coastal Flooding-Coastal flooding is generally

due to ocean-based storm systems. Hurricanes,

tropical storms, and extratropical storms, such as

"northeasters," are the principal causes, with

flooding occurring when storm tides are much

higher than the normal tide. This is a "storm

surge" and exhibits high water velocity and wave

action. The maximum intensity of a storm tide occurs at high tide, so storms that persist through several tide cycles, such as "northeasters," are

the most severe.

The velocity and range of coastal floods vary in

part with the severity of the storm of which they

are a part. The damaging effects of coastal

flooding are caused by a combination of the storm

surge, rain, wind, waves, erosion, and battering by debris.

The extent and nature of coastal flooding is also

related to physiographic features of the terrain and

the characteristics of the adjoining body of water.

Pacific coastal areas are principally vulnerable to

earthquakes, tsunamis (seismically induced tidal

waves) and other natural forces that can trigger excessive erosion, mud slides, and flash flooding. Great Lakes coastal areas are subject to erosion

and severe winter storms. The Atlantic and Gulf

Coasts are consistently exposed to the forces of

hurricanes, lesser tropical storms, and

northeasters.

10
Coastal flooding is most frequent on the Atlantic and Gulf Coasts, which are made up of a succession of barrier islands, beaches, and dunes.

These physiographic elements are maintained in

dynamic balance as sand is moved by wind,

waves, and ocean currents. This self- replenishing

beach-dune system resists storm surges and provides a buffer for inland areas.

In coastal areas, as shown in Figure 1.27, the

removal of beach sand and the leveling of dunes,

along with the construction of seawalls, jetties, and

piers, are common practice. These can destroy

the shoreline's natural protection system, further

increasing the impacts of surges and high winds.

and wave action. These factors, combined with the

potential for scour and erosion and high debris impact forces, represent a significant hazard to the

typical manufactured home set-up. The susceptibility of coastal areas to extremely high winds

presents additional hazards to manufactured

homes sited in this environment, as shown in

Figure 1.29.

Figure 1.29 Wind Damage

Figure 1.27 Coastal Area

Manufactured homes in coastal flood -hazard

areas, as shown in Figure 1.28, are subject to the

same flood forces experienced in riverine flooding

environments. However, in some cases, the

coastal forces occur at greater magnitude and

severity.

Figure 1.28 Elevation in Coastal Zone

Coastal areas represent a unique hazard to the

typical manufactured home installation due to the

potential for very high forces from velocity water

In the coastal environment, the combined effects of flood and wind forces must be evaluated

to determine adequate anchoring and elevating

techniques. Subsequent chapters of the manual

will address the impact of flood and wind forces

on manufactured homes.

REGULATORY REQUIREMENTS

As an initial step in the installation of a

manufactured home in a flood hazard area,

federal, state, and local regulatory requirements

must be identified and assessed. Much information regarding flood hazards and construction in

flood-prone areas is available through a wide variety of public agencies at all levels of government

(see Appendix C).

Federal Regulations-The National Flood Insurance Program (NFIP) is the Federal Government's principal administrative mechanism for

reducing flood losses. Established by Congress

in 1968 and broadened and modified since then,

the NFIP is administered by the Federal Insurance

Administration (FIA) of the Federal Emergency

Management Agency (FEMA). The program offers

property insurance for buildings and their contents

in flood-prone areas where conventional insurance

had been generally unavailable.

The NFIP provides insurance to communities

that agree to implement comprehensive land use

planning and management to reduce flood

11
damage in their jurisdictions. Community response

to this requirement generally involves the adoption of zoning, building code, and other development regulations that place various requirements

on new construction and on substantial improvements to existing construction in identified

flood hazard areas. Some communities, either in

recognition of the benefits of a strong floodplain

management program or in response to specific

problems, adopt regulations that are considerably

more restrictive than the minimum NFIP

requirements.

FEMA establishes "Special Flood Hazard

Areas" that depict those areas inundated by the

100-year flood as shown in Figure 1.30.

cases, to the elevation of the building in relation

to the base flood. Higher rates are charged for

buildings subject to greater hazard. This differential rate structure provides a financial incentive for

homeowners to reduce their susceptibility to flood

damage.

The NFIP is administered in two phases: the

Emergency Program and the Regular Program.

The function of the Emergency Program is to

make flood insurance readily available to property owners throughout flood-prone communities.

The operation of the program is simple and direct.

The FIA notifies a community that it has been identified as flood prone by providing the community

with a Flood Hazard Boundary Map (FHBM), as

shown in Figure 1.31.

Figure 1.31 Flood Hazard Boundary Map

Figure 1ONE C

-U- -~~~~~~~~~ 

Figure 1.30 Special Flood Hazard AreasThis "base flood" is a flood that has a one percent chance of being equalled or exceeded in any

given year. Over a 30-year period, there is at least

a 26 percent chance that this flood will occur.

The 100-year flood standard is used by every

federal agency in their administration of floodrelated programs. This standard has also been

adopted by many states and is used administratively in the operations of their own programs involving floodplain management.

The NFIP's flood insurance premium structure

has been developed based upon studies of the

degree of property exposure to flood hazards.

Flood hazard areas are divided into different zones

according to the degree of risk. The insurance

rates vary according to the zone and, in certain

This map is a preliminary delineation of special

flood hazard areas within the community with a

definite likelihood of inundation. No elevations are

shown.

A community receiving such a map may participate in the program by completing an application to FIA. Upon approval of the application,

limited amounts of insurance become available in

that community. The community is required to

apply minimal floodplain management regulations

based on the FHBM and is encouraged to

reasonably use any additional data that may be

available from other sources to establish the flood

elevations.

A community generally enters the Regular Program after the completion of a detailed technical

study of flood hazards. The study includes a determination of elevations of floods of varying intensity, including the base flood, areas inundated by

the various magnitude of flooding, and flood boundaries. This information is presented on a Flood

Insurance Rate Map (FIRM) and Flood Boundary

and Floodway Map (FBFM), as shown in Figures

1.32 and 1.33.

12
State and Local Regulations-Some states

have regulations, as shown in Figure 1.34, applying to the placement of manufactured homes in

flood hazard areas.

Figure 1.32 Flood Insurance Rate Map

Figure 1.34 State and Local Regulations

Figure 1.33 Flood Boundary and Floodway Map

FIRMs generally show flood-prone areas as

either A-Zones or V-Zones. Riverine flood-prone

areas and coastal flood-prone areas subject to

storm surges with velocity waves of less than three

feet during the 1 00-year flood are generally classed as A-Zones. "Coastal high hazard areas" are

shown on FIRM's as V-Zones. The V-Zone is the

portion of the floodplain subject to storm surges

with velocity waves of three feet or more during

the 100-year flood.

Based on this information, regulatory standards

that are more detailed than Emergency Program

requirements are adopted and enforced by the

participating community. These standards include

requirements that influence the location and manner of installation of manufactured homes.

The technical guidelines contained in this

manual can be used in complying with the NFIP

performance standards. Also, local community officials should be contacted to determine what NFIP

regulations govern the placement of manufactured

homes in flood hazard areas.

New Mexico, for example, requires manufactured home dealers to provide the purchaser with

a notice to check with the local government to

determine installation requirements in a flood

hazard area. In Texas, North Carolina, and several

other states, when more than one-fourth of the

manufactured home frame members are more

than three feet above grade, the foundation

system must be designed by a registered professional engineer or architect.

The appropriate state government departments

responsible for manufactured housing installation,

floodplain management, water resources, building

codes, or-coastal zone management should be

consulted prior to installation of a manufactured

home in a flood hazard area.

Each state also has designated a state coordinating agency to assist in the implementation of

the NFIP. This agency is a focal point for information on flood insurance, floodplain management,

and coordination of the diverse state agencies with

responsibilities for riverine and coastal floodplains.

A listing of state coordinating agencies is included

in Appendix B.

The authority of each state's coordinating

agency varies and can best be determined through

direct contact. These agencies can be important

sources of physical data, information on community eligibility for flood insurance, state regulations,

references to other agencies and, in some instances, technical assistance.

Local governments play the key role in

floodplain management. Most local jurisdictions

13
have specific zoning ordinances and regulations

pertaining to manufactured homes and manufactured home developments. in addition, they can

provide sources of flood hazard data and other

regulatory information. Local offices that may be

of assistance include Departments or Offices of

Public Works, Building, Engineering, Zoning, and

Planning.

Building Codes-Building codes can be a

mechanism for reducing flood losses through the

inclusion of sections specifically intended to

mitigate flood damage.

The Building Officials and Code Administrators'

(BOCA) BasiclNational Building Code, the International Conference of Building Officials' (ICBO)

Uniform Building Code, and the Southern Building

Code Congress International's (SBCCI) Standard

Building Code, shown in Figure 1.35, all include

sections requiring structural integrity of foundations, walls, floor slabs, and retaining walls that

may be subjected to flooding.

Figure 1.35 Building Codes

These model codes are used in many parts of

the country and are developed with technical data

from many organizations and individuals through

an annual code revision process. Some communities may have similar or more stringent requirements that specifically address manufactured

homes. State and local guidance should be used

to determine what flood-related codes are in effect for a particular area.

Chapter II describes the effects of flood and

wind forces on manufactured homes.

14
CHAPTER 11

FLOOD AND WIND HAZARDS

FLOOD AND WIND HAZARDS

Flooding and wind have various characteristics

and parameters that determine the magnitude of

forces produced by these natural hazards.

Because a manufactured home installation can be

affected by flood and wind induced forces acting

on the body of the unit, the supporting foundation

structure, and the anchoring system, these forces

and their application to manufactured homes

become critical and must be recognized.

HAZARDS FROM FLOODS

Flood Forces-Floodwaters cause pressures or

forces on the surfaces of manufactured homes

and their supporting foundations. These forces are

of three basic types: hydrostatic, hydrodynamic,

and debris impact.

Hydrostatic forces are caused by water either

above or below the ground surface, free or confined, that is essentially stagnant. These forces

result at any point of floodwater contact with a

structure. Hydrostatic pressures are equal in all

directions and always act perpendicular to the surface on which they are applied. As Figure 2.1 indicates, the loads can act vertically on structural

members, such as floors and foundation slabs,

and can act laterally on upright structural

elements, such as walls, piers, piles, or posts.

Figure 2.1 Hydrostatic Forces

Hydrostatic forces have the ability to cause

manufactured home flotation, floor and wall collapse, and foundation/anchoring system failure.

Hydrodynamic forces, depicted in Figure 2.2,

are those caused by the flow of floodwater around

and against the manufactured home or foundation. Hydrodynamic forces are lateral in nature and

apply directly against the structure. They include

drag forces as water flows around the structure

and negative pressure caused by eddies on the

downstream side.

Figure 2.2 Hydrodynamic Forces

Hydrodynamic forces have the ability to cause

foundation system failure, increase erosion and

scour around foundation system components, and

cause damage to the manufactured home structure, as shown in Figure 2.3.

Figure 2.3 Damage From Hydrodynamic Forces

15
Debris Impact Force-Floods also present an

additional hazard to structures by creating the

potential for debris impact as shown in Figure 2.4.

Debris impact forces are produced by solid objects

carried by the floodwaters striking the structure.

These forces are extremely difficult to predict and

quantify with a high degree of accuracy, yet their

effect on structures is significant enough to warrant consideration.

Figure 2.4 Damage From Debris

Debris impact forces include isolated objects of

normally encountered sizes, such as logs; conglomerates of floatable objects, such as ice floats;

and extremely large masses, such as a collapsed

structure. Forces are produced by objects both

striking and resting against a structure. For the

purpose of design, assumptions on object weight

and impact speed must be made. Chapter IV

presents specifics on debris impact forces.

The magnitude of these three types of flood

forces is dependent on several flood related

parameters such as velocity, depth, duration, rateof-rise, and frequency.

Flood Hazard Parameters-Flood hazard

parameters are used to determine the degree of

hazard that can be expected at a site and provide

necessary information for evaluating and choosing damage reduction and site design strategies.

Depth of flooding is the difference between the

water surface elevation at the time of flooding and

the normal grade elevation of the flooded area.

Flood depths determine the magnitude of the

hydrostatic forces that act on a manufactured

home. There is a direct relationship between depth

of water and amount of hydrostatic pressure.

Greater depths of water exert greater pressure on

a structure. A water depth of five feet, for example, exerts 312 pounds of pressure per square foot

of surface. This pressure can cause buoyancy,

lateral displacement, and overturning of a

manufactured home as well as structural damage

to the floor and wall assemblies. In addition, depth

of flooding is a very important consideration when

elevating a manufactured home. A two foot depth

of inundation can potentially result in damages up

to about 80 percent of the value of the manufactured home.

The conventional manufactured home installation may provide adequate protection from inundation in some flooding situations, where depths

of flooding do not exceed the 24- to 36-inch foundation height of the conventional installation.

In deeper flooding situations the height of the

conventional manufactured home installation is exceeded. Once the floodwaters inundate any portion of the manufactured home, the probability of

significant damage increases. In addition to

damage resulting from inundation, differential

water levels between the interior and exterior of

the manufactured home result in uplift forces on

the manufactured home. These forces can result

in movement of the manufactured home off its

foundation supports and cause other structural

damage to the manufactured home. This situation

can be expected where there is a rapid rate-of-rise

of floodwaters resulting in insufficient time for the

water to enter the manufactured home and

counteract the buoyancy forces.

Velocity of flooding is the time rate of the linear

motion of floodwaters usually measured in feet per

second. Flood velocities vary from point to point

in a floodplain and determine the magnitude of

hydrodynamic forces affecting a manufactured

home. Velocity floodwaters will result in significant

lateral forces on the typical manufactured home

installation. Lateral forces on the foundation supports, along with erosion and scour of the foundation, can result in failure of the foundation if improperly designed.

When flood depths exceed the foundation

height, lateral forces from velocity floodwaters on

the walls can move the manufactured home off its

foundation supports, resulting in inundation and

damage to the home. Conventional installation

techniques are inadequate to withstand the

amount of lateral force which can be expected

from velocity floodwaters.

The potential for debris impact forces is also

much greater in a velocity flooding situation,

because the amount and size of debris will vary

from location to location. The magnitude of the

debris impact force upon a manufactured home

or the foundation supporting the home is directly

proportional to the velocity of the floodwater. Basic

assumptions concerning the size of debris and

velocity of impact must be made when designing

a foundation in a high velocity flood-prone area.

The potential for scour and erosion of the foundation also increases with velocity. If scour and

erosion are of concern, protection for the founda16
tion or increasing the embedment of the foundation supports should be considered.

Rate-of-Rise is a measure of how rapidly water

depths increase during flooding. A slow rise of

floodwaters will allow seepage of water into a

manufactured home thereby counteracting uplift

forces.

When floodwaters rise quickly, this balance may

not occur in time, resulting in buoyancy or failure

of the manufactured home floor. Rate-of-rise is

also relative to the amount of warning time prior

to flooding. This is important in planning for

emergency evacuation and in determining the

feasibility of emergency loss mitigation

procedures.

Duration of flooding is the amount of time from

inundation of an area to the recession of floodwaters. Duration influences how long the manufactured home and its foundation will be subject to

hydrostatic and hydrodynamic pressures, the

strength of soils and building materials, the degree

of seepage, and the length of time that a structure may be inaccessible.

Frequency of flooding, the rate of recurrence

of floods at a particular location, is primarily a consideration in selecting the installation site.

Repeated occurrences of flooding overtime, even

when a single event does not result in the total

loss of a manufactured home, can produce a

cumulative wear-and-tear effect on a typical installation. Flood frequency data is useful in the

design of foundation and elevating systems in that

it is an essential element in formulating assumptions as to how often these systems will be exposed to flood forces.

Debris load, the amount and type of debris carried by floodwaters, can result in significant impact forces against a structure.

Debris can also adversely affect a structure by

blocking channels and interfering with natural

drainage thereby increasing the magnitude of

other flood parameters. Ice poses significant

debris hazards, particularly during early spring

floods.

Manufactured homes are especially vulnerable

to damage caused by debris impact. In addition,

typical foundations, such as unreinforced

masonry, offer little resistance to battering from

debris. An inadequately elevated and anchored

manufactured home can itself become part of the

debris load during a flood. Further information concerning flood parameters is shown in Appendix C.

In addition to flooding, the manufactured home

installation will be subjected to wind forces,

sometimes occurring concurrently with flooding,

thereby placing additional loads on the manufactured home and its foundation.

WIND HAZARDS

High winds, most notably in coastal and mountainous areas, will impose forces on a manufactured home and the structural elements of its foundation. Damage potential is increased when the

wind forces occur in combination with flood forces.

In addition, as the manufactured home is elevated

to minimize the effects of flood forces, the loads

on the elevated structure itself, due to wind, are

increased. Figure 2.5 shows annual extreme

fastest wind speed 30 feet above ground for the

100-year mean recurrence interval.

Figure 2.5 Wind Speed Map

Wind forces exert pressure on a manufactured

home and its supporting foundation. Structural

components, such as walls and roofs, are affected

by winds and are designed to withstand certain

wind forces. Figures 2.6 and 2.7 illustrate these

lateral forces and their results.

Figure 2.6 Lateral Wind Force

17
they are 15 psf and nine psf respectively as shown

in Figure 2.9.

9/15 psf

Figure 2.7 Wind Damage

Uplift affects the roof of the manufactured home

and its anchoragelconnection to the foundation

system, as shown in Figure 2.8.

fl I I L t t LT T 

Figure 2.8 Wind Uplift Force

Wind forces also affect all connections within

the manufactured home as well as those securing the manufactured home to the foundation

system.

Wind is also of particular importance as it affects the loads imposed on the manufactured

home itself. The Manufactured Home Construction and Safety Standards (MHCSS) currently provides for structural integrity of manufactured

homes under two conditions-lateral and uplift

loads imposed by wind.

In hurricane zones, the lateral and net uplift

design loads are 25 pounds per square foot (psf)

and 15 psf respectively; in non-hurricane zones,

Figure 2.9 Wind Design Loads

This is important in site considerations because

wind and flood forces can be additive, thereby taxing the structure, its foundation system, and any

anchoring mechanisms.

If the manufactured home is located above the

flood elevation, the anticipated wind loads would

still be applied. However, they become increasingly more important to the foundation design as

elevation of the manufactured home is increased.

When considering siting techniques and design

criteria, the details on flooding and wind conditions

must be determined because they will dictate what

techniques can be used to reduce flood hazard

damage. As previously discussed, these include

depth, velocity, duration, rate-of-rise, and frequency of flooding and the anticipated forces due

to wind. With this information chapters three and

four can be used to develop an effective design

strategy for flood loss reduction.

18
CHAPTER III

ELEVATION AND ANCHORING TECHNIQUES

ELEVATION AND ANCHORING

TECHNIQUES

Site-specific flood information is critical when

considering a siting strategy for a manufactured

home or manufactured home community. In some

cases, the information gathered may ultimately

result in a decision against siting in the area or

may indicate that no additional flood-related

safeguards are necessary.

This chapter outlines general techniques which

focus on elevating the manufactured home in

riverine or coastal flooding areas. Chapter IV provides specific criteria and calculational procedures

to address the capability of the design techniques

to withstand anticipated flood and wind loads.

ELEVATION ON FILL

Earth fill, shown in Figure 3.1, is a widely used

elevation technique that, with proper construction

practice and materials, can be a means of

elevating a manufactured home above flood levels.

ability at some sites to connect the filled area to

higher ground for emergency evacuation in a

flood, and protection of building elements from

deterioration caused by exposure to floodwaters.

If the desired elevation is achieved by earth fill

and floodwater is not anticipated under the

manufactured home, a conventional installation

can be implemented, as shown in Figure 3.2, in

accordance with the manufacturer's instructions

and state or local installation standards as discussed in Chapter I.

Figure 3.2 Earth Fill and

Conventional Installation

Figure 3.1 Earth Fill

Fill should not be used in areas subject to floodwaters having a velocity greater than 10 feet per

second (fps) due to the risk of scour and erosion

and subsequent foundation failure. In addition, fill

should not be used where it will constrict the flow

of floodwaters and cause increased flood elevations or velocity.

Advantages of fill include its appearance, ability to improve access to the manufactured home,

If elevation by earth fill is being considered as

a partial solution with floodwater anticipated over

the fill, as shown in Figure 3.3, the manufactured

home will have to be elevated and anchored using techniques presented later in this chapter. In

this situation, the earth fill will have to be selected

and placed in consideration of soil saturation and

any anticipated floodwater velocity.

In the manufactured home community shown

in Figure 3.4, new sites were being developed. A

review of flood history showed recurrent flooding

with low velocity and three to four foot depths. By

elevating the new sites with two feet of earth fill,

a conventional installation was used and the occupied portion of these homes is now protected

during flooding.

19
horizontally beyond and contiguous to the top line

of the earth fill, should be cleared of standing trees

and snags, stumps, brush, down timber, other

vegetation, and all objects on and above the

ground surface or partially buried as shown in

Figure 3.6. The area should also be stripped of

topsoil and debris.

Figure 3.3 Earth Fill and Elevated Foundation

Figure 3.4 Earth Fill Site Plan

The earth fill elevation approach is widely used

throughout the United States in low velocity

"backwater" areas. Another example is shown in

Figure 3.5.

Figure 3.5 Earth Fill

Prior to filling, the area upon which earth fill is

to be placed, including a five foot strip measured

After Fill

Figure 3.6 Fill Areas

Earth fill material should be of a selected type,

preferably granular and free-draining. The fill

should be placed in layers not exceeding 12

inches with each layer compacted with rollers or

vibrating compacting equipment. Earth fill selection and placement should include consideration

of the effects of saturation from floodwaters on

slope stability, uniform and differential settlement,

and scour potential.

The minimum distance from any point of the

building perimeter to the edge of the fill slope

should be either 25 feet or twice the depth of fill

at that point, whichever is greater.

Slopes for granular fills should be no steeper

than one and a half feet horizontal to one foot vertical unless data justifying steeper slopes are

developed. For slopes exposed to flood velocities

of less than five feet per second, grass, vine cover,

weeds, brush, and similar vegetation undergrowth

will provide adequate scour protection. For

velocities up to 10 feet per second, stone or rock

slope protection should be provided. Fill should

not be used in areas subjected to velocities higher

than 10 feet per second.

Depending upon state or local regulations, individual sewage systems (septic systems) may be

prohibited in watersheds of the public water supply

system. Where septic systems are allowed the use

of earth fill will most likely be precluded due to its

impact on the performance of the individual

sewage system. This is because the additional

earth fill will affect the absorption rate of the

system. If earth fill is to be considered in conjunction with individual sewage systems, the local

building department should be consulted.

20

ADM
ELEVATED FOUNDATIONS

Elevated foundations are structural techniques

for elevating a manufactured home above a

specified flood level. Such techniques, as shown

in Figure 3.7, include piers, posts, piles, and

similar structural arrangements. These elevated

foundations are designed to withstand anticipated

flood and wind loading and manufactured home

weight while serving to reduce flood loss by raising the manufactured home above the anticipated

flood level.

It is important to note that for any foundation

design that elevates the manufactured home

above the anticipated flood level, the manufactured home must be anchored to the foundation

or ground to resist anticipated wind forces. This

is discussed in the "anchoring" section of this

chapter.

As discussed in Chapter 1 and shown in Figure

3.8, a conventional manufactured home installation consists of blocking on piers placed at intervals to support the steel frame of the home.

Figure 3.8 Conventional Installation

Components

Figure 3.7 Elevated Foundations

21
This generally results in elevation of the

manufactured home two to three feet above grade.

The use of techniques to achieve additional elevation should not override the minimum requirements for steel frame support and set-up as

specified in the manufactured home's installation

manual and ANSI Standard A225.1-1982. In

general, spacing of supporting members should

not exceed 10 feet and additional intermediate

supports may be required for concentrated loads

or areas adjacent to large wall openings. In some

cases, perimeter blocking may be required by the

manufacturer and must also be provided in any

elevated installation.

Piers-As shown in Figure 3.9, a pier foundation

system uses brick, concrete masonry units (CMU),

or cast-in-place concrete. Lateral forces are

resisted through vertical steel reinforcement, bracing, and anchoring and augmented by the dead

weight of the pier and footing.

....... I

Figure 3.9 Pier Designs

Generally piers have a low resistance to lateral

forces. This, in addition to the potential for scour

and erosion of the footing, makes the use of piers

inappropriate in areas subjected to floodwater

velocity.

Built-up piers are generally effective for elevations up to 10 feet, depending on their size, reinforcement, soil conditions, and anticipated lateral

forces. In general, height should be limited to a

maximum of 10 times their least dimension, with

square piers preferable to rectangular piers to

minimize affected loads.

In all but a few cases built-up piers should be

a minimum of 12 inches by 12 inches and reinforced with a minimum of four No. 5 steel bars.

These minimum requirements apply whether the

pier is freestanding or laterally braced. In cases

where large loads are anticipated, the pier cross

section should be increased or additional reinforcement such as cross bracing added. A larger

cross section can be obtained by using piers two

feet or more in width with the long dimension

placed parallel to any anticipated flow, as shown

in Figure 3.10,

grade.

buried at least 30 inches below

Flood Floew Direction

Re-n fo r... ............ ............. ........-..

Fat7"aI - f^,--- -- - -- - -- - - -- - -- - -- - - ; -

Figure 3.10 Large Pier Design

Built-up piers should be laid with type M or S

mortar. Hollow concrete masonry units should be

filled with concrete or high strength mortar after

reinforcement as shown in Figure 3.11.

Concrete grouting

Steel

Relnftrcing

Figure 3.11 Filling of Piers

Cast-in-place concrete piers can also be used

and are essentially reinforced concrete columns.

They are cast in forms set in machine or handdug holes. The holes can be widened or belled

at the base to form a footing integral with the pier,

or a separate footing can be constructed as shown

in Figure 3.12. In the few cases where soil bearing capacity may be adequate to support the

maximum vertical load and no flood velocity is

anticipated, the footing can be eliminated and

loads left to end bearing and friction between soil

and pier.

22

-- --- --I-- -- 

i i
As shown in Figure 3.13, posts replace the

blocking arrangement described for a conventional

installation and carry the weight of the manufactured home by bearing-on undisturbed soil or a

concrete bearing pad.

Figure 3.13 Post Foundation

Figure 3.12 Cast-in-place Pier

The dimensions, reinforcement, and spacing of

cast-in-place concrete piers depend on the manufactured home support spacing requirements as

well as anticipated flood and wind loads.

Pier footing sizes are a direct function of soil

bearing capacity and loading. Depth of pier

footings depends on local frost penetration levels

and expected flooding, wind, and erosion levels.

Local codes may provide specific requirements for

the size and depth of footings based on local soil

conditions. At a minimum the bottom of the footing

must be at least 30 inches below grade or to the

frost line, whichever is greater.

As with all elevation techniques, footings in

areas where the soil bearing capacity is not known

can be unstable and should, therefore, be designed with the guidance of a soils engineer.

Design calculations, suggested construction

details, and anchoring criteria for pier foundations

are presented in Chapter IV, "Design of Elevated Foundations."

Posts-Posts may be used to elevate a

manufactured home above an anticipated flood

level. The height of the posts, design requirements, and the need for additional anchoring will be determined by anticipated flooding conditions, wind loads, and the type of terrain and soil

on the manufactured home site.

Lateral forces from moving water, wind, and

debris are resisted by embedment in the soil, post

size, horizontal cross beams, and by the connection of the steel I-beams of the manufactured home

chassis to the foundation. Posts are a suitable

elevation method in areas where the soil has the

bearing capacity to resist the manufactured home

foundation loads and where high velocity floodwater is not anticipated. Recommended post sizes

and depths of embedment for various wind and

roof load zones and heights above grade are

presented in Chapter IV, "Design of Elevated

Foundations."

Post foundations can be of wood, concrete, or

steel with members set in predug holes or connected to poured concrete piers. Posts can be

round, square, or rectangular and may range from

four to 12 inches in cross section. Rectangular

posts are easier to frame into and are generally

stronger for a given cross sectional area.

Post foundation holes may be excavated by

hand or machine; however, holes deeper than six

feet generally require machine assistance. The

post design, method of installation, and depth to

which posts should be embedded are addressed

in Chapter IV and depend on many conditions including: the type of soil; the depth of the frostline;

anticipated lateral loads from floodwater; debris

impact; wind forces; anticipated erosion from

water flow; and the size of the posts. Post holes

should be a minimum of eight inches larger in

diameter than the greatest dimension of the post

section, thereby allowing for proper alignment and

backfilling.

23
Based on the design loads and the allowable

soil bearing capacity, the post can be set on undisturbed earth at the bottom of the hole as shown

in Figure 3.14. In this case, the bearing capacity

of the soil will govern the elevated foundation

design as presented in Chapter IV, "Design of

Elevated Foundations."

Figure 3.14 Post Set on Soil

Where additional support is needed because of

inadequate soil capacity, bearing capacity can be

improved by pouring a concrete bearing pad at the

bottom of the hole as shown in Figure 3.15. This

pad should have a diameter at least twice the post

diameter and be one post diameter thick (a

minimum eight inches thickness is recommended).

After proper post alignment in the hole is

achieved, clean, well-compacted backfill is

necessary to ensure a post configuration with good

lateral stability and resistance against wind and

flood loads.

Common backfill materials are sand, gravel,

crushed rock, pea gravel, soil cement, concrete,

and earth with granular fills that provide good

drainage. Backfill materials should be dampened

and mechanically tamped during construction to

provide adequate compaction.

Backfilling the hole with concrete, as shown in

Figure 3.16, rather than gravel or sand adds

stability to the structure and increases the bearing area. Soil cement is an economical alternative

to concrete and attains nearly equal strength in

all soils except clay. Soil cement is made by mixing the earth removed from the hole with cement

in a ratio of one part cement to five parts earth,

Figure 3.15 Post Set on Bearing Pad

plus water as directed by the manufacturer. To

achieve the best results, all organic matter should

be removed from the earth, and it should be sifted

to remove all particles larger than one inch.

Wood Post

-Backtill wl

Concrete

Grade

Figure 3.16 Post Backfilled with Concrete

Flood and wind forces are less likely to overturn

or uplift posts if the posts are anchored to a foun24
dation. Two ways to anchor posts are to embed

them in concrete or to fasten them to metal straps,

angles, plates, etc., that are themselves anchored

in concrete footings, piers, or pile caps.

Figure 3.17 shows one method of anchoring

wood posts in concrete. Spikes or lag bolts 5/8 to

3/4 inch in diamenter are driven into the post

around its base. The post is placed into the hole,

secured to bracing restraints to prevent movement

and the concrete is poured.

A calculational procedure, design guidelines,

and examples for determining the adequacy of a

particular post system design for elevating

manufactured homes under differing loads is

presented in Chapter IV.

Piles-Piles, as shown in Figure-3.1-9, are vertical supports made of wood, steel, or reinforced

concrete which can be driven into the ground to

support a manufactured home. The use of properly

driven and embedded piles is an elevation technique that is appropriate in flood-prone areas subject to high water velocity and depth and having

poorly drained soils with low bearing capacity,

such as coastal areas.

Spikes or Lag Screws

Figure 3.17 Post Anchoring

Above-grade wood posts supported by concrete

piers may also be considered as shown in Figure

3.18. In this case, the entire length of the post is

accessible for maintenance; however, the connection between post and pier must be designed to

resist all applied loads.

Wood Post

Hooked Rod

Pier to Footing

Reinforced Foot

Figure 3.19 Piles

Round timber piles, which are frequently used,

are generally available in longer lengths than

square timbers; and for lengths greater than 25

feet, round piles are frequently the only piles

available. A minimum tip diameter of eight inches

and a butt or top diameter (at the floor beam level)

of 11 inches or more are recommended for round

piles.

There are several methods available for embedding piles. Pile driving is an excellent method due

to the strength of the pile and earth interface to

resist vertical and horizontal loads. An alternative,

the drop hammer, consists of a heavy weight that

is raised by a cable attached to a power-driven

winch. The weight is then dropped five to 15 feet

ing

25

Grade

Figure 3.18 Post Anchoring
onto the end of the pile. Drop hammers, however,

must be used with care because they can damage

wood piles.

A much less desirable but frequently used

method of inserting piles into sandy soil is "jetting." Jetting involves passing a high pressure

stream of water through a pipe advanced

alongside the pile. The water blows a hole in the

sand into which the pile is continuously pushed

or dropped until the required depth is reached.

Sand is then tamped into the cavity around the

pile and the end of the pile pounded with a sledge

hammer or other available weight. Unfortunately,

jetting loosens the soil around the pile. Decreased

friction due to loose soil results in a lower load

capacity. To increase the load capacity, jetted piles

must be inserted deeper into the ground than

driven piles.

If the soil content has sufficient clay or silt (has

a higher shear strength), a hole can be excavated

by an auger or other means and will stay open in

order to drop in a pile. Sand or pea gravel is then

poured and tamped into the cavity around the pile.

This technique does not provide as much load

resistance as driving the pile into the ground, and

longer piles are necessary. Some final driving with

a sledge hammer can be helpful.

As with post foundations, piles are often partly

backfilled with concrete to improve their resistance

to lateral loads. As shown in Figure 3.20, the area

at grade around each pile can be replaced by a

concrete collar extending several feet below

grade.

Grade

-Reinforced

Concrete

Collar

V. 

Figr 3 - -C . r 

Figure 3.20I Pile with Concrete Collar

Such collars provide protection from minor erosion, add more deadweight to the structure, and

increase the pull-out resistance of the pile.

The necessary pile size and depth of pile

embedment depends on the number of piles used,

size and weight of the manufactured home, wind

and flood loads, soil bearing capacity, and potential for scour and erosion. Calculations and suggested construction criteria are presented for piles

in Chapter IV.

Horizontal Beams-In a post or pile elevated

foundation system, horizontal beams must be provided to support the manufactured home. As

shown in Figure 3.21, these horizontal members

tie opposing posts or piles together and distribute

any flood, wind, or roof live loads to those vertical

members and into the ground.

Figure 3.21 Horizontal Members

Although steel channel sections could be used

for this foundation component, sawn timber beams

are considered preferable based on their availability. Chapter IV presents details on the required

number and size of these horizontal beams to

carry various imposed loads.

Bracing Elevated Foundations-The vertical

and horizontal members of an elevated foundation must be braced, as shown in Figure 3.22, to

resist flood and wind induced loads. Even in areas

where low loading is anticipated, bracing will provide added assurance that the elevated foundation will withstand the impact of floating debris or

excessive flood or wind forces.

Although bracing placed underneath a manufactured home may be struck by floating debris or

subject to other flood forces, the negative effects

of these on a structure's survivability are generally

outweighed by the beneficial effects of the bracing.

Knee bracing, shown in Figure 3.23, is effective in increasing lateral strength of posts or piles

to resist all loads.

26
in a like manner to the adjacent post or pile just

below the manufactured home. Although diagonal

bracing is more likely to be struck by floating

debris than knee bracing, this is generally

outweighed by the greater lateral stability diagonal

bracing provides, especially in higher elevated

manufactured homes.

Figure 3.24 Diagonal Bracing

Steel rods, as shown in Figure 3.25, can

sometimes be used to diagonally brace wood

posts or piles. The rods are fitted through drilled

holes filled with wood preservative and fastened

with nuts and cast beveled washers. Welded connections or drill holes can be used to provide rod

bracing in steel post or pile foundations.

Figure 3.23 Knee Bracing

Lumber no less than two inches nominal

thickness should be used with bolted connections

because of the resistance to pullout forces by the

bolted connection. Knee bracing should be bolted

to and run between the horizontal beam supporting the manufactured home and to the post or pile.

Diagonal bracing, as shown in Figure 3.24, is

bolted at the base of one post or pile and fastened

Figure 3.25 Diagonal Bracing with Steel Rods

The severe flood and wind forces encountered

in coastal zones may require the use of additional

special bracing techniques which should be

designed in conjunction with qualified engineering consultants.

Perimeter Enclosure (Skirting)-The perimeter enclosure is a weather-resistant material

which encloses the area from the bottom of the

manufactured home to grade. It is primarily used

27

Figure 3.22 Bracing
to improve the appearance of the manufactured

home.

Because floodwaters reach the perimeter

enclosure before the floor level of the home, the

design must take into account anticipated flood

forces. Commonly used enclosures are not designed to resist flood or wind forces. The

enclosure, as shown in Figure 3.26, may be subjected to either of these forces in addition to impact from floating debris and must not transfer

these loads to the elevated foundation or the

manufactured home. Consequently, the design

must ensure that the enclosure disconnects during flooding and does not transfer any forces to

the manufactured home or its foundation.

A perimeter wall is essentially an extended foundation wall which acts as a deep beam in resisting

forces in the plane of the wall. From a structural

standpoint, the most important consideration is the

ability of the wall to resist lateral forces. For this

reason, the perimeter walls must provide a means

of allowing floodwaters to enter the enclosure,

thereby eliminating lateral loads. To minimize drag

forces, the walls should be parallel to anticipated

flood flow.

Supports for the manufactured home would be

horizontal beams, as previously discussed, located

across the span between the walls. As an option,

steel I-beams could be used. Note that the

manufacturer's recommended chassis I-beam

support spacings must not be exceeded and,

where required, perimeter blocking between the

manufactured home and perimeter wall must be

provided.

ANCHORING

Conventional Tie-Down or Anchoring

Systems-Conventional tie-down or anchoring

systems are designed to resist the lateral and uplift

forces resulting from wind. The system is made

up of two distinct components, tie-downs (ties) and

anchors. There are two types of ties: (1) the overthe-top tie as shown in Figure 3.28, and (2) the

frame tie as shown in Figure 3.29.

Figure 3.26 Perimeter Enclosure

Perimeter Walls -In areas with floodwater

velocity less than five feet per second (fps) and

where soil bearing capacity is adequate to support imposed loads, perimeter walls, as shown in

Figure 3.27, can be feasible.

Figure 3.28 Over-the-Top Tie

Figure 3.27 Perimeter Walls Figure 3.29 Frame Tie

28
The first resists vertical loads; the second resists

vertical and lateral loads. Frame ties will always

be required to resist lateral and uplift forces. If the

manufactured home is designed for over-the-top

ties they can be used in conjunction with frame

ties.

Ties are made of rust-resistant steel cable or

straps which fasten the home to anchors embedded in the ground. In combination over-the-top

and frame ties, the cable or strap is secured to

the ground anchor with a yoke-type fastener and

a tensioning device, or with clamps and turnbuckles as shown in Figure 3.30.

As indicated previously, ties are just one component of a two component anchoring system. Anchors are the component set in the ground to

which ties are connected. This effectively transfers

wind loads, both lateral and uplift, to the ground.

As shown in Figure 3.32, these loads can be

significant enough to dislodge anchors.

Figure 3.32 Failed Anchors

There are several types of ground anchors

which can be used, including screw augers, expanding anchors, slab anchors, and concrete

deadmen as shown in Figures 3.33 to 3.35.

Grade

Figure 3.30 Tie Components

In frame ties, the strap is secured to the ground

anchor and manufactured home I-beam as shown

in Figure 3.31.

- - SCoil D-itl C' Strap

To Frae Head 1le of Stop Th.ood

ASh-~n.

D-uil "A"

To "I" Beo-

< S > Coil D-il "D"

Foe Hod Anchor

Mveil "B" (I P.D:

D-Fil "C "

(Typ. Fom- SIr~p B.

\ F-Thi, S-cion -f Coi CLGo- Ar-ond l-B-e

__ Fre Hood

Minimum Rod

Diameter 5/su

oil "D" I

.Anchor H-ad)

Figure 3.31 Tie Components

Minimum Depth

4 Feet

4 Minimum Diameter, Type A

4 Minimum Diameter, Type AA

Figure 3.33 Screw Auger

29
E

3

-t

In

Poured In Place

Concrete Dead Man

-Backfill Must Be

Wsell Tamped

C

F

U

LI

Q

-C

U1

co

eCE

I

-A

Figure 3.34 Concrete Deadman Anchor

-2500 PSI ornreater

Concrete compressive

Strength

Figure 3.35 Anchors in Concrete Slab

Because different soils provide different holding

strengths, the same type of anchor may not perform uniformly in all situations. For example, a six

inch diameter screw auger, which might withstand

a pull of 5,000 pounds in stiff clay, may withstand

only 2,500 pounds in sandy clay.

The holding power of ground anchors can be

determined by conducting pullout tests or by consulting with an anchor manufacturer who should

be able to provide data on anchor capacity for the

various kinds of soils anticipated. If soil conditions

are such that the required degree of holding power

is not attainable, or if the anchor selected cannot

withstand anticipated loads, use of larger sized anchors or additional anchors and ties will be

necessary.

Anchoring and Elevated Foundations-The

necessity of elevating the manufactured home

above the expected levels of flooding is critical.

As previously discussed, elevating the manufactured home can be accomplished using a variety of techniques such as fill, piers, posts, or piles. Once elevated above the expected flood level, it

is necessary that the manufactured home be anchored to resist wind forces. Depending on the

type of elevated foundations used, the manufactured home can either be anchored to the ground using devices such as ground anchors or anchored directly to the elevated foundation.

Pier foundations can have limited resistance to

lateral loads. It is, therefore, important to minimize

the transfer of lateral wind loads to this type of

foundation. Devices independent of the foundations, such as ground anchors, must be used as

shown in Figure 3.36.

Figure 3.36 Anchoring on Pier Foundation

A post or pile foundation has much greater

resistance to lateral loads, and all lateral loads can

be transferred directly to the foundation. The

manufactured home can be anchored directly to

the foundation using two basic methods. If a

manufactured home is designed for over-the-top

ties and frame ties, the ties can be connected

directly to the elevated foundation as shown in

Figure 3.37.

If the manufactured home is designed for frame

ties only, the I-beams can be connected directly

to the horizontal beam as shown in Figure 3.38.

30
EVACUATION

The temporary relocation of manufactured

homes from a flood-prone site prior to flooding has

been practiced successfully in some areas, as

shown in Figure 3.39. Such action may be a viable

strategy for manufactured home parks or subdivisions that were established prior to the availability of flood elevation data, where manufactured

homes are below the base flood elevation.

Figure 3.37 Anchoring on Post or Pile

Foundation

Figure 3.38 Frame Ties

In all cases the intent of anchoring is to ensure

the manufactured home does not become dislodged from the elevated foundation due to the impact of wind forces.

The type and number of manufactured home anchoring connections for elevated post and pile

foundations is presented in Chapter IV.

Figure 3.39 State and Local Evacuation Plan

However, it must be recognized that this procedure is heavily dependent on the mobility of the

manufactured home and requires an extremely

comprehensive evacuation strategy. In addition,

flood insurance coverage under the NFIP may not

be available for manufactured homes which have

either not been elevated or properly anchored in

accordance with the community's adopted ordinances for participation in the NFIP.

It is important that a plan to relocate manufactured homes during a flood event be coordinated

with and involve local emergency management

authorities. It is crucial that an evacuation strategy

be pre-planned, that is, the procedures must be

developed and in place prior to a flood being imminent. Also, such a plan is ideally executed as

a group effort rather than as an individual

endeavor so that "logjams" and civil disorders can

be avoided or minimized.

A primary and major consideration in developing an evacuation plan is the availability and

capacity of evacuation routes and temporary

storage areas. It is absolutely critical that the

movement of manufactured homes will in no way

interfere with any concurrent evacuation of inhabitants of an area threatened by flooding. This

31
is especially important in areas with limited escape

routes or areas where mass evacuations may be

necessary, such as hurricane zones. Adequately

protected sites must be available at which the

manufactured homes can be temporarily stored

until it is determined that they can be returned to

their permanent sites.

Another important element of an evacuation

plan is the need for an effective early-warning

system. Considerable time is required for the actual implementation of an evacuation, therefore

adequate warning is essential. Lead time required

will vary by situation and is composed of several

factors, some tangible and some unpredictable.

Once the decision is made to evacuate, the plan

must be initiated. As with any emergency situation, communication problems may be encountered. Participants, including owners of the

manufactured homes, must be alerted. The status

of evacuation routes and temporary storage areas

must be assessed. Towing devices, such as

trucks, tractors, crawlers, etc., must be obtained

and brought to the flood zones. The manufactured

homes will require preparation for transport.

Utilities must be disconnected, contents must be

secured, and separation from any foundation

system and wind anchors must be accomplished.

Coupling to the towing device is also involved.

Time required for actual transport will vary depending upon distance to temporary site, number and

condition of towing units, weather and traffic conditions, etc.

In addition to the time required to perform these

necessary tasks, additional time will likely be required to resolve problems such as mechanical

failure or lack of necessary personnel and equipment. Sufficient warning time to be provided by

a warning system is a function of all of the above

considerations and the number of manufactured

homes to be evacuated.

The feasibility of an evacuation strategy is also

affected by the availability of necessary equipment

and personnel. Towing units must be procurable,

and special equipment and tools will be necessary

to prepare the manufactured home for transport.

Jacks will be required for separation from foundation, hand tools will be necessary for utility

disconnection and preparation work, and special

equipment, such as portable lights and air compressors to inflate tires, should be considered. Ample, capable personnel must be available to coordinate efforts and execute the actual procedures.

This includes coordination, equipment operation,

manufactured home preparation, and troubleshooting.

Owners may be incapable of performing

evacuation tasks themselves as certain skills may

be required, particularly where safety considerations are involved, such as the disconnection of

gas and electricity services. Representatives of

utility services should, at a minimum, be involved

in the planning process of the evacuation.

The success of the evacuation is also directly

related to the condition of the manufactured

homes themselves. Structural integrity must be

such that the unit can be transported without

damage. The manufactured home's transport

system, including the chassis, axles, wheels, tires,

and hitch must be well maintained. The system

must either be in place and operational or capable

of being readily assembled. The manufactured

home must be easily separable from any foundation, skirting, wind anchors, and structural

elements such as porches and additions. Quick

and safe utility disconnect is also a requirement.

The actual evacuation plan must be developed

based on the number of manufactured homes to

be transported. Other factors must also be quantitatively assessed: evacuation routes and storage

areas, towing units and available equipment and

personnel. The status of these parameters, in addition to the warning system, must also be addressed. The pl # should be functional, simple,

and easily mar ged. A priority basis should be

established-those manufactured homes most exposed to potential flood damage should be removed first.

Participants must be familiar with overall procedures and individual tasks. To make the plan

complete, consideration must also be given to

returning the manufactured homes back to their

permanent sites when it is deemed to be safe. This

could involve considerable restoration work.

Before development of an evacuation plan is

undertaken, it should be determined that no other

alternatives are practicable. The absolute conclusion that must be made is that evacuation can be

accomplished safely without jeopardizing life or

property.

32
CHAPTER IV

DESIGN OF ELEVATED FOUNDATIONS

This chapter provides information with which

elevation techniques discussed in Chapter III can

be evaluated to determine their ability to resist anticipated loads. In addition, the chapter presents

an overview of flood-induced loads, tables of

calculated forces, governing design equations,

and specific technical information which can be

used to determine an appropriate elevated foundation design for a manufactured home.

The capability to withstand flood-induced loads

depends upon not only the site design and structural capacity of the foundation, but also upon the

manufactured home itself. Therefore, it is critical

to assure that the design criteria under which the

manufactured home was constructed are not exceeded through the transferal of flood-induced

loads to the manufactured home itself.

The minimum design criteria for manufactured

homes required by the HUD's Manufactured

Home Construction Safety Standards (MHCSS)

are as follows:

Roof live load

Roof wind net

uplift

Wall wind load

40, 30 or 20 pounds per

square foot (psf) depending

upon anticipated location

15 or 9 psf depending upon

anticipated location

25 or 15 psf depending upon

anticipated location

Floor live load 40 psf

Dead Load 20 psf

The maps shown in Figures 4.1 and 4.2

geographically depict the roof live load and wind

load zones.

Figure 4.1 Roof Live Load

IfG s Standard Wind Zone I- 15 PSF Horizontal- 9 PFPplft Ulf X

Figure 4.2 Wind Load

Generally, manufactured homes have the ability

to resist only a very limited range of flood forces

before the design capacity of the manufactured

home is exceeded and significant damage results.

For example a nine to 12 inch floodwater differential between the inside and outside of an anchored

manufactured home can cause an upward floor

deflection, failure of the floor decking, and floodwater inundation.

The technical information in this chapter includes consideration of the design capacity of the

manufactured home. Any foundation "over

design" would not improve the strength capabilities of the manufactured home design. The foundation system recommendations, therefore, are

compatible with the manufactured home design

criteria.

Each manufactured home siting situation will be

unique. The amount of information necessary to

provide specific technical guidance for all possible siting and flood loading situations would be

overwhelming. This chapter, therefore, provides

general guidance and technical information which

can be effectively used in determining the limits

of a particular candidate design. In this chapter,

example calculations are provided to assist in the

use of various design charts and tables. The user

is cautioned, however, that the examples provided

apply only to the specific conditions stated for each

design problem. Extrapolations from solutions provided for specific examples are not recommended.

As previously discussed, state regulations,

such as those shown in Figure 4.3, may require

design by a professional engineer.

33
Figure 4.3 State and Local Regulations

A. FLOOD FORCES AND THEIR

APPLICATION

A manufactured home installation can be affected by flood and wind-induced forces acting

directly on the manufactured home, and on its

elevated foundation system. This portion of the

manual introduces and applies various flood and

design forces to form the basis for the general

design guidance for elevated foundation techniques presented in Section B of this Chapter.

Hydrostatic Forces

Hydrostatic forces are those caused by both free

and confined water occurring above or below

grade. These forces are equal to the product of

the water pressure and the area of the surface on

which they act. In addition, hydrostatic forces are

equal in all directions and act perpendicular to any

surface with which floodwaters come in contact.

In the case of manufactured homes, hydrostatic

forces include lateral forces (those acting in a

horizontal direction), uplift or buoyancy forces

(those acting vertically upward), or a combination

of both as shown in Figure 4.4.

Lateral Forces

The lateral force of water acting against the wall

of a manufactured home is related to the water's

depth and specific weight.

During a flooding condition, water against a

manufactured home creates a hydrostatic pressure distribution as follows:

p = wxh

Where:

p =pressure in pounds per square foot

w =specific weight of water-62.4 pounds

per cubic foot

h = distance in feet measured vertically

downward from the water surface to the

point of action

The resultant lateral hydrostatic force acting per linear foot on the submerged portion of the

manufactured home is the total area of the

pressure distribution given by:

Ff = 1/2 pxh = w/2wxh2

Where:

Ff = lateral force in pounds per linear foot

h = distance in feet from water surface to

bottom of manufactured home chassis

I-beams

A triangular distribution based on the amount

of submerged area of the manufactured home

results, as shown in Figure 4.5. The lateral

hydrostatic force is assumed to act at a point 2h13

down from the water surface. This force can be

significantly intensified if perimeter enclosures are

installed since the distance h is then measured

to the bottom of the enclosure.

Figure 4.5 Lateral Forces

Figure 4.4 Horizontal and Vertical Forces

For example, if a manufactured home without

perimeter enclosures was submerged in water to

a point two feet above the bottom of the chassis

I-beams, the resulting lateral force would be 124.8

pounds per linear foot. For a 60-foot long manufac34
tured home, this translates into nearly four tons

of pressure.

Table 4.1 provides the total lateral forces and

point at which they act (center of gravity) for

various water depths. The shaded areas in the

table, corresponding to water column heights

greater than or equal to 12 inches, highlight where

these forces become overly excessive and exceed

the design capacity of a manufactured home. Consequently, further elevation should be considered.

Buoyancy

In backwater or no-flow conditions, the critical

hydrostatic force on a manufactured home is buoyancy which is dependent upon the difference in

water height inside and outside the manufactured

home as shown in Figure 4.6. As the manufactured home is raised above grade, the flood depth

relative to the manufactured home floor elevation

will decrease, as will the buoyancy force.

Elevating the home above potential flood levels

is the only practicable method for eliminating

all anticipated buoyancy forces on the

manufactured home.

The buoyancy force is related to the weight of

water and height of water displacement and is

calculated as follows:

Fb = wxAh

Where:

Fb = Buoyancy force in pounds per

square foot

w = Weight of water-62.4 pounds per

cubic foot

Ah = Difference in height of water column inside and outside in feet

Buoyancy is equal to the weight of water times

the difference in water height outside and inside

the submerged surface. Water one foot above the

bottom of an unflooded manufactured home

interior would yield a buoyancy force of one foot

x 62.4 pounds per cubic foot or 62.4 psf as shown

in Figure 4.7. When the buoyancy force is greater

than the weight of a manufactured home, which

is approximately 20 pounds per square foot of floor

area, the capacity of a conventional anchoring

system is exceeded and flotation can occur. In

view of these force considerations, manufactured

homes must be elevated above anticipated flood

levels.

Figure 4.6 Buoyancy Forces

Table 4.1

LATERAL FORCES

(pounds per linear foot of

(Fe)

home length)

Height of Water Colum (inches)

1 2 3 4 5 6 9 12 15 18 24 30 36 48

F4 0.22 0.87 1.95 3.47 5.42 7.80 17.55

c 0.66 1.33 2.00 2.67 3.33 4.00 6.00

Fk = 1/2 w x h2 (lb./lin. ft. of home)

Where:

w = 62.4 lb./cu.ft.

h = height of water column in feet, and

c = center of gravity in inches through which

Ftacts (2.h/3) measured down from water surface.

35
Figure 4.7 Buoyancy Force

Based on a manufactured home weight of 20

pounds per square foot of floor area, it is possible

to determine the net buoyancy forces associated

with differences in floodwater height.

Table 4.2 is the gross buoyancy force (Fb) exerted per foot length of the manufactured home

due to a particular difference in anticipated water

depth (h) inside and outside the manufactured

home.

The Table includes up to 48 inches of water

height difference for the purposes of highlighting

the magnitude of forces. Based on the floor design

capacity, any buoyancy above a nine to 12 inch

water height differential can cause the floor decking to pull off the floor joists, thereby causing water

inundation into the home. Those areas in the

tables are shaded to highlight the practical limits

in relation to buoyancy and the need to consider

additional elevation.

The risk of flotation is largely dependent on the

rate of water rise. With a fast rise, there is insufficient time for water to seep into the manufactured

home, resulting in flotation from increased buoyancy forces. In slow rising flood conditions,

floodwater can enter the manufactured home at

a rate more equal to the water rise, thereby

counteracting any buoyancy forces but causing

substantial inundation damage. Elevation above

anticipated flood levels is again highly important

to minimize damage to the manufactured home.

Hydrodynamic Forces

Hydrodynamic forces are those imposed on any

object which restricts a moving fluid. In the case

of a manufactured home, hydrodynamic forces are

caused by floodwater impact against the manufactured home and foundation. Drag forces, as

shown in Figure 4.8, are one type of hydrodynamic

force and are dependent upon the velocity of the

floodwater, the drag coefficient of the surface

restricting the flow and the depth to which the surface is submerged.

Figure 4.8 Drag Forces

Table 4.2

BUOYANCY FORCES (Fb)

1 2

Difference in water height inside and outside the manufactured hao (in.)

3 4 5 6 9 12 15 18 21 24 30 36 42 48

62.4 124.8 187.2 249.6 312.0 374.4

145.6 218.4 291.2 364.0 436.8

249.6 374.4 499.2 624.0 748.i

291.2 436.8 582.4 728.0 Am3" 8

Fb = h x v x W (lb./lin. ft. of hole)

Where: h = (hl-h 2O ft.; v = 62.4 lb./cu. ft.; W= width (ft.)

36

Hai#

Width

(ft.)

12

14

24

28

92.8

124.8

145.6
Drag forces are calculated as follows:

Fd = Cd x A x p x V2

2

= Drag force in pounds

= Coefficient of drag-1.31 for

a manufactured home

e = Mass density of water-1.94

slugs per cubic foot

V = Floodwater velocity in feet

per second

A = Projected vertical area

submerged in square feet,

including the chassis I-beam

For example a 60 foot long manufactured' home

submerged in six inches of water having a velocity

of five feet per second acting on the side of the

manufactured home chassis I-beam, as shown in

Figure 4.9, would be impacted by the following

drag force:

Fd = 1.31 x (60 x 0.5) x 1.94 x (5)2 = 953 pounds F d = ~~~2

Note that the bottom of the chassis I-beam

is the point at which drag forces initially occur.

Since the drag force is uniform over the side of

Figure 4.9 Drag Force

the manufactured home, the drag force per square

foot can be determined as follows:

953 pounds 31.77 psf.

(60 ft. x 0.5 ft.) -

Table 4.3 provides drag forces as a function of

water velocity and height. Areas in the table where

drag forces exceed the capability of a manufactured home design are shaded.

Table 4.3

DRAG FORCES (Fd)

(pounds per linear foot of home length)

V Water Height (in.)

(fps) 1 2 3 4 5 6 9 12 15 18 24 30 36 48

1.0

2.0

3.0

4.0

5.0

6.0

8.0

10.0

12.0

15.0

.11 .21 .32 .42 .53

Fd = 1.31 x A x 1.94 x V2 (lb./lin. ft. of home)

2

.64 .95 1.27 1.59 1.91 2.54 3.18 3.81 5.08

Where: A = projected submerged vertical area in square feet

V = floodwater velocity in feet per second

37

Where: Ed

Cd
If, for example, one foot of water depth flowing

at six feet per second were anticipated, the drag

force would be 45.75 pounds per foot of length.

As the water velocity increases, the drag force

will increase significantly because of the V2 term

in the equation. In the above example note that

a doubling of velocity from six fps to 12 fps yields

a drag force four times as great or 182.98 pounds

per square foot of impacted area.

Drag forces due to water impacting the

manufactured home can impose a significant increase in the loads on an elevated foundation. For

this reason the manufactured home must be

elevated above anticipated flooding. In addition,

drag forces will affect any manufactured home

foundation enclosure and transfer the drag forces

to the elevated foundation. Provisions for allowing water to flow under the manufactured home

should, therefore, be employed to reduce drag

forces. In some higher velocity situations, any

manufactured home foundation enclosure should

be designed to break away before the forces cause

overloading or failure of the elevated foundation

system.

Another important consideration is the difference in height of the water upstream and

downstream of the manufactured home. Water

height will be greater on the upstream side; subsequently, the buoyancy and lateral forces will be

greater on the upstream side. This imbalance of

forces on the manufactured home will create a

tendency for the manufactured home to overturn

as shown in Figure 4.10.

Figure 4.10 Overturning

The difference in height between the upstream

and downstream side of the home is dependent

upon water velocity and the position of the

manufactured home in relation to flow. If the

manufactured home is placed parallel to the flow

as shown in Figure 4.11, the drag forces are reduced due to a smaller area being exposed to flow,

thereby reducing the tendency for the manufactured home to overturn.

Figure 4.11 Parallel to Flow

If the manufactured home is placed perpendicular to flow, the area of the manufactured home

affected by the flow, as shown in Figure 4.12, increases as do the resulting drag force and the

height differential between upstream and

downstream flow.

Figure 4.12 Perpendicular to Flow

Due to lack of data available on height differential related to flow velocity, manufactured home

size and position in relation to flow, no specific

calculations or design techniques can be offered.

If, however, the manufactured home is elevated

such that floodwaters are not expected to reach

the manufactured home, any tendency to overturn will be eliminated. It is important to note,

however, that an elevated manufactured home

must be anchored to the elevating foundation or

ground to prevent overturning from wind.

38
Impact Forces

Impact forces occur due to floating debris, ice,

and other water-borne objects hitting the manufactured home or elevated foundation as shown in

Figure 4.13. The magnitude of these forces will

be dependent upon the mass of debris and the

velocity of the water in which they are carried.

Figure 4.13 Impact Forces

Impact forces are calculated as follows: Fi F m x dVb ~dt --- 

Where: Fi = impact force in pounds

m = mass of the water displaced

by the object in slugs

Vb = velocity of the object in feet

per second

t = time in seconds

dVb = acceleration (deceleration) of

dt the object in feet per second

squared

The impact force acts horizontally at the flood

level and can be assumed to be the impact force

produced by a 1000 pound object traveling at the

velocity of the floodwater and acting on one square

foot of the manufactured home or elevated foundation. It can be assumed that the velocity of the

object goes to zero in one second and the Impact

Force equation can be restated as:

Fi =31 xVb

Table 4.4 provides impact forces as a function of

water velocity. For instance, at a velocity of 10 feet

per second the impact force would be 310 pounds

acting on one square foot of surface.

IMPACT FORCES (Fi)

(Pounds per square foot)

Velocity fps)

1 2 3 4 5 6 7 8 9 10 151!

31 62 93 124 155 186 217 248 279 310 465

Fi = 31 x Vb (lb./sq. ft.)

Where: Vb = velocity of the object in feet per

second

Where floating debris is anticipated, the impact

force on the manufactured home elevated foundation must also be taken into account in the

design. Since the manufactured home itself is not

designed to withstand these forces, the importance of elevating above anticipated flood depths

is again apparent.

Scour

Scour is an important consideration for the

manufactured home elevated foundation.

The calculation for determining scour depth at

the elevated foundation members as shown in

Figure 4.14 is based on their shape and width as

well as the water velocity, water depth, and type

of soil.

Figure 4.14 Scour

39

Table 4.4
The scour depth at square and circular piers,

posts, and piles can be calculated as follows:

S = d[2.2(a)0.65 X (_V )0.43

Where: S = depth of scour hole in feet

d = depth of flow upstream of structure

in feet

a = diameter of post, pier, or pile in

feet

V = velocity of flow approaching the

structure in feet per second

g = acceleration of gravity-32.2 feet

per second squared

Table 4.5 has been prepared for numerous

velocity and depth values and typical vertical

member diameters and applies to average soil

conditions (2,000-3,000 psf bearing capacity). For

loose sand and hard clay, the values may be

increased and decreased respectively. However,

the assistance of a soils engineer should be sought

where highly erodable soil conditions exist (e.g.

loose sand) or where scour depth values lower

than those provided in Table 4.5 are used. In

reviewing the scour depths in Table 4.5, note their

magnitude and potential impact on piers which are

placed on grade.

Table 4.5

DEPTH OF SCOUR (feet)

Vertical Member Diameter

Velocity (in.)

(fps) 4 6 8 10 12

1 .75 1.00 1.25 1.50 1.75

2 1.00 1.25 1.50 1.75 2.00

4 1.50 1.75 2.00 2.25 2.75

6 1.75 2.00 2.50 2.75 3.25

8 2.00 2.25 2.75 3.25 3.50

10 2.25 2.50 3.00 3.50 4.00

15 2.50 3.00 3.50 4.00 4.50

The determination of maximum potential scour

is critical in designing an elevated foundation

system to ensure that failure during and after

flooding does not occur due to any loss in bearing capacity or anchoring resistance around the

posts, piles, or piers elevating the manufactured

home. The anticipated scour depth must,

therefore, be added to the preflood height above

grade in determining the necessary post, pile, or

pier design as shown in Figure 4.15. Without the

inclusion of scour depth in elevated foundation

design, failure and collapse of the foundation

system is more likely after a flood.

Before Scour After Scour

Figure 4.15 Addition of Scour Depth

In coastal areas scour depths can be significant

due to both localized scour as calculated above

and area erosion resulting from the overall effects

of storm surge. Since area erosion is difficult to

predict, local regulatory officials should be contacted for further information and historical

perspectives.

Based upon the water velocity and type of soil,

scour and erosion would increase the height above

grade of the vertical member since the grade level

would be reduced due to scour and erosion.- As

this occurs, the depth of burial of the vertical

member also decreases an identical distance. This

can result in an elevated foundation failure

because the loss of natural supporting soils would

change the conditions under which the elevated

foundation system was designed.

40
To account for this, the vertical member length

used for the purpose of determining an acceptable

design must be increased. This vertical member

length is used in Section B of this chapter in determining the size of the vertical member needed to

carry anticipated loading.

For example, consider an area where a

manufactured home is to be placed on a six foot

high foundation using eight inch posts. If the anticipated flood velocity was six fps, the scour depth

from Table 4.5 is 30 inches. For the purpose of

determining the necessary post length, the scour

depth of 30 inches would be included in the vertical member length as shown in Figure 4.16.

Design Loads

Design loads (roof live, wind, dead, floor live)

which are over and above those caused by

flooding, must also be considered in that any

elevated foundation design must be capable of

also withstanding these loads. The magnitude of

these loads is shown in Tables 4.6 to 4.9 and will

be used in the evaluation of elevated foundation

design strategies. Note that roof live loads and

wind loads are not additive. Hence, the elevated

foundation design must be based on the greater

of these loads.

sfpo

2'-6' ~Depth 011M&

Figure 4.16 Addition of Scour Depth

Table 4.6

BUILDING LOADS (pounds)

12 Foot Wide Manufactured Home

Length (feet)l

32 40 48 52 56 60 66 72

Roof Live2

20 psf 9520 11760 14000 15120 16240 17360 19040 20720

30 psf 14280 17640 21000 22680 24360 26040 28560 31080

40 psf 19040 23520 28000 30240 32480 34720 38080 41440

Dead3 7680 9600 11520 12480 13440 14400 15840 17280

Floor

Live 4 15360 19200 23040 24960 26880 28800 31680 34560

Wind5

(lateral)

15 psf 4680 5850 7020 7605 8190 8775 9653 10550

25 psf 7808 9750 11700 12675 13650 14625 16088 17550

Wind6

(Upl ift)

9 psf 5526 6750 7974 8586 9198 9810 10728 11646

15 psf 9210 11250 13290 14310 15330 16350 17880 19410

1. Length refers to the "box" length of the manufactured home.

2. Roof live loads based upon roof area including a one foot roof eave

around the manufactured home perimeter.

3. Dead loads based upon gross floor area and 20 psf design load.

4. Floor live loads based upon gross floor area and 40 psf design load.

5. Wind loads (lateral) based upon windward longitudinal wall area

(9.75' x length') and stated wind force.

6. Wind loads (uplift) based upon roof area including a one foot eave

around the manufactured home perimeter. A 2.5 wind load factor

is applied to the one foot eave.

Table 4.7

BUILDING LOADS (pounds)

14 Foot Wide Manufactured Home

Length (feet)l

32 40 48 52 56 60 66 72

Roof Live2

20 psf 10880 13440 16000 17280 18560 19840 21760 23680

30 psf 16320 20160 24000 25920 27840 29760 32640 35520

40 psf 21760 26880 32000 34560 37120 39680 43520 47360

Dead3 8960 11200 13440 14560 15680 16800 18480 20160

Floor

Live 4 17920 22400 26880 29120 31360 33600 36960 40320

Wind5

(Lateral)

15 psf 4680 5850 7020 7605 8190 8775 9653 10530

25 psf 7800 8710 11700 12675 13650 14625 16088 17550

Wind6

(Utplift)

9 psf 6192 7560 8928 9612 10296 10980 12006 13032

11 psf 10320 12600 14880 16020 17160 18300 20010 21720

1. Length refers to the "box" length of the manufactured home.

2. Roof live loads based upon roof area including a one foot roof eave

around the manufactured home perimeter.

3. Dead loads based upon gross floor area and 20 psf design load.

4. Floor live loads based upon gross floor area and 40 psf design load.

5. Wind loads (lateral) based upon windward longitudinal wall area

(9.75' x length') and stated wind force.

6. Wind loads (uplift) based upon roof area including a one foot eave

around. the manufactured home perimeter. A 2.5 wind load factor

is applied to the one foot eave.

41
Table 4.8

BUILDING LOADS (pounds)

24 Foot Wide Manufactured Hlme

Length (feet) 1

R40 48 52 56 60

Roof Live2

20 psf 21840 26000 28080 30160 32240

30 psf 32760 39000 42120 45240 48360

40 psf 43680 52000 56160 60320 64480

Dead 3 19200 23040 24960 26880 28800

Floor

Live 4 38400 46080 49920 53760 57600

Wind 5

(lateral)

15 psf 5850 7020 7605 8190 8775

25 psf 9750 11700 12675 13650 14625

Wind6

(Lplift)

9 psf 11610 13698 14742 15786 16830

15 psf 19350 22830 24570 26310 28050

1. Length refers to the "box" length of the manufactured home.

2. Roof live loads based upon roof area including a one foot roof eave around the manufactured home perimeter. -

3. Dead loads based upon gross floor area and 20 psf design load.

4. Floor live loads based upon gross floor area and 40 psf design load.

5. Wind loads (lateral) based upon windward longitudinal wall area (9.75' x length') and stated wind force.

6. Wind loads (uplift) based upon roof area including a one foot eave around the manufactured home perimeter. A 2.5 wind load factor

is applied to the one foot eave.

Table 4.9

BUILDING LOADS (pounds)

28 Foot Wide Manufactured Home

Length (feet) 1

40 48 52 56 60

Roof Live 2

20 psf 25200 30000 32400 34800 37200

30 psf 37800 45000 48600 52200 55800

40 psf 50400 60000 64800 69600 74400

Dead3 22400 26880 29120 31360 33600

Floor

Live 4 44800 53760 58240 62720 67200

Wind 5

(Lateral)

15 psf 5850 7020 7605 8190 8775

25 psf 9750 11700 12675 13650 14625

Wind6

(Uplift)

9 psf 13230 15606 16794 17982 19170

15 psf 22050 26010 27990 29970 31950

1. Length refers to the "box" length of the manufactured home.

2. Roof live loads based upon roof area including a one foot roof eave around the manufactured home perimeter.

3. Dead Iads based upon gross floor area and 20 psf design load.

4. Floor live loads based upon gross floor area and 40 psf design load.

5. Wind loads lateral based upon windward longitudinal wall area (9.75' x length') and stated wind force.

6. Wind loads (uplift) based upon roof area including a one foot eave around the manufactured home perimeter. A 2.5 wind load factor

is applied to the one foot eave.

For example, the building loads for a 14 ft. by 66 ft. manufactured home located in a 40 psf roof live

load zone and 25 psf (lateral) and 15 psf (uplift) wind zone would be calculated as follows:

Roof Live Load

Dead Load

Floor Live Load

Wind Load

(lateral)

Wind Load

(uplift)

= (width + 2 ft. eaves) x (length + 2 ft. eaves) x40 psf =

(14 + 2) x (66 + 2) x 40 psf = 43,520 pounds

= (width x length) x 20 psf = (14 x 66) x 20 psf = 18,480 pounds

= (width x length) x 40 psf = (14 x 66) x 40 psf = 36,960 pounds

= (height x length) x 25 psf = (9.75 ft. x 66 ft) x 25 psf = 16,088

pounds

= {[[(width + 2 ft. eaves) x (length + 2 ft eaves)] - [(width x length)]] x

2.5 x 15 psf} + (width x~length) x 15 psf = {[{(14 + 2) x (66 + 2)] -

[(14 x 66)]] x2.5 x15 } + (14 x 66) x 15 psf = 6150 pounds + 13860

pounds = 20,010 pounds

42
The flood and design loads, as shown in Figures 4.17 and 4.18, can be summarized and shown as follows:

FORCE

Roof Live

Load

Dead Load

Floor Live

Load

Buoyancy

Wind (uplift)

VALUE

20, 30, or 40 psf of roof

area

20 psf of gross floor area

40 psf of gross floor area

62.4 lb./ft. 3 x height differential inside/outside

home

9 or 15 lb./ft. 2 x (roof

area + 1 ft. eaves at 2.5

x load)

IMPACTED PORTIONS OF THE

ELEVATED FOUNDATION

Vertical load affecting the foundation bearing

capability, any horizontal support members and

their connections.

Vertical load affecting the foundation bearing

capability, any horizontal support members and

their connections.

Vertical load affecting the foundation bearing

capability, any horizontal support members and

their connections.

Vertical load affecting floor capacity, anchoring of

structure, capacity of the foundation system and

connections.

Vertical load affecting floor capacity, anchoring of

structure, capacity of the foundation system and

connections.

31.2 lb./ft. 3 x (height of

water column acting on

vertical surface)2

Horizontal load affecting the wall capacity, anchoring of structure, foundation design and

connections.

Lateral (wind) 15/25 lb./ft. 2 x (windward

longitudinal wall area)

Horizontal load affecting the wall capacity, anchoring of structure, foundation design and

connections.

1.2707 x (velocity)2 x

height of water column

on the vertical surface

31 x flood velocity

Horizontal load affecting the wall capacity, anchoring of structure, foundation design and

connections.

Horizontal load affecting the wall capacity, anchoring of structure, foundation design and

connections.

43

Lateral

(flood)

Drag

Impact
For example, a 14 ft. by 66 ft. manufactured

home not impacted by flooding and located in

eastern North Carolina (southern roof live load

zone and hurricane wind zone) would have the

following loads as shown in Figure 4.19. Note

again roof live and wind loads are not additive.

16,089 lb.

1. Roof Live Load

2. Dead Load

3. Floor Live Load

4. Wind Load (lateral)

5. Wind Load (uplift)

Figure 4.17 Home Elevated Above Flood

Waters

0 1

-A.

..- :-x-:-gc-x-pc-yib :6

X ; tz ;~"z'~

- '44\N" 

Roof Live Load

Dead Load

Floor Live Load

Wind Load (lateral)

Wind Load (uplift)

Drag

Lateral

Impact

Buoyancy

Figure 4.18 Home Not Elevated Above Flood

Waters

1. Roof Live Load

2. Dead Load

3. Floor Live Load

4. Wind Load (lateral)

5. Wind Load (uplift)

20,010 lb.

36,960 lb.

10,480 lb.

r

= 21,760

= 18,480

= 36,960

= 16,088

= 20,010

Figure 4.19 Design Loads

In the above situation with the floor of the

manufactured home located three feet above

grade and impacted with a four foot water depth

above grade (that is, one foot of water depth acting on the manufactured home) at 15 feet per

second the following flood forces as shown in

Figure 4.20 would also occur.

2,059 lb. -g-[ L

l 5ps

6. Drag

7. Total Lateral Load

8. Impact

9. Buoyancy

= 18,870 lbs. = 2,059 lbs. = 465 psf

= 57,658 Ibs.

Figure 4.20 Flood Loads

44

21,760 lb.

Ibs.

Ibs.

lbs.

lbs.

lbs.

6

ii 

-I

#4

1.

2.

3.

4.

5.

6.

7.

8.

9.

__------s ... ------ - -W-zz,4,.-..-g-z-z==z{{f==z==,=.z.z.zg==4==zzzffz' \ Al

' 1211111'
The following section provides calculational procedures and design methods which can be used

to determine appropriate strategies to withstand

or counteract these forces.

Note, as previously stressed, elevation of the

manufactured home above anticipated flooding

significantly reduces or eliminates the flood

loading on the manufactured home. This, in turn,

greatly enhances the probability for survival of a

flood occurrence.

B. EVALUATION OF ELEVATED

FOUNDATIONS

An elevated foundation must be adequately supported in the ground, tied effectively to the

manufactured home, and have the ability to withstand flood and wind induced loads, dead loads,

roof live loads, and floor live loads. As previously

discussed, the imposition of hydrostatic, hydrodynamic, and impact forces on the manufactured

home will add additional loads to the elevated

foundation. The magnitude of these loads, as

shown in Figure 4.21, is sufficient to seriously

damage a manufactured home not elevated above

anticipated flooding.

Figure 4.21 Magnitude of Loads

Ground anchors and tie-downs can hold the

manufactured home in position only under extremely low velocity and depth conditions during flooding. From a design standpoint, the practical limitations on anchoring and tie-down to

counteract flood forces can be summarized as

follows:

Water

Velocity

0 to 5 fps

5 to 10 fps

10 fps

Tie Angle

To Vertical

0

30

45

0

30

45

Maximum

Flood Height

Above Floor

10"

9,,

8"

8"

71"

6"

None-Manufactured home

must be elevated above

anticipated flooding.

The above information indicates that flood

heights of more than one foot will result in forces

that cannot be effectively counteracted by ground

anchors and tie-downs. Because of this, and also

because even a small amount of depth above the

floor can substantially damage a manufactured

home, the use of anchoring alone, without additional elevation is not effective in minimizing future

flood losses. The optimum strategy is to elevate

the manufactured home to a height where no

flood forces are anticipated to affect the

manufactured home.

Appendix E provides additional details on

ground anchor spacings necessary to resist

buoyancy and drag forces and should be consulted for further information. A review of this

appendix will serve to validate the limitations of

ground anchoring and tie-downs in flooding

conditions.

Piers, posts, and piles can be used to elevate

a manufactured home above anticipated flood

levels. When the manufactured home is elevated

above anticipated flooding, flood forces act only

on the elevated foundation. The elevated foundation, however, must support the manufactured home, and withstand all other anticipated

forces.

As previously discussed, a registered design

professional may be required to design the

elevated foundation depending upon state or local

regulations and the height above grade.

Design Forces and Loads

The flood forces to be considered in the design

of an elevated foundation are buoyancy, lateral,

impact, and drag forces as discussed in Section

A of this chapter. Generally, when compared to

forces imposed by wind, dead load, roof live load,

and floor live load, the flood forces on the foundation are small if the manufactured home is

elevated above the anticipated flood level. As the

manufactured home elevation is increased, the

45
non-flood forces on the elevated foundation are

increased as shown in Figure 4.22. This affects

the design of the elevated foundation.

44U44444I4U

-i1

Gs = dead, floor live, and roof live loads

associated with the manufactured

home

Wd = dead load Wr = roof live loa

Wig = floor live load

Figure 4.23 Roof Dominated Loads

Figure 4.22 Forces Increase With Elevation

Figures 4.23 and 4.24 show the loads applied

to an elevated foundation supporting a manufactured home.

dead floor live and wind loads

associated with the manufactured

home

Wd = dead load Wa = wind load (I

Wff = floor live load

Wu = wind load (uplift)

Figure 4.24 Wind Dominated Loads

lateral)

46

I1

Wll4

lWd

I

1-MM-1 - t WTM

id
Table 4.10 provides the calculated maximum

vertical loads for typical manufactured home

widths (12 feet and 14 feet) and chassis I-beam

spacing 751/2-, 82-, and 99-inches based on the

spacing of vertical members along the length of

the manufactured home. The elevated foundation

design must support these loads. Note that design

roof live loads and wind loads do not occur

simultaneously.

Because a double section manufactured

home is essentially two connected single section manufactured homes, this chapter

presents design criteria based on typical single

section widths. When a double section

manufactured home is being considered, the

recommendations provided herein should be

followed for each section.

Table 4.10

MAXIMUM VERTICAL MEMBER LOAD (pounds)

Longitudinal Manufactured Home Width (ft.)

Spacing (ft.) 12 14

Wind Load = 15 psf Chassis I-Bean Spacing (in.)

Roof Live Load = 20 psf 75 1/2 82 99 75 1/2. 82 99

5 2500 2900

6 3000 3480

7 3500 4060

8 4000 4640

9 4500 5220

10 5000 5800

Wind Load = 25 psf

Roof Live Load = 20 psf

5 2744 2669 2520 3044 2969 2900

6 3293 3203 3024 3653 3563 3480

7 3842 3737 3528 4262 4157 4060

8 4391 4271 4032 4871 4751 4640

9 4940 4805 4536 5480 5345 5220

10 5489 5339 5040 6089 5939 5800

Roof Live Load = 30 psf

Wind Load = 15 or 25 psf

5 2850 3300

6 3420 3960

7 3990 4620

8 4560 5280

9 5130 5940

10 5700 6600

Roof Live Load = 40 psf

Wind Load = 15 or 25 psf

5 3200 3700

6 3840 4440

7 4480 5180

8 5120 5920

9 5760 6660

10 6400 7400

47
For example, a 14-foot wide manufactured

home having a 99-inch chassis I-beam spacing

located in a 30 psf roof live load area and eight

foot spacing of vertical support members has a

5,280 pound load at each point where the elevated

foundation supports the manufactured home as

shown in Figure 4.25.

a1

8l

I'

5280# *5280# AL A I 

4-t t 1 5280# _ -t280#

,5i-- lflssnI# _ r

52 88 #

-r528# ,5280# _ ~\_ I 

L 99",

14'-D" 

Figure 4.25 Vertical Member Load

The design of an elevated foundation to resist

these loads is dependent on the specific elevation

technique selected, height above grade, size of

foundation components, connections, and spacing of supporting members lengthwise along the

manufactured home. Spacing of vertical

members must not exceed the recommended

spacing in the homeowner's installation manual

or 10 feet, whichever is less. In addition, any requirements for perimeter blocking in the

homeowner's installation manual must also be

completed.

Prior to selecting and designing a vertical

member to elevate the manufactured home, the

bearing capacity of the soil on the site must be

determined. This is critical to assure that adequate

support for any elevated foundation is provided by

the soil.

Estimating the bearing capacity of soil is difficult

in that soil characteristics may be altered later by

a change in moisture content. Soil classification

of a general nature can be shown as follows:

SOIL

Clay, soft

Clay, firm

Clay, stiff

Loose sand, wet

Firm sand, wet

Packed earth

Loose earth

Gravel 1"

Gravel 21/2"

BEARING PRESSURE (psf)

600-1200

1500-2500

3000-4500

800-1600

1600-3500

2700-3000

2300-2700

3000-3300

2700-3000

In general soils can be divided into five

classifications; very soft, poor, average, good, and

very hard. Bearing capacities are respectively

800 - 1200 psf, 1200 - 2000 psf, 2000 - 3000 psf,

3000 - 4100 psf, and 4100+ psf.

Where a general visual inspection and consultation with a soils engineer is not sufficient to

estimate the bearing capacity of the soil, a load

test should be conducted. The load test should be

conducted when the soil is wet.

After the soil bearing capacity is determined,

that value can be evaluated against the maximum

vertical member load to determine the required

bearing area of the vertical member as follows:

Vertical Member Load (lb.)

Soil Bearing Capacity (psf)

bearing = area

(sq. ft.)

For example, if the previous example shown in

Figure 4.25 was to be located where the soil bearing capacity was found to be 1800 psf, the required

bearing area under each vertical member would

be as follows:

5280 lb = 2.93 sq. ft.

1800 psf

'A bearing pad 21 inches by 21 inches would be

acceptable since it provides an area of 3.06 sq. ft.

The following sections provide details on the

design of components needed to resist these loads

under various conditions. These include vertical

member (piers, posts and piles), horizontal supports, diagonal bracing, and connections associated with an elevated foundation design.

Vertical Support Members

Piers-Flood and wind loads are resisted by piers

through their structural capacity. Cast-in-place

concrete and built up concrete masonry unit or

brick piers, because of their larger surface area

and limited anchorage in the ground, are more

susceptible to flood forces than posts or piles.

48
Consequently, they are more dependent on

surface soil conditions than posts or piles and their

performance is greatly affected by erosion and

scour which undermines their stability upon which

the elevated manufactured home is dependent.

The use of a pier system should, therefore, not

be considered in areas where any flood velocity

is anticipated.

Piers can be used in backwater (no-flow) flood

areas to elevate the manufactured home above

the anticipated flood level. Where the required

elevation is less than three feet to the bottom of

the chassis I-beam, a conventional installation can

be used as previously discussed.

Based on an analysis of anticipated loads and

the capacity of various pier designs, Table 4.11

provides pier designs acceptable for elevating a

manufactured home.

2-1 i21 $j_

Based on the pier spacing along the length of

the manufactured home and height of the pier, one

or more acceptable pier designs is available. For

other pier designs or conditions not addressed

herein, Appendix D on design calculations can be

consulted.

Pier Design A, as shown in Figure 4.26, consists

of eight inch by eight inch cast-in-place concrete

with four No. 4 steel reinforcement bars. Pier

Design A is acceptable for the conditions labeled

A as shown in Table 4.1 1, based on a minimum

concrete strength of 2500 psi.

Pier Design B, as shown in Figure 4.27, consists

of eight inch by 12 inch concrete masonry units

(CMU) with four No. 4 steel reinforcement bars.

Pier Design B is acceptable for the conditions

labeled A or B as shown in Table 4.11 based on

a minimum CMU strength of 1500 psi.

12" 4"min.

i4 

4"min.

_ &

8.s

# 3 t~~#

Ties @ 10" a.c.

"_2 r

4 1 , 4 4 ,,r, D | 8O"min.

t>

Figure 4.26 Pier Design A Figure 4.27 Pier Design B

49

I- -

WL ..

#5 Dowels -.

3-#4 Reinf.

each wag_

1 -6"

8"min.
Pier Design C, as shown in Figure 4.28, consists

of eight inch by 16 inch concrete masonry units

with four No. 5 steel reinforcement bars. Pier

Design C is acceptable for the conditions labeled, A, B, or C, as shown in Table 4.1 1, based on

a minimum CMU strength of 1500 psi.

Pier Design D, as shown in Figure 4.29, consists of 12 inch by 12 inch cast-in-place concrete

or common brick with four No. 5 steel reinforcement bars. Pier Design D is acceptable for the conditions labeled A, B, C, or D as shown in Table

4.11, based on a minimum concrete strength of

2500 psi.

16" 4"min.

_- _ I

4" min. _ F

8-.

Figure 4.28 Pier Design C

Table 4.11

MINIMUM ACCEPTABLE PIER DESIGNS'

IPer rmnufactured hane section

2Total pier length including depth below grade to the top of the footing.

50

#5 Dowels

-. . ..

3-#4 Reinf.

each w'ay -

1 2"min.
4" min.

12"

Ties @1I On o.c.

#5 Dowels

3-#4 Feinf. _ ,'

eachwf m _oj < ,,,., I> 

^X:iff-' "d 12"min.

Figure 4.29 Pier Design D

3-#4 Reinf.

j each way -

Figure 4.30 Pier Design E

51

2-1

F 4"min.

12"

1 /4"'O Wire

circularties

@ 12"o.c.

8"min.

12"min.

4 

I

i
Pier Design E, as shown in Figure 4.30, consists of 16 inch by 16 inch (two eight inch by 16

inch) copcrete masonry units with four No. 5 steel

reinforcement bars. Pier Design E is acceptable

for the conditions labeled A, B, C, D, or E as shown

in Table 4.1 1, based on a minimum CMU strength

of 1500 psi.

Pier Design F, as shown in Figure 4.31, consists

of 16 inch by 16 inch cast-in-place concrete with

four No. 5 steel reinforcement bars. Pier Design

F is acceptable for all conditions shown in Table

4.11 based on a minimum CMU strength of 1500

psi.

4'min.

2-1/2 _.. -.. .

Ties @ 16' o.

it, ,+"e.-i.-6

#5 DoMels lM

3-#4 Reinf!

each way , 

12"min.

Figure 4.32 Typical Ground Anchor Detail

W~~~~I~ =~~~~ Il V==-

I' lo Lf 1.ii DaM lo a AL Dsa j

V lrDy o l T gatr g < Li, ->\

Figure 4.33 Longitudinal Spacing

of Ground Anchors

Figure 4.31 Pier Design F

Ground anchors, over and above those required

in state anchoring regulations, must be provided

as shown in Table 4.12. Ground anchor location

should be between two and three feet from the pier

toward the center of the manufactured home and

installed as shown in Figures 4.32 and 4.33.

Table 4.12

GROUND ANCHOR SPACING

Height Above Grade (ft.)

Wind

Zone 1 2 3 4 5 6 7 8 9 10

I (10 pSa)

20 Foot on Center

II (25 psf) 10 Foot on Center

To provide additional stability and resistance to

anticipated loads, all pier designs must have the

bottom of the footing at least 30 inches below

grade or to the frostline, whichever is greater.

The bearing capacity of the soil must be greater

than the maximum vertical load through the piers.

Based on the information in Table 4.10, the maximum vertical load is determined and must be

added to the weight of each pier and footing. In

calculating the vertical load, a weight of 150

pounds per foot of pier should be used and 400

pounds for each footing. Hence, a ten foot pier and

footing would weight 1900 pounds. If the bearing

capacity of the soil is exceeded, the footing area

must be increased or additional piers added.

52
Further support and resistance to lateral loads

is provided by two inch by six inch pressure treated

wood cross bracing. The pier design, anchorage,

cross bracing, and footing details needed to

'elevate a manufactured home are then combined

to form the elevated foundation as shown in Figure

4.34.

Chassis I-Beam

-2-1/2" fAnchor bolts,

1.-a- long with 2- hook

2'H4' Pressure

treated lumber

Figure 4.35 Posts and Piles

settle or sink into the ground. Prior to selecting

the vertical members (posts or piles), the bearing

capacity of the soil on the manufactured home site

must be determined. This value will become the

maximum allowable design load for selection of

the vertical members.

Pile foundations, as opposed to other vertical

wood members, rely on a combination of end bearing and side friction caused by the pile driving process to resist both vertical and horizontal loads.

The frictional capability of piles is heavily dependent on local soil conditions and driving methods.

For this reason, a full scale pile load test at the

manufactured home site is the most reliable

method of determining pile capacity. The results

of such a test can be used in lieu of the vertical

member capacity values presented in Tables 4.13.

to 4.16. If pile capacity tests are not conducted,

then the information on vertical member capacity

in Tables 4.13 to 4.16 must be used.

Tables 4.13 to 4.16 provide data on square and

round vertical wood member capacity as a function of total vertical member length, diameter, wind

load, and manufactured home width. Total vertical

member length as used in these tables includes

height above grade, depth of embedment, and

scour considerations. Note that for a double section manufactured home, these values are

applicable, and two side-by-side foundation

systems must be constructed.

53

Figure 4.34 Elevated Pier Design

Where floodwaters are anticipated to rise above

the floor of the manufactured home, then additional ground anchoring must be provided to

secure the manufactured home against vertical

and lateral flood forces. Anchoring by connection of the manufactured home to the piers will

not resist flotation. Note, as previously discussed and shown in Appendix E, additional ground

anchoring to resist flood loads will be effective in

only limited cases. Additional elevation by increasing pier height will eliminate this concern.

Posts and Piles-The vertical members (posts or

piles) of an elevated foundation must support

manufactured home loads which are subsequently

resisted by end bearing on undisturbed soil or concrete footings as shown in Figure 4.35.

As previously discussed, the first consideration

in designing an elevated foundation is the capability for the soil to support the anticipated loading

imposed by the vertical support members. Without

such consideration, the entire foundation can
The information in these tables can be used to

determine the vertical member size necessary to

support a given manufactured home installation.

Based on the wind and roof live loads, manufactured home width, and total member length including any scour considerations, the vertical

member capacity is provided and the size of vertical member chosen.

Based on the support spacing, wind and roof

live loads, manufactured home width, and chassis

I-beam spacing, a vertical member load is chosen

from Table 4.10. Based on the length of the verTable 4.13

'VERTICAL MEMBER CAPACITY (pounds per member) 1

Wind Design = 15 psf

BHne Width = 12 feet

Vertical Menber Diameter (in.)

Length2 Square 3 Round4

(ft.) 4 6 8 10 12 4 6 8 10 12

1 2250 7477 16576 299Z5 47750 2065 6161 13084 23144 36548

2 1319 4660 10844 20364 33572 1189 3731 8262 15142 24645

3 933 3384 8058 15433 25886 834 2675 6037 11251 18590

4 720 2657 6411 12425 21064 642 2085 4756 8951 14924

5 585 2187 5323 10398 17756 522 1708 3924 7432 12465

6 492 1855 4550 8939 15346 09 1445 3339 6354 10702

7 423 1611 3969 7840 13512 378 1253 2906 5549 9376

8 368 1422 3521 6981 12069 331 1105 2570 4924 8342

9 324 1272 3162 6284 10905 293 988 2304 4427 7514

10 289 1148 2869 5717 9946 262 892 2088 4016 6835

11 260 1045 2624 5242 9130 237 813 190& 3677 6269

12 236 956 2416 4840 8444 216 745 1756 3390 5783

13 216 877 2238 4493 7853 198 687 1626 3144 5370

14 199 809 2082 4191 7337 183 635 1513 2931 5012

15 - 751 1944 3926 6885 169 591 1414 2744 4698

16 - 700 1820 3691 6483 158 551 1326 2579 4420

17 - 655 1706 3481 6124 - 517 1247 2432 4173

18 - 615 1605 3291 5802 - 486 1174 2300 3952

19 - 579 1513 3119 5510 - 458 1109 2181 3752

20 - 547 1431 2960 5245 - 434 1050 2072 3570

1Maximum capacity limited to bearing capacity of soil; i.e., if soil capacity is 4000 psf, that becomes the limiting factor in determining configuration or end bearing area must

be increased.

2Length is determined as total height above and below grade with depth of embedment

greater than or equal to the height above grade but not less than six feet plus the

addition of scour depth as previously discussed.

3Rough dimensions. Nominal dimension will be 1/2 inch less per side on square posts.

4Rough dimension as diameter of round member.

54
tical member, including anticipated scour depth,

a vertical member size is chosen which is capable

of supporting that load consistent with the soil

bearing capacity of the site.

Consider a 14 foot by 66 foot manufactured

home having a 99-inch chassis I-beam spacing

with a desired elevation of five feet above grade

and a recommended support spacing of eight feet

located in a 25 psf wind load and 20 psf roof live

load zone, where no scour is anticipated and soil

bearing capacity is 3,000 psf as shown in Figure

4.36.

Table 4.14

VERTICAL MEMBER CAPACITY (pounds per member)

Wind Design = 15 psf

Home Width = 14 feet

lMaximum. capacity limited to bearing capacity of soil; i.e., if soil

capacity is 4000 psf, that becomes the limiting factor in determining

configuration or end bearing area must be increased.

2 Length is determined as total height above and below grade with depth of

embedment greater than or equal to the height above grade but not less than

six feet plus the addition of scour depth as previously discussed.

3 Rough dimensions. Nominal dimension will be 1/2 inch less per side on

square posts.

4 Rough dimension as diameter of round menber.

55

Vertical Member Diameter (in.)

Square 3 Round4

Length2

(ft.) 4 6 8 10 12 4 6 8 10 12

1 2454 8049 17674 31674 50245 2261 6672 14048 24678 38749

2 1461 5113 11804 22019 36093 1321 4112 9046 16482 26690

3 1040 3746 8861 16875 28161 933 2972 6671 12373 20355

4 805 2956 7093 13679 23087 720 2327 5284 9904 16450

5 656 2441 5913 11501 19562 586 1912 4374 8256 13803

6 552 2074 5069 9921 16971 493 1620 3732 7078 118-89

7 475 1804 4431 8723 14986 425 1406 3254 6195 10441

8 413 1595 3936 7783 13417 373 1241 2881 5507 9308

9 363 1427 3540 7017 12145 330 1110 2585 4957 8397

10 323 1289 3215 6391 11094 295 1004 2345 4501 7648

11 291 1173 2943 5867 10196 266 915 2144 4125 7022

12 264 1073 2711 5421 9439 242 839 1974 3806 6482

13 241 984 2512 5036 8786 222 773 1829 3532 6024

14 222 907 2337 4701 8215 205 715 1702 3294 5625

15 - 841 2182 4405 7713 190 664 1591 3085 5276

16 - 783 2043 4143 7267 177 619 1492 2901 4966

17 - 732 1915 3908 6868 - 580 1403 2736 4691

18 - 687 1800 3695 6509 - 545 1321 2588 4443

19 - 647 1696 3502 6184 - 514 1247 2454 4219

20 - 611 1604 3323 5887 - 486 1180 2331 4016

; I I ll
The vertical member load, based on wind load,

roof live load, home width, dead and floor live

loads, and the longitudinal spacing of vertical

supports, is derived from Table 4.10. For the

manufactured home installation in this example,

the load is 4,640 pounds per vertical member as

shown in Figure 4.37.

Referring to Table 4.16 on vertical member

capacity, it is found a 12-inch diameter round or

square post is acceptable based on the vertical

member capacity and length of 11 feet (five feet

above grade plus six feet embedment) as shown

in Figure 4.38.

Because the vertical member load in psf is

greater than the soil bearing capacity of 3,000 psf,

a bearing pad must be provided for each post. The

minimum area is as follows:

4640 lb. = 1.55 sq. ft.

3000 psf

Note that in this example, if a 12 inch square

member were chosen a vertical member length

Table 4.15

VERTICAL MEMBER CAPACITY (pounds per member]

Wind Design = 25 psf

Hcae Width = 12 feet

Vertical Menter Diameter (in.) ,

Square 3 Round4

Length 2

(ft.) 4 6 8 10 12 4 6 8 10 12

1 1657 5722 13071 24168 39323 1503 4630 10095 18250 29354

2 929 3371 8027 15378 25798 831 2664 6013 11209 18523

3 645 2389 5792 11277 19196 574 1870 4282 8088 13530

4 493 1850 4531 8902 15284 438 1-440 3325 6326 10658

S' 399 1510 3720 7354 12697 354 1171 2717 5195 8791

6 335 1273 3156 6-264 10859 297 986 2297 4407 7481

7 287 1101 2738 5456 9485 255 852 1990 3826 6511

8 250 969 2418 4832 8421 223 749 1754 3381 5763

9 221 865 2165 4333 7571 198 668 1568 3028 5170

10 198 780 1959 3929 6876 178 603 1418 2740 4687

11 178 710 1788 3593 6294 161 549 1293 2503 4287

12 162 650 1645 3309 5804 147 503 1189 2304 3946

13 149 597 1521 3067 5385 135 464 1100 2133 3658

14 137 552 1414 2857 5022 125 430 1022 1986 3408

15 - 513 1320 2673 4704 116 400 955 1858 3190

16 - 479 1237 251-1 4424 108 374 895 1745 2998

17 - 449 1161 2366 4174 - 351 842 1644 2828

18 - 423 1093 2236 3950 - 330 794 1554 2675

19 - 399 1033 2119 3749 - 312 751 1473 2538

20 - 377 978 2012 3566 - 296 712 1399 2414

1Maximum capacity limited to bearing capacity of soil; i.e., if soil capacity is 4000 psf, that becomes the limiting factor in determining configuration or end bearing area must

be increased.

2Length is determined as total height above and below grade with depth of embedment

greater than or equal to the height above grade but not less than six feet plus the

addition of scour depth as previously discussed.

3Rough dimensions. Nominal dimension will be 1/2 inch less per side on square posts.

4Rough dimension as diameter of round member.

56
of 16 feet (eight feet above grade and eight

embedment) could be used because its capacity

at 4906 pounds is greater than the vertical member

load of 4640 pounds. If a six foot spacing were

used, in lieu of an eight foot spacing, the load per

post in this case is reduced to 3,480 pounds and

a 10 inch square post could be used with a subsequent reduction in bearing pad area.

By changing the vertical member spacing,

determining the applicable load and comparing

that load with different vertical member capacities,

as shown in this example, a number of elevated

design strategies can be determined for a particular site.

As previously discussed, state and local regulations for manufactured home installations may

impose a height limitation on elevated foundations

unless they are designed by a registered professional engineer. Where elevations over state or

local height limitations are anticipated, the

materials in this chapter can be used as general

guidance but should not be used in lieu of obtaining an elevated foundation design by a

registered professional engineer.

Table 4.16

VERTICAL MEMBER CAPACITY (pounds per member) 1

Wind Design = 25 psf

Hane Width = 14 feet

Vertical Menber Diameter (in.)

Square3 Round4

Length2

(ft.) 4 6 8 10 12 4 6 8 10 12

1 1804 6170 13986 25698 41596 1641 5016 10863 19529 31259

2 1022 3686 8730 16640 27791 916 2923 6567 12189 20066

3 713 2628 6345 12303 20866 635 2062 4706 8859 14776

4 547 2024 4983 9759 16703 486 1593 3666 6958 11693

5 443 1670 4103 8087 13925 393 1298 3003 5729 9674

6 371 1410 3487 6904 11940 330 1094 2543 4869 8250

7 319 1221 3029 6023 10450 284 945 2205 4233. 7191

8 278 1075 2678 5341 9290 249 832 1945 3744 6373

9 245 960 2399 4794 8362 220 743 1740 3357 5722

10 219 866 2172 4350 7603 197 670 '1574 3039 5192

11 198 788 1984 3981 6963 178 610 1437 2778 4752

12 180 721 1825 3668 6426 163 560 1321 2557 4377

13 165 663 1689 3401 5966 150 516 1222 2369 4058

14 152 613 1570 3169 5566 138 478 1137 2206 3783

15 - 569 1466 2966 5216 128 444 1062 2064 3542

16 - 531 1373 2787 4906 120 415 996 1939 3330

17 - 498 1289 2627 4631 - 390 936 1827 3141

18 - 468 1213 2483 4383 - 367 883 1728 2973

19 - 441 1145 2352 4160 - 346 834 1638 2821

20 - 417 1084 2233 3958 - 328 791 1556 2683

1Maxinum capacity limited to bearing capacity of soil; i.e., if soil

capacity is 4000 psf, that becomes the limiting factor in determining

configuration or end bearing area must be increased.

2 Length is determined as total height above and below grade with depth of

embedment greater than or equal to the height above grade but not less than

six feet plus the addition of scour depth as previously discussed.

3Rough dimensions. Nominal dimension will be 1/2 inch less per side on

square posts.

4 Rough dimension as diameter of round menber.

57
20 psi

Figure 4.36 Manufactured Home

Design Conditions

MAXIMUM VERTICAL MEMIRER LOAD, f-,,uni

Figure 4.37 Vertical Member Loads

VERTICAL MEMBER CAPACITY (pounds per member}

Wind Design = 25 psi 1im Width = 14 feet

1 6604f'I 50 166 24 411568 84' 2S~t'~iB~~li4$ 1

i 6? 54766 21671 m 64"62 5'' 8754 1 O 08

Ihhxir 'p26I4ty 4i6ited to bea 1ing 1 1 '6ty of soi; I 5 'e., if s J I spac $ty is 4000 p "f, that 6664s the limiting fa'tor in det' 'l82g -anfiguretioan or end bearig area must be incresed.

2Legth Jis det'1ni22 1 d totl height bv1 4 nd belw grde ith depth o3 -lbed-ret greater thsn or equal to the height ab-v grade bat bar le.. than six feet pins the addition of srour depth a. previously dlscu~ssd.

Cough dne..ion.. 6 i80 nal di2 enso S ill be 112 i6ch less per side 1n $gsquParests.

411ough d 6i61"2s15 as di-ter of r'6nd Th66b1r.

Figure 4.38 Vertical Member Selection

In addition to wood posts or piles, steel pipe, steel structural sections, and concrete piles can

be considered for elevated foundations. A presen- tation on the design methods for these materials

is beyond the scope of this manual and assistance

of a registered professional engineer is recommended if these materials are considered.

C. BRACING SUPPORT AND

CONNECTIONS FOR ELEVATED

FOUNDATIONS

Having determined the size1 length, and spac- ing of the vertical members, the need for and

design of horizontal supports, diagonal bracing,

and connection details must be determined. In addition, where ground anchors are not used, the

manufactured home must be securely connected

to the elevated foundation. Figure 4.39 indicates

where loading is applied to an elevated foundation system and what components of the elevated

foundation must withstand anticipated loading.

Horizontal Support Beams

Where posts or piles are used to elevate the

manufactured home, a structural member (hori- zontal support beam) must be connected between

vertical members and run underneath the mandfactured home to support the design loads as

shown in Figure 4.40.

Horizontal cross members must be capable of

supporting the dead load and floor live load of the

manufactured home, as well as wind and roof live

loads. At a five to 10 foot spacing of posts or piles the total weight including dead (20 psfc, live (40

58
Figure 4.39 Elevated Foundation Loading

psf), snow (20, 30, 40, psf) or wind (15, 25 psf) is

shown in Tables 4.6 to 4.9.

Using these loads, the minimum horizontal

beam sizes shown in Table 4.17 must be used to

support the manufactured home and are based

on sawn timber having an allowable bending

stress (Fb) of 1250 psi, corrected for load duration, treatment, and service condition. Minimum

beam sizes are determined as follows:

Sx

P x e

Ffb

Where: Sx = minimum section modulus of

beam (in3)

P = load from chassis I-beam (lb)

e = distance from chassis I-beam

to horizontal beamlvertical

member connection (assumes

15 inches from side of

manufactured home to the

center line of the vertical

member)

FPb = allowable wood bending

stress (psi) corrected =

1207.5 psi

Figure 4.40 Horizontal Support Beam

For the purposes of the horizontal beam capacity and installation, the distance from the side of

the manufactured home to the center line of the

vertical member must not be more than 15 inches.

For instance, a 14 foot wide manufactured home

located in a 20 psf roof live load and 25 psf wind

load area having chassis I-beams 75-1/2 inches

apart and longitudinal post spacings along the

manufactured home of eight feet, would require

four three-by 14-inch horizontal members between

each post running under the manufactured home

as shown in Table 4.17. Where other vertical

member spacings or wood having a different bending stress is used, the equation previously shown

can be used to calculate the minimum section

modulus needed, and the required timber size

determined.

Longitudinal beams placed above the horizontal support beams as shown in Figure 4.41 can

provide additional support and will be needed if

the manufacturer requires perimeter supports for

the floor system. These transfer certain loads away

from the center of the horizontal support beam.

Where longitudinal beams are used the vertical

member capacity presented in Tables 4.13 to 4.16

must be reduced by five percent.

59
Cross Bracing

To improve the lateral stability of an elevated

foundation system using vertical members bracing will be needed along the length of the elevated

foundation between vertical members as shown

in Figure 4.42. This becomes increasingly more

important as the height above grade increases.

Tables 4.18 and 4.19 provide minimum recommended sawn timber sizes for cross bracing. The

number of spans between vertical members (bays)

needing cross bracing is shown along with the

minimum sawn timber size. The required net area

of the cross bracing is calculated using the

allowable working stress in tension parallel to grain

for the wood type used as follows:

An = P

Ft

Where:

An = Net area (sq. in.)

P = Axial load (lbs.)

Ft = Allowable working stress in tension

parallel to grain (psi)

Based on a Ft of 531.3 psi (550 psi corrected

for service condition, treatment and load duration

factors), the minimum sawn timber sizes shown

Table 4.17

MINIMUM SAWN LUMBER HORIZONTAL BEAM

Vertical Member Manufactured Hone Width (ft.)

Spacing (ft.) 12 1 14

Wind Load = 15 Chassis I-Beam Spacing (in.)

Roof Live Load = 20 psf 75 1/2 82 99 75 1/2 82 99

5 20 3"x12"1 20 3"'x12"1 20 2"xJ.4" 20 3"x14"1 20 3"x14' 20 3"1x14"

6 40 2"lx12" 20 3"lx12" 20 2'x 14" 40 2"x14"1 40 2"x14" 20 3"x14"

7 20 3"x14"1 20 3"x14"1 20 31x12" 40 3"x121 4Q 3"x12" 40 2"x14"1

8 40 2"IX14" 20 31X14" 40 2"X12" 40 3"x14' 40 3"x12"' 40 21x14"'

9 40 2"x14" 40 21x14"1 20 3"x144 40 3"x14" 40 3x14" 40 3"x12"'

10 20 31XI61 40 3"X14"1 20 3X14 1

1' 40 3"x14" 40 3"x141 40 3"x141"

Wind Load = 25 psf

Roof Live Load = 20 psf

5 20 3"x12" 20 Shxi2ii 20 3x12"1 40 2"x14" 20 3"x14"1 40 2"x12"

6 4 2"x12"1 20 3"x12"' 40 2"xlO" 40 2"'x141" 20 3"x141- 40 2"x12"

7 40 21x12"1 20 3"x141 40 2"x121 40 3Yx12' 40 3"x12' 40 2"x14"

8 4 21"X12"' 20 3"X14l 40 21X1 2" 40 31x141 40 3"x121 40 2"x14"

9 40 3"x121 40 2"x14"1 20 3"x14"1 40 31x14' 40 3"x14" 40 3"'x12"

10 40 3"X12" 40 2"IX14" 20 3"X14"1 40 3x14"1 40 3Xx141 40 3"x14"

Roof Live Load = 30 psf

5 20 3"x14"' 20 3"x12"' 20 21x141 40 2"x141" 40 2"x14"' 20 3"x14"

6 20 3"xI14" 20 3"x14"1 40 2"x12' 40 3"x12"1 40 3"x12" 40 2"x14"

7 40 2"x14' 40 2"x14"1 20 3"x141 40 3"x14"1 40 3"x121 40 3Px12"

8 40 3"X122 40 2"1X144' 20 3IX14t' 40 31x14"1 40 3x141 40 3"x14"

9 4 3lx12"l 40 3"x121 40 2"x14"' 40 3"x14" 0 3"x14"' 40 3"x14"

10 40 3"X141 40 31"X14" 40 3X12" 40 3"x16"1 40 3"x161 40 3'x14"

Roof Live Load = 40 psf

5 20 3"x14"' 20 3"x14"1 20 3"x121 40 3Px12' 40 2'txl4"' 20 3"x14"

6 40 2"x14" 20 3"x14"1 40 2"x12"' 40 3"x14"' 40 3"x14" 20 31x16"

7 40 3"x12 40 2"x14"1 20 3"x141 40 3"x14"1 40 3"x14' 40 3"x12"

8 40 3Px121 40 3"x12"1 40 2x14"x __ 4@ 3"x14" 40 3"x14"

9 4@ 3'x141 40 3"x12' 40 2"x141" __ _ 40 3"x14"

10 40 3'x14"1 40 3'x141 40 3"x121 __ __ __

60
Figure 4.41 Longitudinal Support Beams

in Tables 4.18 and 4.19 are recommended.

Diagonal bracing members should be placed between the posts under the home (across the width)

at a minimum at each end and at the center span

as shown in Figure 4.43.

For double section manufactured homes, both

bays under the manufactured home must be

braced using the width of one section of the

manufactured home as appropriate.

Figure 4.42 Cross Bracing

For instance a 14 foot wide manufactured home

in a 25 psf wind load area would, as shown in

Figure 4.44, require two-by six-inch sawn timber

diagonal bracing.

Table 4.18

MINIMUM SAWN TIMBER CROSS BRACING

(for 15 psf wind load)

Manufactured Height of Vertical Members Above Grade (ft.)

Hone Length

(ft.) 3 4 5 6 7 8 9 10

32 3 Braced Bays with 2"x 3" Menbers

40

48 3 Braced Bays with 2"x 4" Memrbers

52

56 3 Braced Bays with 2"x 611 Menbers

60

4 Braced Bays with 2"1x 4"1 Men-bers 

66

72 4 Braced Bays with 2"x 6" Menbers

61
A 28 foot wide manufactured home in the same

area would, as shown in Figure 4.45, require use

of two-by six-inch sawn timber diagonal bracing.

Figure 4.43 Cross Bracing Locations

Table 4.19

MINIMUM SAWN TIMBER CROSS BRACING (for 25 psf wind load)

Altoufact.red leight of Vertical Iatber Above Grade (ft.)

tarrlegth [I .3 4 5 ST 10

I Bracd B-y

32 w[ I h 2"x 4v

40

48 3 SBrced Bay. aith 2`a S' Rbters

52

56 3 B-rord ays aith 2¢x ' M5 b- lol

69

4 Soraed Bas." Os a il 2rs 6

4arters

72 4I Seraed Says with 2's 5' Masters

Figure 4.44 Cross Bracing-Single Section

Figure 4.45 Cross Bracing-Double Section

End Bracing

In addition to cross bracing across the width of

the manufactured home to counteract longitudinal

wind loads, the end of the manufactured home will

also experience wind loads. The end bays at each

end of the manufactured home must, therefore,

be braced as shown in Table 4.20, based on the

same governing equation for wood cross bracing

previously presented.

Table 4.20

MINIMUM SAWN TiMBER BRACING AT EACH END

SWhere the wind load = 15 psf, 2" by 311 sawn timber bracing

at each end is acceptable at all vertical nnrber spacings and

height above grade values shown.

Connections

In the design of an elevated foundation system,

there will be numerous connections which will

differ depending upon the type of elevated foundation design chosen and the loads anticipated.

Connection points are shown in Figure 4.46 and

can be summarized as follows:

(1 ) Vertical member and horizontal support

beams.

(2) Manufactured home I-beams to horizontal

support beams.

62

4 5 6 7 8 9 10

Wind Lead = 25 psf Height of Vertical Nattber Above Grade (ft.) Vertical AIniter

Spacing (ft.)

S

6

7

S

9

10

3
(3) Diagonal bracing and vertical members.

(4) Longitudinal

members.

support beams and vertical

(5) End bracing and vertical members.

Figure 4.46 Elevated Foundation Connections

Depending upon the design loads and type of

elevated foundation and member sizes, the required connections will be subjected to different

loads. These connections must withstand the

loads so failure of the elevated foundation design

does not occur.

Horizontal Beam

The total load on a horizontal beam is based on

vertical support spacings, wind and roof live loads,

as well as manufactured home width. The bolts

connecting the horizontal beams to the vertical

members, as shown in Figure 4.47 must be

capable of carrying that load, thereby transmitting

it to the vertical member.

Tables 4.21 and 4.22 provide the recommended

number of bolts and their diameter for each

horizontal beam to vertical member connection.

These values are based on design values for

bolts loaded at both ends with loading perpendicular to grain for wood having a design value

of 890-970 psi in wet service conditions. Note that

all bolts must be of corrosion resistant metal and

bolt holes drilled to one-sixteenth inch diameter

greater than the required bolt diameter. To mainFigure 4.47 Bolts at Horizontal Beam and

Vertical Member

Table 4.21

NUMBER AND DIAMETER OF BOLTS

AT CONNECTION OF HORIZONTAL BEAMS

TO SQUARE VERTICAL MEMBERS

Vertical Menber Manufactured Home

Spacing (ft.) Width (ft.) 12 14

Wind Load = 15 or 25 psf

Roof Live Load = 20 psf

5 4@5/81 4@5/8"

6 4 e 5/8" 4 e 5/8"

7 4@ 5/8" 4 C 7/8"

8 4 e 5/8" 4 a 7/8"

9 4 C 3/4" 4 C 7/8"

10 4@ 7/8" 4 C 7/8"

Roof Live Load = 30 psf

5 4% 5/8" 4 @ 5/8"

6 4 C 5/8" 4 @ 7/8"

7 4 C 5/8" 4 @ 7/8"

8 4 C 7/8" 4 C 7/8"

9 4 @ 7/8" 4 @ 7/8"1

10 4 7/8" 4 @ 7/8"

Roof Live Load = 40 psf

5 4@ 5/8" 4 C 7/8"

6 4 @ 5/8" 4 @ 7/8"

7 4 @ 7/8" 4 a 7/8"

8 4 @ 7/8" 4 a 7/8"

9 4C 7/8 4 @ 1"

10 4C 7/8" 4 @ 1"

63

@0(I

L

o RD r

l' . ' t)

' ; t . . 1 t S

l "' ' ' IS so l -.H i ' .:B.::.:3 'Ba l : ::::! ,:::::: ::? 1:-: l : : :' -:R:R:-v * :: l

f.i i::.:: :j [S:- X

I ,,, ,,, K
Table 4.22

NUMBER AND DIAMETER OF BOLTS

AT CONNECTION OF HORIZONTAL BEAMS

TO ROUND VERTICAL MEMBERS

Vertical NMatber Manufactured Hoane Spacing (ft.) Width (ft.) -

Wind load = 15 or 25 psf

Roof Live Load = 20 psf

5 3 e 5/83' 3 0 3/411

6 3 0 3/4? 3 3/4"1

7 3 e 3/4" 3 @ 7/8"

8 3 e 7/8?? 3 @ 7/88"

9 3 @7/81? 301? 10 301" 301"

Roof Live Load = 30 psf

5 3o e5/83 3 @3/4"?

6 3 @ 3/4" 3 0 7/8"

7 3 0 7/3t 3 @ 7/831

8 3 @7/811 30@1? 9 301?I 30j1?

10 301?? 301 1/4??

Roof Live Load = 40 psf

5 3 0 5/8tt 3 7/8t' 6 3 0 3/4" 3 0 7/8"

7 3@7/831 30I1?

8 3 0 I 3 @ Jlt

9 301?? 30 1/4?? 10 301 1/4"? 301 1/4??

tain the structural capacity of the vertical member,

notching is not recommended.

Placement of the bolts at each connection is

critical and from a structural standpoint must meet

the following minimum spacing criteria.

Table 4.23 provides the minimum bolt spacing distances, as shown in Figure 4.48, as a function

of the required bolt diameter for square vertical

members.

Table 4.24 provides the minimum bolt spacing distances, as shown in Figure 4.49, as a function

of the required bolt diameter for round vertical

members.

Note that in some cases the horizontal beam or

vertical member size may not facilitate the use of

these minimum spacing criteria. In those cases,

the vertical support spacing should be decreased

and additional vertical members included, thereby distributing the loads over more connections and

allowing the use of smaller. diameter bolts.

Chassis I-Beams

The connection of the chassis I-beams to the

horizontal support beams will be subject to uplift and lateral loads from wind and flood forces.

Table 4.23

MINIMUM BOLT SPACING DISTANCES

SQUARE VERTICAL MEMBERS

BDLT SIZE Dimension (in.) Minimim Vertical Mininum

(in.) a b c e f g h Merber Dimension Beam Depth (d) (44) (54) (7d) (4d) (4d) (4d) (2 1/2d) (2h + b) (e + f + g)

5/8 2 1/2 3 1/8 4 3/8 2 1/2 2 1/2 2 1/2 1 9/16 6 1/4 7 1/2

3/4 3 3 3/4 5 1/4 3 3 3 17/8 7 1/2 9

7/8 3 1/2 4 3/8 6 1/8 3 1/2 3 1/2 3 1/2 2 3/16 8 3/4 10 1/2

Table 4.24

MINIMUM BOLT SPACING DISTANCES

ROUND VERTICAL MEMBERS

BOLT SIE Dimension (inches Minimnn Vertical Minirnrn

(in.) a b c e f g Menber Dimension Beam Depth (d) 1 1/24) (4d) (7d) (4d) (4d) (1 1/2d) (2a) (e + 2f + g)

5/8 1 5/8 2 1/2 4 3/8 2 1/2 2 1/2 15/16 1 7/8 8 7/16

3/4 1 1/8 3 5 1/4 3 3 1 1/8 2 1/4 10 1/8

7/8 1 5 15 3 1/2 6 1/8 3 1/2 3 1/2 1 5/16 2 5/8 11 13/16

1 1 1/2 4 7 4 4 1 1/2 3 13 1/2

1 1/4 1 7/8 5 8 3/4 5 5 1 7/8 3 3/4 16 7/8

64
Table 4.25

Figure 4.48 Minimum Bolt Spacing Distances

for Square Vertical Members

Where no direct contact of the manufactured

home with floodwaters is anticipated four lag

screws should be used at each chassis Ibeam/horizontal beam connection as shown in

Table 4.25.

L - b

MINIMUM NOMINAL LAG SCREW SIZE

(inches)

Support Spacing Wind Load

(ft.) 15 psf 25 psf

5 1/2 X 2 1/2 X 3

6 1/2 X 3 1/2 X 4

7 1/2 X 3 1/2 X 4

8 1/2 X 3 1/2 X 4

9 1/2 X 3 1/2 X 5

10 1/2 X 4 1/2 X 5

Figuro 4.50 shows how these are to be applied

at each chassis i-beam/horizontal beam connection. Other types of connections can be considered

but must be designed to withstand both withdrawal

and lateral loads imposed by uplift and lateral wind

forces.

Figure 4.49 Minimum Bolt Spacing Distances

for Round Vertical Members Figure 4.50 Chassis I-Beam Lag Screws

65
Where floodwaters will directly affect the

manufactured home, the use of frame clips and

lag screws will not be capable of withstanding the

buoyancy forces created. In those cases, anchoring of the manufactured home with ground

anchors must be considered or additional elevation considered to eliminate anticipated flood

forces on the manufactured home.

Cross Bracing

Connections between wood cross bracing and

vertical members as shown in Figure 4.51 must

be designed to withstand loads imposed on the

elevated foundation.

The following equation determines the minimum

number of bolts required at each cross bracing to

vertical member connection.

Number of Bolts =

at Each Connection

Load

Bolt Capacity

from Table 4.26

Where: Load =

LFd =

No. Bays =

load on each member

(lb.) calculated as (L x

F~d)(2 x No. Bays)

manufactured home

length (ft.)

diagonal loading from

Table 4.27 (lb./ft.)

Number of cross braced

bays across the width

of the manufactured

home

Table 4.26 provides the capacity in pounds for

various bolt diameters based on height of vertical

members above grade and vertical member

diameter.

Table 4.26

Figure 4.51 Cross Bracing Vertical Member

Connection

BOLT CAPACITY (pounds)

Height Above Bolt Dianeter Vertical 3Mber Diameter (in.) Grade (ft.) (in.) 4 6 8 10 12

3 1/2 568 - - - - 5/8 861 891 888 - -

3/4 1100 1273 1277 1273 -

4 1/2 554 - - - - 5/8 826 87D 864 - - 3/4 1043 1234 1240 1232 -

5 1/2 537 - - - - 5/8 783 845 835 - - 3/4 977 1185 1195 1184 -

6 1/2 519 - - - - 5/8 739 817 803 - - 3/4 910 1134 1147 1131 -

7/S 1056 1455 1546 1529 1517

7 5/8 697 788 771 - - 3/4 847 1083 1099 1080 - 7/8 976 1367 1477 1457 1441

8 3/4 790 1034 1054 1031 - 7/8 905 1287 1413 1389 1370

I 1016 1525 1777 1802 1769

IC 3/4 698 951 974 947 - 7/8 791 1153 1300 1272 1249

I 884 1340 1617 1647. 1609

The anticipated diagonal loading due to lateral

wind load per foot length of manufactured home

is shown in Table 4.27 and is based on height

above grade and wind load.

For example, consider a 60 foot manufactured

home located in a 25 psf wind load zone with 8

inch vertical members six feet above grade and

four braced vertical member bays. The load per

diagonal member is calculated as follows:

Load _ 60 ft. x 265 lb./ft.

4 x 2

= 1,987.5 lb.

66
PER FOOT

Table 4.27

DIAGONAL LOADING

OF MANUFACTURED HOME

(pounds)

Using Table 4.26 for the six foot above grade

height and eight inch post diameter, it is found that

3/4 inch bolts have a capacity of 1,147 pounds

each. The minimum number of bolts is calculated

as follows:

1987.5 lb. = 1.73 or 2

1147 lb./bolt

Therefore, using two three-quarter inch bolts at

each diagonal wood bracing to vertical member

connection will be acceptable.

End Bracing

For end bracing at the ends of the manufactured

home, the size and number of bolts is determined

in the same manner as presented for cross bracing. The load is determined using the manufactured home width as follows:

Load = Width x Fd

2

For instance, if the 60 foot manufactured home

in the previous example was 14 feet wide, the load

per diagonal member is calculated as follows:

Load = 14 ft. x 265 lb./ft.

2

= 1855 lb.

From Table 4.26 for the six foot above grade

height and eight inch post diameter, a threequarter inch diameter bolt has a capacity of 1,147

lb. The required number of bolts is calculated as

follows:

1855 lb. _-= 1.6 or 2.0 bolts

1147 lb./bolt per connection

The diagrams shown in Figures 4.52 and 4.53

provide minimum dimensions for placement of

bolts in the diagonal and vertical member connection. Bolt holes should be one-sixteenth inch larger

than the diameter of the bolt and all connection

hardware must be of corrosion resistant material.

Figure 4.52 Two Bolt Connection

Figure 4.53 Four Bolt Connection

67

Height Above Wind Load

Grade (ft.) 15 psf 25 psf

3 149 248

4 151 252

5 155 257

6 159 265

7 164 273

8 170 283

10 183 305
Longitudinal Support Beams

As previously discussed, any manufactured

home requiring perimeter blocking must be supported by a longitudinal support beam. While the

chassis 1-beams of the manufactured home are

connected to the horizontal support beams the

longitudinal support beams may also be subjected

to lateral wind loads. These loads are transferred

to the connection of the longitudinal support beam

to the post as shown in Figure 4.54.

Figure 4.54 Longitudinal Support

Beam Loading

At each longitudinal support beam and vertical

member connection two lag screws at one half

inch by five inches should be used as shown in

Figure 4.55 to maintain the structural stability of

this connection.

D. ADDITIONAL DESIGN

CONSIDERATIONS

Having determined which siting and elevated

foundation design will withstand given conditions,

it is necessary to identify other construction and

siting considerations which must be considered

in implementing any elevated foundation design.

These include such items as set-up, enclosures,

utilities, location of mechanical equipment, and

access/egress to and from the manufactured

home.

Jacking

Jacking is the typical technique to elevate a

manufactured home onto a foundation system. If

the foundation must be placed in the ground, the

Figure 4.55 Longitudinal Support Beam

Connection

foundation should be constructed first and the

manufactured home elevated to its planned height.

Equally spaced hydraulic jacks may be located

under the steel frame adjacent to the planned

foundation points, which are then moved under

the home. A minimum of eight jacks should be

used with additional jacks provided as follows:

Jacks required = W x L x 20 lb./ft. 2

C

Where: W = Width of home (ft.)

L = Length of home (ft.)

C = Jack capacity (lb.)

Maximum jack extension is also a consideration.

Where the elevation height exceeds the jack

extension, a two step procedure consisting of jacking, blocking and jacking on top of elevated pads

may be required. Temporary steel beam supports

are then run perpendicular to and underneath the

homers I-beams. They extend across the actual

foundation site and are supported on both sides.

The home is then moved across these beams

lowered onto and secured to the elevated

foundation.

68
Where jacking cannot be used to raise the

manufactured home to the required elevation, the

use of a crane should be considered.

Utility Service

Water, sewer, and gas services enter manufactured housing under the floor and originate from

grade beneath the home. To connect the manufactured home to these services, utility pipes must

extend from grade to the floor of the manufactured

home. This location makes them extremely

susceptible to flood inundation (leakage) and

damage from floating debris. To minimize damage

to these pipes, they should be placed in waterproof

risers which are located adjacent to the elevated

foundation members on the downstream side of

the flood flow as shown in Figure 4.56.

Figure 4.57 Overhead Utility Service

located in a waterproof enclosure or elevated

above the anticipated flood level.

Methods to support these tanks above grade

include separate elevated foundation systems or

support from a platform built off the manufactured

home foundation system. To minimize the potential for debris impact and damage, tanks and their

supporting foundation should be located on the

downstream side of the manufactured home.

Figures 4.58 and 4.59 show suggested methods

of elevating utility service tanks.

Figure 4.56 Utility Service Protection

Underground telephone and electric service

should also be enclosed in a waterproof riser and

protected as above for other utility service. If electrical and telephone service is supplied from

overhead lines, the service connection to the

manufactured home must be located above anticipated flooding as shown in Figure 4.57.

Applicable codes and regulations must also be

followed when designing and installing utility service. These provisions may also require waterproofing of all connections, backflow preventors

on water and sewer service, and the use of certain specific waterproof materials in and for these

services.

Propane and fuel oil tanks used to supply

energy for heating or other services should be

Figure 4.58 Separate Utility Service Tank

Foundation

Mechanical Systems

Heating and air-conditioning equipment installed inside the manufactured home will generally be located directly under or on the floor.

Elevation of the manufactured home above potential floods will, therefore, eliminate their being

69
c L o H x :k 1 I

ME /@\', 

Based on an evaluation of the site, other

measures may be warranted which will further

reduce the potential for damage. These measures

may, however, require additional costs over and

above those necessary to comply with the

minimum NFIP requirements. However, they will

afford a greater probability of flood damage loss

reduction which may offset or exceed the

necessary costs to attain the additional protection.

Chapter V discusses the costs associated with

improving the flood loss reduction potential for a

manufactured home siting technique(s).

Figure 4.59 Connected Utility Service Tank

Foundation

damaged. Where external components, such as

an outdoor compressor/evaporator unit are

installed, they must also be elevated above anticipated flooding. As with utility service tanks,

these components are best located adjacent to the

manufactured home on a separate platform supported by the elevated structure.

Access and Egress

A clear access and egress path to the manufactured home must be provided to allow evacuation

and rescue efforts necessitated by a flood. Access

and egress should be addressed during the

evaluation of a potential site to ensure feasible

alternates exist. Necessary considerations may

also include bridges, walkways, and areas of safe

refuge which allow safe evacuation during

flooding.

Having considered Chapters l-IV of the manual

a determination on the usability of a particular

flood-prone site for manufactured housing should

be possible. Where use of the land is feasible, at

least one candidate design may be available for

consideration. Such a site design could include

site modifications, elevation of the manufactured

home, anchoring or evacuation based upon an

assessment of the flood forces and capacity of any

candidate designs.

The design work sheet provided in Appendix F

should be used in developing candidate designs

in conformance with the manual.

Having developed a candidate design(s), the

ability of the design(s) to survive anticipated flood

occurrences should be assessed. At a minimum,

the design would have to comply with the provisions of the National Flood Insurance Program

(NFIP) which include certain tie-down and elevation requirements as previously discussed.

70

a =CM- 1i ..
CHAPTER V

ECONOMICS

Costs associated with developing a site and

elevating a manufactured home on the site are

important factors in selecting an elevation strategy.

These costs consist of those necessary to improve

the site, erect the elevated foundation design

developed pursuant to Chapter IV, and properly

secure the manufactured home on that foundation.

This chapter presents a format and guidance

for estimating the first costs associated with those

site design options found acceptable from a

technical standpoint. Cost ranges are provided

with which a general estimate for a particular

design can be developed. Due to the number of

possible siting and flood situations, it is difficult

to provide specific cost data on each particular

design option. Moreover, the variance in labor

rates, distance to the site, cost of materials, etc.,

further highlight the difficulty in developing and

stating a firm fixed cost for each elevation

technique.

In evaluating the economic aspects of an

elevated manufactured home installation, the cost

of a conventional installation must be recognized.

This establishes a baseline cost with which

additional improvement and elevation costs can

be compared. The additional costs associated with

improving the site and elevating the manufactured

home (i.e., fill, elevation, anchoring, etc.) can then

be considered in relation to the cost of a

conventional installation.

Conventional Installations

As discussed in Chapter I, a conventional

installation (without consideration of flooding)

consists of piers on which the manufactured home

rests and ground anchors to secure the

manufactured home on top of the piers. The piers

will generally be two feet in height and located

under the manufactured home in accordance with

the manufacturer's installation instructions. In

addition, ground anchors will generally be installed

every 10 feet along the length of the manufactured

home.

The cost for a conventional installation two feet

to the bottom of chassis I-beam will generally be

as follows:

Cast-in place concrete

piers

Built-up unreinforced

masonry piers

$100-$150/pier

$ 50-$100/pier

Jacking and placement

Ground anchors and tie

downs

$200-$300/section

$ 35-$75/anchor

Conventional installation costs, not including

miscellaneous clearing, grading work, steps, utility

connections, etc., are shown for various length

single section manufactured homes in Table 5.1.

For various length double section manufactured

homes, conventional installation costs are shown

in Table 5.2.

As previously discussed, these cost ranges may

not represent local conditions, labor and material

rates, as well as conventional installations which

may not utilize piers and ground anchors. Using

these costs as a baseline, the following costs for

fill and additional elevation through piers, posts,

and piles can be placed in perspective. It is

important to note that, in general, as the height

above grade increases, the incremental cost for

each additional foot of elevation decreases. This

can be largely attributed to labor and other fixed

costs which are essentially the same regardless

of height. Therefore, additional elevation will

provide a relatively low cost safety factor against

unusually high flooding.

Earth Fill

Earth fill can be used to elevate a manufactured

home in areas where flood velocity is not greater

than 10 fps. Prices for any site work vary

considerably due to the present condition of and

need to clear the site, distance from sources of fill,

quality of fill, and costs of the fill material. The cost

for earth fill material will generally range from

$2.00 to $4.00 per cubic yard, not including any

additional site work required. In some cases, the

total cost of elevating with fill may be as much as

$10.00 to $12.00 per cubic yard. For instance, the

Residential Cost Handbook by Marshall and Swift

show costs from $2.43 to $3.78 for soft earth and

$3.51 to $5.94 per cubic yard of fill for hard earth.

If earth fill is considered for use in elevating a

manufactured home local excavating and hauling

contractors should be contacted for an accurate

estimate.

Based on a cost of $2.00 to $4.00 per cubic yard

of earth fill, the ranges in Table 5.3 are shown for

various length single and double section

manufactured homes.

71
Table 5.1

CONVENTIONAL INSTALLATION COSTS FOR SINGLE SECTION

(dollars)

Manufactured Hlme Length (ft.)

48 52 56 60 66 72 76

Concrete Piers 1000-1500 1000-1500 1200-1800 1200-1800 1400-2100 1400-2100 1600-2400

Masonry Piers 500-1000 500-1000 600-1200 600-1200 700-1400 700-1400 800-1600

Ground Anchors 350- 750 350- 750 420- 900 420- 900 490-1050 490-1050 560-1200

Jacking 200- 300 200- 300 200- aoo 200- 300 200- 300 200- 300 200- 300

TOTAL GOST 1050-2550 1050-2550 1220-3000 1220-3000 1390-3450 1390-3450 1560-3900

Cost/Ft. Length 21.88-53.13 20.19-49.04 21.79-53.57 20.33-50.00 21.06-52.27 19.31-47.92 20.53-51.32

Table 5.2

CONVENTIONAL INSTALLATION COSTS FOR DOUBLE SECTION

(dollars)

Manufactured Ham Length (ft.)

32 40 48 52 56 60 66

Concrete Piers 1600-2400 1600-2400 2000-3000 2000-3000 2400-3600 2400-3600 2800-4200

MDsonry Piers 800-1600 800-1600 1000-2000 1000-2000 1200-2400 1200-2400 1400-2800

Ground Anchors 280- 600 280- 600 350- 750 350- 750 420- 900 420- 900 490-1050

Jacking 400- 600 400- 600 400- 600 400- 600 400- 600 400- 600 400- 600

TOTAL GJST 1480-3600 1480-3600 1750-4350 1750-4350 2020-5100 9020-5100 2290-5850

Cost/Ft. Length 46.25-112.50 37.00-90.00 36.46-90.63 33.65-83.65 36.07-91.07 33.67-85.00 34.70-88.64

Table 5.3

EARTH FILL COSTS1

(dollars]

Depth of Manufactured Manufactured HamE Length (ft.)

Fill Hane Width

(ft.) (ft.) 32 40 48 62 56 60 66 72 76

1 12 - 82- 164 94- 188 102- 204 108- 216 114- 228 124- 248 134- 268 140- 280

14 - 88- 176 104- 208 110- 220 118- 236 124- 248 136- 272 146- 292 152- 304

24 106- 212 126- 252 146- 292 156- 312 16- 332 176- 352 192- 384 - -

28 118- 236 140- 280 164- 328 174- 348 186- 372 198- 396 214- 428 - -

2 12 - 184- 368 214- 428 228- 456 242- 484 256- 612 278- 556 299- 598 312- 624

14 - 200- 400 232- 464 246- 492 262- 524 278- 556 300- 600 324- 648 340- 680

24 234-468 278- 556 320- 640 342- 684 362- 724 384- 768 416- 832 - -

28 260-520 308- 616 356- 712 380- 760 402- 804 426- 852 462- 924 - -

3 12 - 312- 624 358- 716 382- 764 404- 808 428- 856 462- 924 496- 992 520-1040

14 - 336- 672 386- 772 410- 820 436- 872 460- 920 498- 996 536-1072 560-1120

24 388- 776 456- 912 524-1048 558-1116 592-1184 624-1248 676-1352 726-1452 -

28 430- 860 504-1008 578-1156 616-1232 654-1308 690-1380 746-1492. 802-1604 -

4 12 - 465- 930 531-1062 564-1128 597-1194 631-1262 680-1360 730-1460 763-1526

14 - 498- 996 569-1138 604-1208 640-1280 676-1352 729-1458 782-1564 818-1636

24 569-1138 664-1328 759-1518. 806-1612 853-1706 901-1802 972-1944 - -

28 626-1252 730-1460 843-1686 887-1774 939-1878 991-1982 1069-2138 - -

5 12 - 644-1288 733-1466 778-1556 822-1644 866-1732 933-1866 1000-2000 1044-2088

14 - 687-1374 782-1564 830-1660 877-1754 924-1848 996-1992 1067-2134 1114-2228

24 778-1556 902-1804 1027-2054 1089-2178 1151-2302 1213-2426 1307-2614 - -

28 852-1704 988-1976 1124-2248 1193-2386 1261-2522 1329-2658 1431-2862 - -

IThe nunber of cubic yards of fill can be derived by dividing the first value in each

range above by 2 since the cost for the first entry is based on $2.00/ou.yd.

72
The amount of earth fill needed can be entered

in Part II of the Design Worksheet shown in Appendix F. The total cost can be estimated from

Table 5.3 above or the cost per cubic yard can be

secured from a local excavation and hauling contractor and applied to the amount of fill required.

Piers

As presented in Chapter IV, piers can be castin-place, built-up concrete masonry units, or brick.

The cost of piers will vary due to local material and

labor costs and will also be affected by constant

fixed costs such as contractor equipment set-up.

For instance, the Dodge Manual for Building Construction Pricing and Scheduling shows costs of

$8.86 to $16.76 per linear foot for a cast-in-place

concrete pier 12 inches square, while Marshall and

Swift show $22.50 to $31.00 per linear foot. These

costs do not cover footings, excavation, contractor set-up, and reinforcement costs.

For the purpose of estimating the cost of a pier

foundation, the general cost ranges per pier shown

in Table 5.4 can be used.

Based on the desired height above grade and

number of piers needed to elevate the manufactured home, Part II of the Design Worksheet

in Appendix F can be completed for the purposes

of estimating the design costs. If piers are a design

option, it is strongly suggested that a local contractor cost estimate be secured.

Assuming piers are installed at 10 foot intervals,

the cost per foot of manufactured home, based

on height above grade, is shown in Table 5.5.

Where additional ground anchors are used, a

cost of $35.00 to $75.00 per ground anchor should

be used.

Post and Piles

Depending upon the anticipated height, spacing, and anticipated loads, a post or pile design

may be an option. The cost of these members will

vary depending upon their length, diameter,and

depth of embedment and will also be impacted by

contractor fixed costs, such as equipment set-up.

Of critical importance is the type of soil and ease

with which these members can be secured in the

ground. Marshall and Swift, for instance, show

$8.20 installed for 10 inch and $10.15 for 12 inch

creosoted wood piles plus fixed set-up costs.

Based on estimates of $10.00 to $15.00 per foot

of length, the general costs shown in Table 5.6

can be used.

Based on the desired height above grade and

number of posts or piles needed to elevate the

manufactured home, Part II of the Design Worksheet in Appendix F can be completed for the purpose of estimating the design costs. Not that a

review of local costs will most likely indicate a small

difference in pile costs regardless of diameter such

that use of a larger diameter pile will provide additional support and may allow a reduction in the

number of vertical members.

Assuming posts and piles are installed at 10 foot

intervals, the cost per foot of manufactured home,

Table 5.4

PIER COSTS

(dollars)

Height of Pier (ft.)

3 4 5 6 7 8 9 10

Cast-in-Place 150-210 200-280 250-350 300-420 350-490 400-560 450-630 500-700

Built-up Masonry 100-150 133-200 166-250 200-300 233-350 266-400 300-450 330-500

Table 5.5

COST PER FOOT OF MANUFACTURED HOME

(dollars)

Height of Pier (ft.)

3 4 5 6 7 8 9 10

Single Section 20-42 27-56 33-70 40-84 47-98 53-112 60-126 66-140

Dbuble Section 40-84 54-112 66-140 80-168 94-196 106-224 120-252 132-280

73
Table 5.6

POST AND PILE COSTS

(dollars)

Length of Member (ft.)

10 11 12 13 14 15 16 17 18 19 20

100-150 110-165 120-180 130-195 140-210 150-225 160-240 170-255 180-270 190-285 200-300

based on height above grade and no consideration for scour, is shown in Table 5.7.

Table 5.7

COST PER FOOT OF MANUFACTURED HOME

(dollars)

Height of Post/Pile Above Grade (ft.)

4 5 6 7 8 9 10

Single Section 20-30 22-33 24-36 28-42 32-48 36-54 40-60

Double Section 40-60 44-66 48-72 56684 64-96 72-108 80-120

The design of a wood foundation also necessitates the use of horizontal beams, cross bracing,

and other hardware such as bolts, clips, etc. In

addition, labor costs to construct such an elevated

foundation must be included. Because material

and labor costs vary widely, as will the design of

each wood foundation, it is very difficult to provide

estimates for this aspect of an elevated wood foundation. Based on actual construction costs for

elevated wood foundations, costs associated with

these materials and their installation will be

roughly equivalent to the cost of the installed vertical members. Hence, the total cost for a wood

elevated foundation can be estimated by doubling

the cost for the posts/piles previously presented.

Jacking

The placement of jacks and jacking of a

manufactured home are based on the desired

height above grade. Table 5.8 provides general

estimates based on height above grade.

Miscellaneous

As discussed in Section D of Chapter IV, there

will be other miscellaneous costs associated with

elevating a manufactured home. These include

utilities, equipment, access/egress, etc. Based on

the height above grade, these costs should be

approximately the same regardless of the method

of elevation chosen. Subsequent to determining

the desired height above grade, costs for these

items should be developed with the help of a local

contractor and included in Part II of the Design

Worksheet in Appendix F. Note that even with a

coventional installation, these costs will still be

applicable. Essentially, the only cost increase can

be attributed to materials and labor to extend or

increase the height of the elevated manufactured

home.

Summary of Total Costs

Based on the information presented in this

chapter, it is possible to estimate the relative costs

of various elevation strategies. As previously

discussed, there are both fixed and variable costs

associated with an installation. In addition, the cost

of a conventional installation must be considered

and subtracted from the elevated design cost to

obtain the real cost of the elevated foundation

design. Based on the information previously

presented, Figures 5.1 and 5.2 have been

prepared to provide a general overview and comparison of costs.

Figure 5.1 provides an estimated cost per foot

length of manufactured home in comparison to

height above grade for single section manufactured homes. Figure 5.2 provides the same information for double section manufactured homes.

Note, as previously discussed, costs for each

particular installation will vary due to a number of

design and site specific considerations. It is,

therefore, strongly recommended that cost

estimates for each elevated design alternative

be secured from as many manufactured home

installation contractors as possible.

Table 5.8

JACKING COSTS

(dollars)

Height Above Grade (ft.)

3 4 5 6 7 8 9 10 I

Single Section 250-350 300-400 350-450 400-500 450-550 500-600 550-650 600-700

Double Section 500-700 600-800 700-900 800-1000 900-1100 1000-1200 1100-1300 1200-1400

74
8

4

1 conventional (low)

2 conventional (high) 3 conventional (low) + fill 4$2.00/cu.yd. 4 conventional (high) + fill @$4.00 cu.yd.

5 piers (low)

6 piers (high)

7 posts/piles (low)

8 posts/piles (high)

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

COST PER FOOT LENGTH OF MANUFACTURED HOME ($/FT.)

Figure 5.1 Single Section General

Comparison of Material Costs

8

I conventional (low)

2 conventional (high)

3 conventional (low) + fill (d$2.001cu.yd. 4 conventional (high) + fill tL$4.00 cu.yd.

5 piers (low)

6 piers (high)

7 posts/piles (low)

8 posts/piles (high)

15 30 45 60 75 90 105 120 135 150 165 180 195 210 225 240 255 270 285 300

COST PER FOOT LENGTH OF MANUFACTURED HOME ($/FT.)

Figure 5.2 Double Section General

Comparison of Material Costs

75

10

9

8

7

6

5

4

3

1-

LL

a

0

0

03

I

U

3I

2

-

jL~

0

co

l-I

-
APPENDIX A

Bibliography

American Institute of Architects, "Elevated Residential Structures,"1983.

American Society of Planning Officials, "Regulations of Modular Housing, with Special Emphasis on

Mobile Homes," 1971.

American Wood Preservers Institute, "Pole Building Design," Sixth Edition, April 1981.

Colorado Department of Natural Resources, Colorado Flood Proofing Manual, October 1983.

Commerce, Department of, Cornell University, "Project Agnes Final Report to the Economic Development Administration," June 1975.

Compliance Systems Publications Inc., The Complete Guide to the H.U.D. Mobile Home Standards

Program, Second Edition, Atlanta, Georgia, 1977.

Cooke, P. W., R. D. Dikkers, H. R. Trechsel, H. K. Tejuja, L. P. Zelenka, National Bureau of Standards,

"Model Documents for the Evaluation, Approval, and Inspection of Manufactured Buildings," NBS

Building Science Series 87, 1976.

Defense, Department of, "Flood-Proofing Regulations," June 1984.

Defense, Department of, "Protecting Mobile Homes from High Winds," (TR-75), February 1984.

Dodge Construction Cost Information System, 1985 Manual For Building Construction Pricing and

Scheduling, Annual Edition No. 20, Princeton, New Jersey: McGraw-Hill Information Systems Company,

1984..

Federal Emergency Management Agency, "Design Guidelines for Flood Damage Reduction," (FEMA

15), December 1981.

Federal Emergency Management Agency, "Flood Insurance Study - Guidelines and Specifications for

Study Contractors," September 1982.

Federal Insurance Administration, National Flood Insurance Programs, Elevated Residential Structures.

Reducing Flood Damage Through Building Design: A Guide Manual, 1976.

Federal Trade Commission, Bureau of Consumer Protection, "Mobile Home Sales and Service," August

1980.

Foremost Insurance Company, "Wind Alert - Mobile Home Safety Fact Kit from Foremost."

Galetschky, Henry, University of Missouri -Columbia, "Mobile Home Challenges for Today and Tomorrow," (UED 25 4/74/1 M), 1974.

Harris, Robert B., University of Michigan, College of Engineering, (04442-2-F), "Wind Forces on Mobile

Homes," June 1962.

Housing and Urban Development, Department of, Design and Construction Manual for Residential

Buildings in Coastal High Hazard Areas, January 1981.

Housing and Urban Development, Department of, "Guidelines for Improving the Mobile Home Living

Environment," August 1977.

77
Housing and Urban Development, Department of, NAHB Research Foundation, Manual for the Construction of Residential Basements in Non-Coastal Flood Environs, (CR-997), March 1977.

Kovacs, William D. and Felix Y. Yokel, National Bureau of Standards, "Soil and Rock Anchors for Mobile

Homes - A State-of-the-Art Report," NBS Building Science Series 107, 1979.

Manufactured Housing Dealer Magazine, "Overview of the Nation's Tie-Down Status," 1979.

Marshall and Swift Company, "Residential Cost Handbook," 1985.

Mobile Home Information Service, "Mobile Home Owners Can Protect Themselves from Hurricane."

National Concrete Masonry Association, A Manual of Facts on Concrete Masonry, 1983.

National Forest Products Association, "National Design Specification for Wood Construction," 1982

edition.

NCSBCSIANSI A225.1, NFPA 501A- Manufactured Home Installations-1982.

Nutt-Powell, Thomas E., "Manufactured Homes - Making Sense of a Housing Opportunity," Auburn

House Publishing Company, 1982.

Pennsylvania Department of Community Affairs, Flood Damage Prevention Handbook, September 1983.

Pennsylvania Department of Community Affairs, Map Initiatives Project Instructional Manual, April 1984.

Pennsylvania Department of Community Affairs, "Regulatory Flood Plain Development," December 1982.

Pennsylvania Department of Community Affairs, "You Can't Prevent the Flood."

Pennsylvania, SRBC, "Planning Guide Self-Help Flood Forecast and Warning System Swatera Creek

Watershed," November 1976.

Pennsylvania, SRBC, A User's Manual for Flood Evacuation Planning of Water and Wastewater Treatment Plants, February 1984 Draft.

Pilkey, Orrin H., Sr., Walter D. Pilkey, Orrin H. Pilkey, Jr., William J. Neal, Coastal Design: A Guide

For Builders, Planners, & Homeowners, New York: Van Nostrand Reinhold Company, 1983.

SEDA-COG, Lewisburg, Pennsylvania, "Reducing Flood Hazards of Mobile Home Parks: Recommendations," July 1979.

SEDA-COG, Lewisburg, Pennsylvania, "Strategy and Techniques to Accelerate the Reduction of Flood

Hazards at Mobile Home Parks."

Susquehanna River Basin Commission, Neighborhood Flash Flood Warning Program Manual, October

1976.

Texas Department of Labor and Standards, Manufactured Housing Division, "Department Rules Pertaining to Manufactured Housing."

Urban Land Magazine, "Planning for Hurricane and Other Coastal Disturbances," January 1984.

Vann, W. Pennington and James R. McDonald, "An Engineering Analysis: Mobile Homes in Windstorms,"

February 1978.

Vellozzi, Joseph W., Ph.D., P.E., Manufactured Housing Institute, "Evaluation of NBSIR 77-1289, The

Measurement of Wind Loads on a Full Scale Mobile Home - Comparison with Wind Tunnel and Full

Scale Test Data for Other Low Rise Structures," November 1978.

78
Vellozzi, Joseph W., Ph.D., P.E., Manufactured Housing Institute, "Review of Proposed 1980 ANSI A58.1

Standard on Wind Forces Relative to Federal Mobile Home Construction and Safety Standards," May

1980.

Waldrip, Travis G., "Mobile Home Anchoring Systems and Related Construction," June 1976.

Yokel, Felix Y., Riley M. Chung, Frank A. Rankin, Charles W. C. Yancey, National Bureau of Standards,

"Load Displacement Characteristics of Shallow Soil Anchors," NBS Building Science Series 142,1981.

Yokel, Felix Y., Charles W. C. Yancey, Christopher L. Mullen, National Bureau of Standards, "A Study

of Reaction Forces on Mobile Home Foundations Caused by Wind and Flood Loads," NBS Building

Science Series 132, 1981.

Yokel, Felix Y., Riley M. Chung, Charles W. C. Yancey, National Bureau of Standards, "NBS Studies

of Mobile Home Foundations," NBSIR 2238, 1981.

79
APPENDIX B

Federal and State Contacts

Federal Emergency Management Agency

Regional National Flood Insurance Program Contacts

Region I

J. W. McCormack

POCH, Room 442

Boston, Massachusetts 02109

(617) 223-2616

Connecticut, Maine,

Massachusetts, New Hampshire, Rhode Island, and

Vermont

Region II

26 Federal Plaza

Room 1349

New York, New York 10278

(212) 264-4734

New Jersey, New York, Puerto

Rico, and Virgin Islands

Region Ill

Liberty Square Building

Second Floor

105 South Seventh Street

Philadelphia, Pennsylvania

19106

(215) 597-7791

Delaware, District of Columbia,

Maryland, Pennsylvania,

Virgnia, and West Virginia

Region IV

1375 Peachtree Street, N. E.

Atlanta, Georgia 30309

(404) 257-2391

Alabama, Florida, Georgia,

Kentucky, Mississippi, North

Carolina, South Carolina, and

Tennessee

Region V

300 South Wacker Drive

24th Floor

Chicago, Illinois 60606

(312) 372-6098

Illinois, Indiana, Michigan,

Minnesota, Ohio, and

Wisconsin

Region VI

Federal Regional Center

Room 206

800 North Loop 288

Denton, Texas 76201

(817) 749-9127

Arkansas, Louisiana, New

Mexico, Oklahoma, and

Texas

Region Vill

911 Walnut Street, Room 300

Kansas City, Missouri 64106

(816) 758-2161

Iowa, Kansas, Missouri, and

Nebraska

Region Vill

Denver Regional Center,

Building 710

P. 0. Box 25267

Denver, Colorado 80225-0267

(303) 322-4380

Colorado, Montana, North

Dakota, South Dakota, Utah,

and Wyoming

Region IX

Building 105

Presidio of San Francisco,

California 94129

(415) 556-9840

Arizona, California, Hawaii, and

Nevada

Region X

Federal Regional Center

130 228th Street, S. W.

Bothel, Washington 98021-9796

(206) 396-0282

Alaska, Idaho, Oregon, and

Washington

State Coordinating Agencies for Flood Insurance

Alabama Department of

Economics and Community

Affairs

State Planning Division

P. 0. Box 2939

3465 Norman Bridge Road

Montgomery, Alabama

36105-0939

(205) 2848735

Alaska Department of

Community and Regional

Affairs

Division of Municipal and

Regional Affairs

949 East 36 Avenue

Suite 400

Anchorage, Alaska 99508

(907) 561-8586

Arizona Department of Water

Resources

Flood Control Branch

99 East Virginia, Second Floor

Phoenix, Arizona 85004

(602) 255-1566

81
Arkansas Soil and Water

Conservation Commission

#1 Capitol Mall, Suite 2D

Little Rock, Arkansas 72201

(501) 371-1611

California Department of

Water Resources

P. 0. Box 388

Sacramento, California 95802

(916) 445-6249

Colorado Water Conservation

Board

State Centennial Building

1313 Sherman Street, Room

823

Denver, Colorado 80203

(303) 866-3441

Connecticut Department of

Environmental Protection

Water Resources Unit

165 Capitol Avenue

Hartfofd, Connecticut 06106

(203) 566-7245

Delaware Department of

Natural and Environmental

Control

Division of Soil and Water

Conservation

Richardson and Robbins

Building

89 Kings Highway

P. 0. Box 1401

Dover, Delaware 19903

(302) 736-4411

District of Columbia

Department of Consumer

Regulatory Affairs

614 H Street, N. W.

Washington, D. C. 20001

(202) 727-7577

Florida Department of

Community Affairs

Division of Resource Planning

and Management

2571 Executive Center Circle

East

Tallahassee, Florida 32301

(904) 488-9210

Georgia Department of

Natural Resources

19 Martin Luther King, Jr.

Drive, S. W.

Room 400

Atlanta, Georgia 30334

(404) 656-3214

Guam Office of Civil Defense

P. 0. Box 2877

Agana, Guam 96910

(011-671) 477-9841

Hawaii Board of Land and

Natural Resources

Department of Land and

Natural Resources

P. 0. Box 373

Honolulu, Hawaii 96809

(808) 548-7539

Idaho Department of Water

Resources

State House

Boise, Idaho 83720

(208) 334-4470

Illinois Department of

Transportation

Division of Water Resources

Local Flood Plain Programs

300 North State Street, Room

1010

Chicago, Illinois 60610

(312) 793-3864

Indiana Department of Natural

Resources

608 State Office Building

Indianapolis, Indiana 46204

(317) 232-4160

Iowa Department of Water,

Air and Waste Management

Wallace State Office Building

Des Moines, Iowa 50319

(515) 281-5029

Kansas State Board of

Agriculture

Division of Water Resources

109 Southwest Ninth Street

Topeka, Kansas 66612

(913) 296-3717

Kentucky Department of

Natural Resources

Division of Water

18 Reilly Road

Fort Boone Plaza

Frankfort, Kentucky 40601

(502) 564-3410

Louisiana Department of

Urban and Community

Affairs

P. 0. Box 44455, Capitol

Station

Baton Rouge, Louisiana 70804

(504) 925-3730

Maine Bureau of Civil

Emergency Preparedness

State House

187 State Street

Augusta, Maine 04333

(207) 289-3154

Maryland Department of

Natural Resources

Water Resources

Administration

Tawes State Office Building

D-2

Annapolis, Maryland 21401

(301) 269-3826

Massachusetts Water

Resources Commission

Division of Water Resources

State Office Building

100 Cambridge Street

Boston, Massachusetts 02202

(617) 727-3267

Michigan Department of

Natural Resources

Water Management Division

P. 0. Box 30028

Lansing, Michigan 48909

(517) 373-3930

Minnesota Department of

Natural Resources

Division of Waters

444 LaFayette Road

St. Paul, Minnesota 55101

(612) 296-9226

82
Mississippi Research and

Development Center

3825 Ridgewood Road

Jackson, Mississippi 39211

(601) 982-6376

Missouri Department of

Natural Resources

1101 R. Southwest Boulevard

P. 0. Box 1368

Jefferson City, Missouri 65102

(314) 751-4932

Montana Department of

Natural Resources and

Conservation

32 South Ewing Street

Helena, Montana 59601

(406) 444-6646

Nebraska Natural Resources

Commission

P. 0. Box 94876

Lincoln, Nebraska 68509

(402) 471-2081

Nevada Division of

Emergency Management

Capitol Complex

Carson City, Nevada 89710

(702) 885-4240

New Hampshire Office of

State Planning

2 1/2 Beacon Street

Concord, New Hampshire

03301

(603) 271-2231

New Jersey Department of

Environmental Protection

Division of Water Resources

P. 0. Box CN 029

Trenton, New Jersey 08625

(609) 292-2296

New Mexico State Engineer's

Office

Bataan Memorial Building

Santa Fe, New Mexico 97501

(505) 827-6140

New York Department of

Environmental Conservation

Flood Protection Bureau

50 Wolf Road, Room 422

Albany, New York 12233

(518) 457-3157

North Carolina Department of

Natural Resources and

Community Development

Division of Community

Assistance

P. 0. Box 27687

Raleigh, North Carolina 27611

(919) 733-2850

North Dakota State Water

Commission

State Office Building

900 East Boulevard

Bismarck, North Dakota 58505

(701) 224-2750

Ohio Department of Natural

Resources

Flood Plain Planning Unit

Fountain Square

Columbus, Ohio 43224

(614) 265-6755

Oklahoma Water Resources

Board

10th and Stonewall, 12th Floor

Oklahoma City, Oklahoma

73105

(405) 271-2533

Oregon Department of Land

Conservation and

Development

1175 Court Street, N. E.

Salem, Oregon 97310

(503) 378-2332

Pennsylvania Department of

Community Affairs

551 Forum Building, Room 317

Harrisburg, Pennsylvania 17120

(717) 787-7400

Puerto Rico Planning Board

P. 0. Box 41119, Minillas

Station

Santurce, Puerto Rico 09940

(809) 726-7110

Rhode Island Office of State

Planning

Statewide Planning Program

265 Melrose Street

Providence, Rhode Island

02907

(401) 277-2656

South Carolina Water

Resources Commission

3830 Forest Drive

P. 0. Box 4440

Columbia, South Carolina

29240

(803) 758-2514

South Dakota Department of

Military and Veterans

Affairs

Division of Emergency and

Disaster

State Capitol

Pierre, South Dakota 57501

(605) 773-3231

Tennessee Department of

Economic and Community

Development

Division of Local Planning

1800 James K. Polk Office

Building

505 Deaderick Street

Nashville, Tennessee 37219

(615) 741-2211

Texas Department of Water

Resources

P. 0. Box 13087, Capitol

Station

1700 North Congress Avenue

Austin, Texas 78711

(512) 475-2171

Utah Office of Comprehensive

Emergency Management

1543 Sunnyside Avenue

Salt Lake City, Utah 841U8

(801) 533-5271

Vermont Environmental

Conservation Agency

Division of Water Resources

State Office Building

Montpelier, Vermont 05602

(802) 828-2761

Virginia State Water Control

Board

P. 0. Box 11143

Richmond, Virginia 23230

(804) 257-0075

83
Virgin Islands Disaster

Preparedness Office

Box 1208

St. Thomas, Virgin Islands

00801

(809) 774-6555

Washington Department of

Ecology

Mail Stop PV11

Olympia, Washington 98504

(206) 459-6288

West Virginia Office of

Emergency Services

Capitol Building, Room EB-80

Charleston, West Virginia

25305

(304) 348-3831

Department of Natural

Resources

Flood Plain-Shoreland

Management Section

P. 0. Box 7921

Madison, Wisconsin 53707

(608) 266-1926

Wyoming Disaster and Civil

Defense Agency

P. 0. Box 1709

Cheyenne, Wyoming 82003

(307) 777-7566

84
APPENDIX C

Sources of Flood Information

To develop an effective manufactured home installation strategy applicable to a flood hazard

area, several hydrologic factors must be

evaluated. These include the regulatory floodplain

boundaries and the anticipated flooding

characteristics for the site such as depth, velocity, duration, rate-of-rise, and frequency. Various

sources of this information are available. Also,

there are methods by which portions of such

information can be individually developed.

As part of its National Flood Insurance Program

(NFIP), the Federal Emergency Management

Agency (FEMA) develops Flood Insurance Studies

(FISs) which will often contain the most current and

detailed information that is available for a community. Such studies can include a Flood Insurance Rate Map (FIRM), a Flood Boundary and

Floodway Map (FBFM) and a FIS report, from

which the following information can be obtained:

* Floodplain and floodway boundaries

* Stream profiles that show the elevations of

the 1 00-year or "base" flood (the flood that

has a one-percent chance of being equalled

or exceeded in any given year) and other

flood events

* Mapped base flood elevations which, in

combination with other data, can be used

to develop flood depths for a specific site

* Flood velocity data

* Flood frequency data

* Flood discharge data

* Historical flood information

FISs can be extremely valuable sources of portions of the information necessary to evaluate a

potential manufactured home site. The specific

hydrologic data elements, and ways to obtain

them, are described below.

Flood Hazard Boundaries-Boundaries for the

different degrees and types of flooding, including floodways, floodway fringe, coastal high

hazard areas, coastal fringe, and shallow

flooding areas, must be identified. Flood hazard

boundaries are significant because they determine the specific flood hazard zones that are

part of a proposed development site or that will

influence development on the site. In addition,

boundaries indicate where floodplain management regulations and flood insurance

requirements, apply to the site. Flood hazard

boundary data can be obtained from FBFMs,

FIRMs, and floodplain maps or can be

developed from topographic maps, zoning

maps, aerial photographs, and related

hydrologic data.

Flood Depths-Flood depths are determined

by the difference between water surface elevation at times of flooding and normal ground surface elevations. This information is important

both in determining the elevations at which flood

damage is likely to occur and in defining the appropriate elevations for flood insurance and

floodplain management regulations. Flood

depths also influence the hydrostatic forces that

are in effect during flooding, including the

horizontal loads that can cause lateral displacement or overturning, and the vertical loads that

can cause uplift and flotation. Flood depths can

be derived by using a FIRM showing base flood

elevations in combination with a topographic

map depicting ground elevations for a particular

site. Flood depth data is also available from

various technical studies that include flood

elevations, water surface profiles, or stream and

coast cross-sections. In the absence of official

reports, information on flood depths can be obtained from site survey and historical records.

Flood Water Velocity-The average and maximum velocity of floodwater is important in determining hydrodynamic forces, which influence

horizontal loads in excess of hydrostatic loads.

Velocity also affects the magnitude of debris impact loads (i.e., force of floating objects carried

by floodwaters), and can increase erosion and

affect soil stability on slopes. Data on water

velocity is listed in Floodway Data Tables often

included in FIS reports and is available from the

various floodplain technical studies or can be

determined by special hydrologic studies.

Velocity can also be calculated by assuming

floodwaters to be at a uniform flow, estimating

some floodplain characteristics, and using Manning's Equation:

85
V = 1.49 (A/P)213 S'1/2

n

where: V = Average flow velocity (feet

per second)

A = Cross-sectional flow area

(feet)2

P = Wetted perimeter of A (feet)

S = Bed slope (feet/feet) (use

average ground surface

slope within a reach from approximately one-half mile

upstream to approximately

one-half mile downstream)

n = Manning's channel

roughness coefficient

Frequency-Frequency of flooding is a major

consideration in the evaluation of potential installation sites. Frequency of flooding is the

probability (in percent) that a random flood event

will equal or exceed a specified magnitude in

a given time period. Manufactured homes sited

at lower elevations near a flooding source will

likely have a higher frequency of flooding than

those located on higher ground away from the

flooding source. Flood frequency information is

included in FIS reports and other technical

floodplain studies. Frequency of flooding can

also be statistically determined using historical

records of flooding at the location under

consideration.

Rate of Rise-The rate of rise of a flood is an

expression of how rapidly water depth increases

during a flooding event. This factor is important

in evaluating buoyancy hazards and investigating the feasibility of an evacuation plan.

The rate of rise of flood- waters can be derived

from a stream flow hydrograph that relates

flooding depth to time for the area under consideration. Information required to determine the

rate of rise may also be obtained from existing

hydrologic studies, on-site investigations,

historical records, and local civil defense offices.

Duration-The duration of flood inundation,

which is a function of the rate of rise and fall

of water, has several important influences on

a manufactured home installation. Duration influences the saturation of soils and building

materials, the amount of seepage, and the

length of time that a manufactured home might

be inaccessible. The various floodplain technical

studies and historical records are sources of information concerning duration of flooding.

The following tables provide sources of

various types of information and assistance.

86
87
APPENDIX DCalculational Procedures for

Elevated Foundation Design

As presented in Chapter IV, the minimum recommended design criteria are based on design conditions and data appropriate for the context of the manual. The intent of Chapter IV is to show the selection of elevated foundation components and materials that when assembled would provide sufficient

design capacity to withstand the anticipated loads. Recognizing that such a presentation reduces the

applicability to certain possible design conditions, this appendix provides the calculational procedures

used as a basis in the manual. For ease of use and consistency with the manual, the presentation in

this appendix follows the outline of Section B of Chapter IV.

For the purposes of this appendix, the following terminology is applicable.

Ac = Vertical member area (in2)

Af = Floor area (ft2)

An = Total net area of wood member (in2)

Ap = Pier area (in2)

Ar = Roof area including eaves (ft2)

a = Chassis I-beam spacing (in)

Cs = Lag screw pullout capacity (lblin)

Di = Dead load of manufactured home (psf)

d = Vertical member diameter (in)

E = Modulus of elasticity (psi)

e = Distance from center to center of chassis I-beam to vertical member (in)

Fb = Bending stress (psi)

F'b = Bending stress (psi) corrected for conditions of service

Fc = Compressive strength of wood (psi)

F'c = Compressive strength of wood (psi) corrected for conditions of service

Fe = Floor live load of manufactured home (psf)

Fr = Load per vertical member where roof load governs (lb)

Ft = Allowable working stress of wood in tension (psi)

Fw = Load per vertical member where wind load governs (lb)

f'm = Concrete strength (psi)

h = Outside height of manufactured home including perimeter rail and chassis I-beam

(ft)

L = Outside length of manufactured home (ft)

Lp = Lag screw penetration length (in)

Lw = Vertical load due to wind (lb)

eI = Total length of vertical member (in)

= Number of vertical members (load points)

P = Vertical member capacity (lb)

Q = Bolt design capacity (lb)

Re = Roof live load (psf

89
s = Vertical member spacing (ft)

Sx = Section modulus (in3)

W = Outside width of manufactured home (ft)

Wf = Lateral wind load (psf)

WYt = Lateral wind load per vertical member (lb)

W*e= Lateral wind load adjusted parallel to bracing (lb)

Wu = Uplift wind load (psf)

W'u = Uplift wind load per vertical member (Ib)

Vertical Member Loads

The vertical load on each elevating member from Table 4.10 must be determined for two design conditions: 1) where roof live load governs and 2) where wind load governs.

For the first condition above, roof live load, dead load, and floor load act vertically downward through

the chassis I-beam. This load (Fr) is determined as follows:

F [(Re)(Ar) + (Df + Ff)(Af)J r=

Where:

Fr = Load per vertical member (Ib)

Rf = Roof live load (psf)

Ar = Roof area including eaves (if 2

)

Df = Dead load (psf)

Ft = Floor live load (psf)

Af = Floor area (ft2)

2v = Number of vertical members (load points)

For the second condition above, the lateral wind load is transferred to the chassis I-beam and the

dead load and floor live load act vertically downward through the chassis I-beam. This load (Fw) is determined as follows:

Fw - Lw + (Df + Ff)(Af)

Where:

Fw = Load per vertical member (lb)

De = Dead load (psf)

Ff = Floor live load (psf)

Af = Floor area (ft2)

27 = Number of vertical members (load points)

Lw = Vertical load due to wind (lb)

The vertical load due to wind is calculated as follows:

LW =[(h/2)(1 2)(Wf)(L)(h)J Lwa

Where:

h = Outside height of manufactured home, including perimeter rail and chassis I-beam (ft)

We = Lateral wind load (psf)

L = Outside length of manufactured home (ft)

9O
a = Chassis I-beam spacing (in)

As discussed in Section B of Chapter IV, the following design conditions were used.

Re = 20, 30 or 40 psf based on roof live load zone (figure 4.1)

Ar = Width (W) X length (L) (including eaves)

De = 20 psf

Fe = 40 psf

Af = Width (W) X length (L)

h = 9.75 ft.

We = 15 or 25 psf based on wind load zone (Figure 4.2)

a = 75 112 in., 82 in., and 99 in.

s = 5, 6, 7, 8, 9, and 10 ft.

Pier Design

As presented in Section B of Chapter IV, various pier designs in Table 4.11 are recommended based

on the anticipated loads which impact the manufactured home and are transferred to the pier. The recommended pier designs were determined using the interaction equation as follows:

1.33 = fa + fb

Fa Fb

Where:

fa = computed axial stress in psi = [W x s x 60] -. Ap

Where:

W = Outside width of manufactured home (ft)

s = Vertical member spacing (ft)

Ap = Cross sectional area of pier (in2)

fb = Horizontal loading in psi

= [Wexhxsxfx12] Sx

Where:

We = Lateral windload (psf)

h = Outside height of manufactured home (ft)

s = Vertical member spacing (ft)

Sx = Section modulus of pier (in3)

e = Length of pier (ft)

Fa = 0.20 f'm

Fb = 0.30 f'm

Where:

f'm = Strength of concrete (psi)

The following design conditions were used in calculating the acceptable conditions under which pier

failure would not occur for the pier designs in Section B of Chapter IV.

W = 12 and 14 ft.

s = 5, 6, 7, 8, 9, and 10 ft.

We = 15 and 25 psf.

f'm = 1500 and 2500 psi.

e = 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, and 15 ft.

Sx = Based on pier designs A-F

Ap = Based on pier designs A-F

91
Vertical Support Members (wood posts/piles)

Depending upon the anticipated loading to be transferred to the vertical support member, it is necessary

to determine the capacity of a given vertical member. In determining the values in Tables 4.13 to 4.16,

it was necessary to apply beam/column theory and solve for the capacity of a given member under

various design and load conditions. This capacity is determined as follows:

=/P ( A1FcI Sf)K

Where:

P = Vertical member capacity (lb)

AC = Area of the vertical member (in2)

= d2 for square members

ird2 for round members =_

Where:

d = diameter (in)

£ = Total length of the vertical member above and below grade (in) including scour allowance

from Table 4.5

Sx= Section modulus of the vertical member (in3) (bd2 ) for square members,

6

7rr2 for round members

4

Where:

r = radius (in)

d = depth (in)

b = base (in)

Fb = Bending stress of vertical member modified for moisture, duration and service conditions

(psi)

K = AIB

Where:

A = (h)(Wf) and

2

B = (DO x FO)(W) + (h212) (WL)_

2 [a a 

Where:

E (w + 30 a- a

2

FIc = Compressive strength of vertical member (psi)

if fid -< 1, then FC = 625 psi wy

if 11 <f/d < K', where KI = 0.671 FC oc

then Pc = 625 (Oc) [1v/3(£/d 4]

if lid = K', then FIc = 625(2/3)

if £/d>K' and s50, then FPc = 0.3E

(ld) 2

E = Modulus of elasticity (psi)

oc = 1.0

92
The following design conditions were used in calculating the data provided in Tables 4.13-4.16

De = 20 psf

Fe = 40 psf

We = 15 and 25 psf

W =12and14ft.

e = 12, 24, 36, 48, 60, 72, 84, 96, 108, 120, 132, 144, 156, 168, 180, 192, 204, 216, 228,

and 240 in.

d = 31/2, 51/2, 71/2, 91/2, and 111/2 in. (square members)

d = 4, 6, 8, 10, and 12 in. (round members)

e = 371/2, 46 and 491/4 in.* for W = 12 ft. and a = 99 in., 82 in., and 751/2 in. respectively

e = 491/2, 58, and 611/4 in.* for W=14 ft. and a=99 in., 82 in., and 751/2 in. respectively

*Includes 30 in. additional width to allow for clearance between manufactured home

side and vertical member.

F'b = 915 psi = [(800 psi) X (1.33 duration) X (0.86 moisture)]

E = 1,000,000 psi

oc = 1.00

h = 9.75 ft.

Horizontal Support Beams

The application of the vertical member loads to the elevated foundation requires that the horizontal

support beams be capable of sustaining the anticipated loading. The recommended horizontal beam

size is determined as follows.

Min. Sx - (Fr)(e) or (Fw)(e) whichever is greater

F'b F'b

Where:

- (b)(d)2 (in 3) Sx = 6

Fr = Vertical force (Table 4.10)

or

Fw

e = Distance from I-beam to vertical member (in)

Fb = bending stress of wood (psi)

Subsequent to a determination of the minimum Sx required for each design condition, the required

size and number of horizontal beams can be found.

The following values were used in this calculation.

Fr = Forces provided in Table 4.10 for the loads, and design dimensions shown

or

Fw

e = 371/2, 46 and 491/4 in.* for w = 12 ft. and a = 99 in., 82., and 751/2 in. respectively

e = 491/2, 58 and 611/4 in.* for w = 14 ft. and a = 99 in., 82 in., and 7511⁄22 in.

respectively

*Includes 30 in. additional width to allow for clearance between manufactured home

and vertical member.

F'b = 1207.5 psi = [(1250 psi)(1.15 duration)(0.84 vertical bending)]

93
Cross Bracing

Cross bracing will improve the stability of the elevated foundation against lateral wind loading. The

required number and size of cross bracing members is determined as follows:

Ane = [(h)(L)(WQJ/Cos 4

Ft

Where:

Ax = Total net area of cross bracing required (in2)

h = Manufactured home height (ft)

L = Manufactured home length (ft)

Wf = Design wind load (psi)

Cos i = Cosine of angle between cross bracing and horizontal beam.

Ft = Allowable working stress in tension of wood (psi)

Subsequent to calculating the minimum net area, the number of vertical members cross braced is

divided into An for each member and a size member chosen having at least that cross sectional area.

The following design values were used in developing the criteria for Tables.4.18 and 4.19.

h = 9.75 ft.

L = 32, 40, 48, 52, 56, 60, 66, and 72 ft.

Wf = 15 and 25 psf.

Cos + = Vertical member height above grade at 3, 4, 5, 6, 7, 8, and 10 ft. with a 17 ft.

space between vertical members under the manufactured home

Ft = 531.3 psi.

End Bracing

There is a need to improve the lateral stability of the elevated foundation against lateral wind loading

on the end of the manufactured home. Each end is required to be braced and the size of end bracing

is determined as follows:

An = [(h)(W)(Wf)]/Cos 4

Ft

Where:

ATZ = Total net area of end bracing required (in2)

h = Manufactured home height (ft)

W = Manufactured home length (ft)

we = Design wind load (psi)

Cos + = Cosine of angle between end bracing and the horizontal

Ft = Allowable working stress in tension of wood (psi)

Subsequent to calculating the minimum net area, the value is divided by two to represent the two

members under tensile load at the loaded end of the manufactured home and a member size chosen

having at least that cross sectional area.

The following design values were used in developing the criteria for Table 4.20.

h = 9.75 ft.

W = 12and 14ft.

We = 15 and 25 psf.

Cos + = Vertical member height above grade at 3, 4, 5, 6, 7, 8, and 10 ft. and vertical

support spacings of 5, 6, 7, 8, 9, and 10 ft.

Ft = 531.3 psi.

94
Connections

Vertical Member to Horizontal Beam

The vertical member loads presented in Table 4.10 are directly transferred to the connection of the

vertical member and horizontal beam. The number of bolts of a given size are determined as follows:

Fr/Q or Fw/Q (whichever is greater)

Fr = Vertical member load (lb) from Table 4.10 or calculated for other conditions

or

Fw

Q = Bolt design capacity (lb)

The bolt design capacity is based on the type of wood, length of bolt in the horizontal beam, diameter

of bolt and direction of load to wood grain.

The values in Tables 4.21 and 4.22 of the manual were derived from the vertical member loads in

Table 4.10 at the stated design conditions and the following bolt design values for spruce-pine- fir, setka

spruce, yellow poplar, and eastern spruce and lodgepole pine.

Q - Bolt Design Value (lb)

Horizontal Beam Size

Bolt Diameter (in) 2@3 in. 4@2 in. 4@3 in.

5/8 990.0 1012.5 -

3/4 1170.0 1295.0

7/8 1290.0 1505.0 1 V5.0

1 1410.0 1687.5 2147.5

1 1/4 1655.0 1985.0 3117.5

To be capable of supporting the imposed load the total bolt capacity (bolt design value x no. bolts)

must be greater than or equal to the vertical load from Table 4.10 or calculated as previously discussed.

Chassis l-beams

The lag screws used to connect the chassis I-beam clips to the horizontal beam must be capable

of resisting uplift and shear loads resulting from wind.

The lateral wind load per vertical member is calculated as follows:

-'e = [(h)(12) (We) (s)(h)

a

Where:

W'f = Lateral wind load per vertical member (lb)

h = Manufactured home height (ft)

We = Lateral wind load (psf)

s = Vertical member spacing lengthwise (ft)

a = I-beam spacing

The uplift wind load per vertical member is calculated as follows:

W'u = [(Wu)(s)(W)] + 2.5Wu[(s)(W + 2)]

2

95
Where:

Wfu = Uplift wind load per vertical member (lb)

W = manufactured home width (ft)

WU = Uplift wind load (psf

s = Vertical member spacing lengthwise (ft)

The pullout capacity (Cs) of 1/2 in. lag screws with wood having a specific gravity of 0.42 is 291 lb.

per inch of shank diameter. To determine the required penetration length (lb) per screw, the following

calculation is used.

LP =W'e + W'u

(4) (291)

In calculating the required penetration length the following values were used.

h = 9.75ft.

Wie = 15 and 25 psf.

s = 5, 6, 7, 8, 9, and 10 ft.

a = 751⁄2/2 in.

W = 14 ft.

Wu = 9 and 15 psf.

Cross and End Bracing

To determine the number and diameter of bolts needed for cross and end bracing, the lateral wind

load, which acts horizontally, must be adjusted to act parallel with the load bearing the bracing member

as follows:

W*e = (h)(Wf)

sin <

Where:

W*t f = Adjusted load (lb./ft. home)

h = Manufactured home height (ft)

WI = Lateral wind load (psf)

sin 4 = sine of the angle between horizontal and bracing member

In this calculation the following values were used.

h = 9.75 ft.

Wf = 15 and 25 psf

sin e = Based on geometry of 12 ft. wide horizontal dimension and height of vertical

member at 3, 4, 5, 6, 7, 8, and 10 ft. (corrected by subtracting 1 ft. to allow

for horizontal beam depth)

Bolt capacity Table 4.25 was developed based on the type of wood previously described for vertical

members and corrected for interpolation between capacity parallel and perpendicular to grain as follows:

Q = [ (QI) (Q//) ] X12

[(Q//)(Cos2e)] + [(Qi)(sin2e)]

Where:

Q = Bolt capacity as loaded (lb)

Q l = Capacity perpendicular to grain (lb)

96
= Capacity parallel to grain (lb)

e = Angle between horizontal and diagonal member

The total number of bolts and size required per connection are determined as follows:

Number bolts = W*I/Q

Having reviewed the methods and procedures utilized, one should appreciate the complexity of design

for elevated foundations. Where additional details are necessary, different conditions apply or further

design detail is required, publications such as those listed in Appendix A should be consulted. As noted

in the manual, state and local regulations should be considered to determine provisions requiring design

by a registered engineer.

97

Q//
APPENDIX E

Buoyancy and Drag Forces

Buoyancy

Based on the anticipated flood height, the

resultant buoyancy force, and the load capacity

of manufactured housing ground anchors, it is

possible to determine the ground anchor spacing

necessary to resist flotation. The vertical tie force

(TV), shown in Table E.1 can be divided into

anchor capacity to yield the anchor spacing needed to resist the anticipated load. Minimum anchor

spacings based on a 2,200 pound ground anchor

capacity for given flood height differences and tie

angles are shown in Table E.2. The shaded areas

in the table indicate situations where the amount

of anchorage is not feasible or would transfer loads

to the floor or other portions of the manufactured

home causing them to fail and damage the

manufactured home. Note these ties are in addition to those recommended by the manufacturer

or ANSI Standard A225.1-1982 for wind

anchorage.

Note that where diagonal ties are used, the required tie spacing decreases and additional ties

will be needed. In reviewing Tables E.1 and E.2,

the effects of water height and tie angle on tie

spacing are dramatically shown. On a 14 foot wide

manufactured home, for instance, the number of

ties increases by a factor of four if the difference

in water height is increased from six to nine inches

and diagonal ties at 45 degrees are used in lieu

of vertical (90 degree) ties.

Limited testing of ground anchors in saturated

soil has been undertaken. Of the testing conducted, a wide range of results have been

achieved. Unless specific test results for the soil

type of the site have been conducted for the particular anchor the above tie spacing values should

be used. Where testing is performed the spacing

values in Table E.2 can be adjusted to yield a new

recommended minimum spacing as previously

shown.

It is important to stress that these criteria for

anchoring only apply to flooding forces due to

buoyancy imposed on a manufactured home.

Where wind forces are also anticipated, additional

anchoring will be necessary and should be consistent with that required by state or local

regulations addressing manufactured housing installations, the manufacturers instructions or ANSI

Standard A225.1-1982.

Consider also the cost associated with anchoring in lieu of elevation. Using the economic information in Chapter V, ground anchors cost $35.00

to $75.00 installed. Referring to Table E.1 for a

15 inch difference in water height and 60 degree

Table E.1

VERTICAL TENSION (TV)

(pounds)

Hon Difference in Water Height (in.) 1

Width

(ft.) 1 2 3 4 5 6 9 12 15 18 21 24 30 36 42 48

12 - - - 4.8 36.0 67.2 160.8 254.4 348.0 441.6 535.2 628.8 816.0 1003.2 1190.4 1377.6

14 - - - 5.6 42.0 78.4 187.6 296.8 406.0 515.2 624.4 733.6 952.0 1170.4 1388.8 1607.2

24 - - - 9.6 72.0 124.4 321.6 508.8 696.0 883.2 1070.4 1257.6 1632.0 2006.4 2380.8 2755.2

28 - - - 11.2 84.0 156.8 375.2 593.6 812.0 1030.4 1248.8 1467.2 1904.0 2340.8 2777.6 3214.4

'Height of floodwater above underside of manufactured home minus height of floodwater inside the

manufactured home (h1 - h2 ).

Fb = h x v x w (Ib./lin. ft. of home) where: h = (h1 - h2) ft;

v = 62.4 lb./cu. ft.; w = width (ft)

T, = (Fb - Wt) (Ib/lin. ft. of home per side) where: Tv = vertical tie force needed to overcome Fb;

2

Wt = weight of home at 20 Ib./sq. ft.

99
Table E.2

MINIMUM SPACING BETWEEN GROUND ANCHORS 1

(feet)

Hore Tie Difference in Water Height (in.) 2

Width Angle

(ft.) 6 9 12 15 18 24 30 36

12 90 32.74 13.68 8.65 6.32 4.98 3.50 2.70 2.19

60 28.35 11.85 7.49 5.47 4.31 3.03 2.33 -

45 23.15 9.67 6.11 4.47 3.52 2.47 - -

30 16.37 6.84 4.32 3.16 2.49 - - -

14 90 28.06 11.73 7.41 5.42 4.27 3.00 2.31 -

60 24.30 10.16 6.42 4.69 3.70 2.60 2.00 -

45 19.84 8.29 5.24 3.83 3.02 2.13 - -

30 14.03 5.86 3.71 2.71 2.14 - - -

24 90 17.69 6.84 4.32 3.16 2.49 - - -

60 15.32 5.92 3.74 2.74 2.16 - - -

45 12.50 4.84 3.06 2.23 - - -

30 8.84 3.42 2.16 - - _ _ _

28 90 14.03 5.86 3.71 2.71 2.14 - - -

60 12.15 5.08 3.21 2.35 - - - -

45 9.92 4.15 2.62 - - - - -

30 7.01 2.93 - -----

'Based on a 2200 pound ground anchor pull out capacity. Where spacing is less than or equal to 24

inches values are not provided. 2Difference in water height (in.) between the inside and outside of the manufactured home.

For soils where the actual ground anchor pull out capacity may be greater than or equal to 2200 pounds,

and less than or equal to 3150 pounds, the values in Table E.2 may be proportionately increased.

Spacing Above x (Actual capacity pounds) New -Spacing = 2200 pounds

For instance if the soil had a pull out capacity of 3150 pounds and a 14 ft. home with a nine inch

anticipated water height were to be anchored with vertical ties, the new spacing would be as follows:

New Spacing = 11.73 x 3150 lbs. = 16.80 ft.

2200 lbs.

2Difference in water height (in.) between the inside and outside of the manufactured home.

100
tie angle, the cost due to additional anchoring is

$14.93 to $31.98 per foot for a 14 foot wide

manufactured home. Comparing this to the cost

for earth fill, piles, or piers, it will be less costly

to elevate the manufactured home two feet while

concurrently reducing potential damage.

Drag

Drag forces will also result in additional anchoring requirements based on water height and

velocity. While buoyancy forces exert an upward

force on the anchoring system, drag forces exert

a horizontal force which must also be counteracted

through additional ties. From the anticipated drag

force, as presented in Section B of Chapter IV and

the capacity of ground anchors, it is possible to

determine the necessary anchor spacing to resist

any drag loads. The drag loads can be divided into

the anchor capacity to yield the required anchor

capacity to resist the anticipated vertical load.

Calculated anchor spacings based on a 2,200

pound ground anchor capacity for given flood

height, differences in tie angles, and velocity are

presented in Table E.3.

The impracticality of anchoring a manufactured

home in floodwaters imposing more than a nine

to 12 inch exterior water height above the finished

floor becomes apparent. Consequently, the

manufactured home must be elevated above anticipated flooding.

Table E.3

MINIMUM SPACING BETWEEN GROUND ANCHORS 1

(feet)

Velocity Tie Height of Water (in.) 2

(fps) Angle

6 9 12 15 18 24 30 36

3.0 60 _- - - - - - -

45 _- - -

30 - ----- 38.47 32.06

4.0 60 - - - - -- 37.48 31.23

45 - - - - - 38.24 30.59 25.50

30 - - - - 36.07 27.05 21,64 18.04

5.0 60 - - - - 39.98 29.98 23.99 19.99

45 - - - 39.16 32.63 24.47 19.58 16.32

30 - - 34.62 27.70 23.08 17.31 13.85 11.54

6.0 60 - - - 33.32 27.76 20.82 16.66 13.88

45 - - 33.99 27.19 22.66 17.00 13.60 11.33

30 - 32.06 24.04 19.24 16.03 12.02 9.62 8.02

8.0 60 - 31.23 23.43 18.74 15.62 11.71 9.37 7.81

45 38.24 25.50 19.12 15.30 12.75 9.56 7.65 6.37

30 27.05 18.04 13.52 10.82 9.02 6.76 5.41 4.51

10.0 60 29.98 19.99 14.99 11.99 9.99 7.50 6.00 5.00

45 24.47 16.32 12.24 9.79 8.16 6.12 4.89 4.08

30 17.31 11.54 8.66 6.93 5.77 4.33 3.46 2.89

12.0 60 20.82 13.88 10.41 8.33 6.94 5.21 4.16 3.47

45 17.00 11.33 8.50 6.80 5.67 4.25 3.40 2.83

30 12.02 8.02 6.01 4.81 4.01 3.01 2.40 2.00

15.0 60 13.33 8.88 6.66 5.33 4.44 3.33 2.67 2.22

45 10.88 7.25 5.44 4.35 3.63 2.72 2,18 1.81

30 7.69 5.13 3.85 3.08 2.56 1.92 1.54 1.28

'For manufactured home sited perpendicular-to flow. Values not provided for spacing 40 feet or more.

2Height of water impacting the manufactured home from the bottom of the chassis I-beam.

101
APPENDIX F

Design Worksheet

The purpose of the design worksheet is to provide a format for developing a manufactured

home installation design on a per section basis. Based on the information presented in the

manual and a determination of the manufactured home and site specifics, the following

worksheet can be completed. It can subsequently be used to determine installation

arrangements and provide a basis for estimating their costs. Where appropriate,

references to information in the manual are presented. To aid in completing the design

worksheet, instructions are also provided. Part I addresses the design of the

manufactured home foundation system. Part II addresses the associated costs for that

design. Part III provides space to develop an installation design plan.

It is suggested that this worksheet be used to develop several designs and cost estimates

so that a comparison can be made to determine an appropriate design strategy. Figure F.1

highlights those areas addressed by the design worksheet. The numbers in Figure F. 1 refer

to specific line numbers (Ln) in the design worksheet.

1 2

- 'Z25 g RE i

Pier Design. Post/Pile Design. Post/Pile Design Pier Design

ia 20 20 18

Figure F.1

103
DESIGN WIIKSHEET

PART I

LINE

No.

(UT ITEMI DESCRI PFION

1. MANUFACIURED 1DJAE LENGTH (L)

2. IVPNUFACTURED LUMIE WIDTH (W)

3. CHASSIS I-BEAM SPACING (a)

4. SUPI-ORT SPACING (s)

5. PERIMETER BLOCKING

6. OVER-THE-TOP TIES

7. MAX. DEPTH OF 100 YR. FLOOD

8. MAX. FLODD VELOCITY

9. WIND LOAD

10. RDOF LIVE LOAD

11. DEPTH '10 RISTL INE

12. TYPE OF SOIL

13. BEARING CAPACITY OF SOIL

14. SCOUR DEPTH

15. MAX.l HT. TO BRYPMI OF

CHASSIS I-BEAM

16. VERT. MEMBER LOAD

17. ELEVATION ON FILL

18. ELEVATION ON PIERS

19. PIER DESIGN

20. ELEVATION ON POSTS/PILES

Z1. MIN. VERT. MEMBER LENGTfH

22. VERT. MEMBER CAPACITY

23. VERT. MEMBER SIZE

24.

25.

VERT. MEBER AREA

VERT. MEMBER UNIT LOAD

26. No. & SIZE IURIZ. BENAS

27. No. & SIZE CROSS BRACING

28. SIZE END BRACING

29. No. & SIZE HDRIZ. BEAM BOLTS

30. CHASSIS I-BEAM LAG SCRESS

31. CROSS BRACING LOAD

32. BOLT CAPACITY

33. No. BOLTS & SIZE PER

CDNNECTION

34. END BRACING LOAD

35. BOLT CAPACITY

36. No. BOLTS & SIZE PER

CONNECI`ION

DATA ENTRY

ft.

ft.

in.

ft.

Y N

Y N

ft.

fps

psf

psf

ft.

psf

ft.

ft.

lb.

Y N

Y N

Y N

ft.

lb.

in.

ft,2

psf

C 11 x "II

2 1 xx

I/t xi"

lb.

lb./bolt

NDTES

MEASURED OVER EXTERIOR OF

MELASURE) OVER EXTERIOR OF

LtD IOF CHASSIS I-BEAAS

sl 0 FT. OR MANUFACRIRE' S

IF YES REDUCE Ln 22 BY 5%

END WALLS

SIDE WALLS

DATA,

AS DISCUSSED IN APPENDIX C

AS DISCUSSED IN APPENDIX C

FIGURE 4.2 (PAGE 33)

FIGURE 4.1 (PAGE 33)

AS DISCUSSED ON PAGE 48

AS DISCUSSED ON PAGE 48

TABLE 4.5 (PAGE 40) (ASSUE 12"0,MEVIBER)

MUST BE ŽLn 7 ENTRY

TABLE 4.10 (PAGE 47)

IF Ln 8 IS siO fps

IF Ln 8 = 0 fps

ENTER A-F FRCM TABLE 4.11 (PAGE 50)

2 X (Ln 14 + Ln 15) NrTE THAT DIST.

BEIJ3V GRADE '6 ft.)

TABLE 4.13 - 4.16. MUST BE ŽLn 16

TABLE 4.13 - 4.16. IF4 121' GO TO

Ln 14 AND RECALCULATE ENTRY AND

Lns 21, Ln 22 and 23

wrr IF )AND d2 IF t

Ln 16 .Ln 24. IF Ln 25- Ln 13, -A]

BEARING PAD, USE PILES OR REDUCE

Ln 4 AND RECALCULATE Lns 16, 22, 23,

24 AND 25 UNTIL Ln 25s Ln 13

FRCV TABLE 4.17 (PAGE 60)

FMfI TABLE 4.18 OR 4.19 (PAGES 61 & 62)

FR$U TABLE 4.20 (PAGE 62)

EfKIM TABLE 4.21 OR 4.22 (PAGES 63 & 64)

FRC1 TABLE 4.25 (PAGE 65)

[TABLE 4.27 x Ln 11 [No. Ln 27 x 21

FRCM TABLE 4.26 (PAGE 66)

a I" Ln 31 .Ln 32 (MUST BE '2)

lb. [TABLE 4.27 x Ln 2J' 2

lb./bolt FRCLI TABLE 4.26 (PAGE 66)

Ca 11 Ln 34.-Ln 35 (MUST BE >2)

PART II

IAXTERIALS DESCRIPTION UNIT COST

CLEARING..~~~~~~~~~~~~~~~~~~~~2 2 C L ................................. E A R I NGi ft.g $ /ft?

EXCAVATION . ........................ uyd. @$ cuu.yd.

EARTH FILL . ._ e __cu.yd. @ $ /cu.yd.

SITE ERADING . ................ cu.yd. @ $ ~/cu.yd.

ANCHQRS ................ .@.... ea.

PIERS- ..... (tLa l-Ln 4]+1)x2= @$ ea.

FDSTS- ..... ([Ln I*Ln 41+1)x2= ________ @ $ ea.

'ITAL COST

=-

=-$

= $7~

= f$

= $~

= $~

104

LINE

No.

(La)

37.

38.

39.

40.

41.

42.

43.
PILES- ................ ([Ln l-Ln 41+1)x2=

HDRIZ. BEFAMS-..([Ln l Ln 4]+1)x No. Ln 26= _

CROSS BRACING- .............. No. Ln 27 x 2=

END BRACING ................ No. Ln 28 x 4=

HDRIZ. BEAM BDLTS-.(No.Ln 29xNo.Ln43or44)=

I-BEAM LAG SCRIWS-...(4x No. Ln 43 or 44)=

CROSS BRACING BOLTS-(No.Ln 33xNo.Ln 27x4)=

END BRACING BOLTS- ........... (No. Ln 36x8)=

JACKING

UTILITIES

STEPS

SITE IMPROVEMENTS

Misc.

MISC.

MISC.

MISC.

TOTAL ESTIMATED MATERIAL COST (ZLn 37-59)

LABCR MAN IUURS

iTmAL ESTIATTEID COST (ELn 60 & 61)

$ _ ea. = _$

e $ 7 -ea. = $ ~ @ $ _ _ _ ea. = $7 _ _

@ $=ea. = _$ -

$ _ _ _ ea. = $_ _

7 $ e6a. =C$- @ 5 _ _ ea. =

@ $ e = $ $a. _

$F~

$7~

@ $ /hr. $~

$ _ _ _ _ _ _

PART III - DESIGN PLAN

. 1. { 37. Clearing - ft2

38. Excavation _ _cu- yd

39. Earth fill - u yd.

40. Site Grading Ccu- gd.

52 Jacking

53. Utilities -_ _ -

54. Steps -. _

55. Site improyements43 Post footing

_ _ Nx x_ N_

Figure F.2

105

44.

45.

46.

47.

48.

49.

50.

51.

52.

53.

54.

55.

56.

57.

58.

59.

60.

61.

62.
DESIGN WORKSHEET - INSTRUCTIONS

PART I

LINE

No.

(L-n)

1. MANUFACTURED HOME LENGTH - Enter outside length (ft.) measured over

exterior of end walls. This value is used in determining input to subsequent line

numbers.

2. MANUFACTURED HOME WIDTH - Enter outside width (ft.) measured over exterior

of side walls. This value is used in determining input to subsequent line numbers.

3. CHASSIS I-BEAM SPACING - Enter the distance (in.) from center to center of the

chassis I-beams. If not available at the time of foundation design, use 75 1/2 inches.

4. SUPPORT SPACING - Enter the maximum distance between supports along length

of manufactured home. Note that vertical member loading may necessitate a

reduction in support spacing.

5. PERIMETER BLOCKING - If the manufacturer requires perimeter blocking, it must

be provided and Line 22 must be reduced by 5 % .

6. OVER-THE-TOP-TIES - If the manufactured home is equipped with over-the-top

ties, they must be secured to the elevated foundation or ground anchors.

7. MAXIMUM DEPTH OF 100 YEAR FLOOD - Enter the depth (ft.) from the lowest

natural grade level to the 100 year flood.

8. MAXIMUM FLOOD VELOCITY - Enter the maximum flood velocity (fps). This value

is used in determining input to subsequent line numbers.

9. WIND LOAD - Based on geographical location and Figure 4.2, determine the

appropriate wind load zone. The corresponding wind load (15 or 25 psf) is entered

and will be used in determining input to subsequent line numbers.

10. ROOF LIVE LOAD - Based on geographical location and Figure 4.1, determine the

appropriate roof live load zone. The corresponding roof live load (20, 30, or 40 psf)

is entered and will be used in determining input to subsequent line numbers.

11. DEPTH TO FROSTLINE - Enter the depth (ft.) from natural grade to the frostline.

This value is used in determining pier height, which is measured from the frostline.

12. TYPE OF SOIL - Enter the type of soil on the site. Where assistance is needed,

contact the local building department or a soils engineer.

13. BEARING CAPACITY OF SOIL - Based on the type of soil in Line 12 and discussion

in Chapter IV, enter the bearing capacity (psf) of the soil.

14. SCOUR DEPTH - Enter the anticipated scour depth (ft.) from Table 4.5 based on the

Line 8 entry. Assume a 12 inch diameter member and if Line 23 results in a smaller

diameter member being needed, Line 13 and subsequent entries may be recalculated.

15. MAXIMUM HEIGHT TO BOTTOM OF CHASSIS I-BEAM - Enter the desired height

above natural grade (ft.) which must be greater than or equal to Line 7. This value

is used in determining input to subsequent line numbers.

106
16. VERTICAL MEMBER LOAD - Enter the vertical member load (lb.) from Table 4.10

based on Line 2, 3, 4, 9, and 10 entries. Note the increase in load as the Line 4 entry

increases. Therefore, consider reducing Line 4, until Line 15 is in the range of 3,500

to 5,000 pounds.

17. ELEVATION ON FILL - If the Line 8 entry is less than or equal to 10 fps, fill as

discussed in Chapter IV can be considered using a conventional set-up with piers and

ground anchors provided that Line 15 is greater than or equal to Line 7. Part II can

be completed for the fill/conventional installation design.

18. ELEVATION ON PIERS - If the Line 7 entry is less than or equal to 10 feet and the

Line 8 entry equals 0 fps, piers as discussed in Chapter IV can be considered.

19. PIER DESIGN - If Line 18 is yes, then use Table 4.11 to determine the appropriate

pier design (A-F) from Figures 4.26 - 4.31. Part II can be completed for the pier

design.

20. ELEVATION ON POSTS/PILES - If the elevation techniques in Lines 17 and 18 are

not feasible, posts/piles as discussed in Chapter IV will be required.

21. MINIMUM VERTICAL MEMBER LENGTH - Calculate as follows: 2 x (Ln 14 + Ln

15). This value is the minimum vertical member length which is used in determining

the required diameter of the vertical member in Line 23. Note that the depth below

grade must be equal to the height above grade but not less than 6 feet.

22. VERTICAL MEMBER CAPACITY - Based on the information in Lines 2, 9, and 21,

use Tables 4.13 - 4.16 to find a vertical member capacity greater than or equal to the

value in Line 16. Note by decreasing Line 4, the value in Line 16 decreases as would

the required value for vertical member capacity.

23. VERTICAL MEMBER SIZE - Enter the vertical member size from Tables 4.13 - 4.16

corresponding to the entry in Line 22. Note that if Line 23 is not equal to 12 inches,

Lines 14, 21, 22, and 23 may be recalculated.

24. VERTICAL MEMBER AREA - Calculate the end area of the Line 23 entry. For

round members area equals 3.14 x radius squared and for square members area

equals diameter squared.

25. VERTICAL MEMBER UNIT LOAD - Calculate as follows: Line 16 divided by Line

24. This is the psf load through the bearing area of the vertical member. If less

than or equal to the Line 13 entry, the soil capacity will support the foundation. If

greater than the Line 13 entry, additional bearing area must be added under the

vertical member, piles must be used, or Line 4 reduced and Lines 16, 22, 23, 24, and

25 recalculated.

26. NUMBER AND SIZE OF HORIZONTAL BEAMS - Based on the entries in Lines 2, 3,

4, 9, and 10, determine the number needed and the size of the horizontal support

beams from Table 4.17.

27. NUMBER AND SIZE OF CROSS BRACING -Based on the entries in Lines 1, 9, and

15, determine the number needed and the size of the cross bracing from Table 4.18

or 4.19.

28. SIZE OF END BRACING - Based on the entries in Lines 4, 9, and 15, determine

the size of the end bracing from Table 4.20.

29. NUMBER AND SIZE OF HORIZONTAL BEAM BOLTS - Based on the entries in Lines

2, 4, 9, 10, and 23, determine the number and size from Table 4.21 or 4.22. Note the

bolt dimension criteria in Tables 4.23 and 4.24 must be followed in installation.

107
30. CHASSIS I-BEAM LAG SCREWS - Based on the entries in Lines 4 and 9 determine

the size of the lag screws from Table 4.25.

31. CROSS BRACING LOAD - Based on the entries in Lines 1, 9, and 15 and Table 4.27

calculate as follows: Load (lb.) = (Table 4.27 value x Ln 1) divided by (No. Ln 27 x

2). This value is used in Line 33.

32. BOLT CAPACITY - Based on the entries in Lines 15 and 23 choose a bolt diameter

and corresponding capacity from Table 4.26.

33. NUMBER OF BOLTS PER CROSS BRACING CONNECTION - Calculate as follows:

Line 31 divided by Line 32 and round up to the next whole number. Note that Line

33 can be recalculated by assuming a different bolt diameter and capacity in Line 32.

34. END BRACING LOAD - Based on the entries in Lines 2, 9, and 15 and Table 4.27,

calculate as follows: Load (lb.) = (Table 4.27 value x Ln 2) divided by 2.

35. BOLT CAPACITY- Based on the entries in Lines 15 and 23 choose a bolt diameter

and corresponding capacity from Table 4.26.

36. NUMBER OF BOLTS PER END BRACING CONNECTION - Calculate as follows:

Line 34 divided by Line 35 and round up to the next whole number.

PART II

37. CLEARING - Determine if the manufactured home site must be cleared and the

area (ftA) entered in Line 37. Obtain cost estimate for clearing ($/ft. 2 ) and

multiply times the area to be cleared to obtain total cost.

38. EXCAVATION - Determine if any areas on the manufactured home site must be

excavated and the volume (cu.yd.) of earth to be removed entered in Line 38.

Obtain a cost estimate for excavation ($/ft.2) and multiply times the volume to be

excavated to obtain total cost.

39. EARTH FILL - Based on the desired volume of fill (cu.yd.) and cost estimate for fill

($/cu.yd.), determine the total cost for fill. Table 5.3 in Chapter V can be consulted

as necessary.

40. SITE GRADING - If grading is necessary to prepare the manufactured home site,

determine the volume (cu.yd.) and cost ($/cu.yd.) and multiply to obtain total cost.

41. ANCHORS - Based on the desired elevation and type of foundation design,

determine the number of ground anchors required. Obtain the installed cost for

ground anchors ($/anchor) and multiply times the number required to obtain total

cost.

42. PIERS - The number of piers required is calculated as follows: [(Ln 1 .+Ln 4) +1] x 2.

Based on the pier design chosen in Line 19, the required length of pier and need for

additional bearing area, obtain a cost estimate for each pier ($/pier). Multiply the

number of piers required by the cost per pier to obtain total cost. Note that wood

cross bracing and bolt costs should be figured in Lines 46 and 50.

43. POSTS - The number of posts required is determined as discussed for piers in Line

42. Based on the post diameter chosen in Line 23, the required length in Line 21 and

need for additional bearing area obtain a cost estimate for each post ($/post).

Multiply the number of posts required by the cost per post to obtain total cost.

108
44. PILES - The total cost for piles is determined in the same manner as discussed for

posts. Note, if a pile pull-out test is required, the cost should be entered as a

miscellaneous cost in Line 56.

45. HORIZONTAL BEAMS - The total number of horizontal beams is determined as

follows: No. Ln 26 x [(Ln I +Ln 4) + 11 Based on the size required from Line 26, a

unit cost ($/beam) should be determined and multiplied by the total number of

beams required to obtain total cost.

46. CROSS BRACING - The total number of cross bracing members is calculated as

follows: No. Ln 27 x 2. Based on the size required from Line 27, a unit cost

($/member) should be determined and multiplied by the total number of members

required to obtain total cost.

47. END BRACING - Based on the size required from Line 28, a unit cost ($/member)

should be determined and multiplied by 4 to obtain total cost.

48. HORIZONTAL BEAM BOLTS -The total number of horizontal beam bolts is

calculated as follows: (No. Ln 29 x No. Ln 43 or 44). Based on the size required

from Line 29, a unit cost ($/bolt) should be determined and multiplied by the total

number of posts/piles to obtain total cost.

49. I-BEAM LAG SCREWS - The total number of chassis I-beam lag screws is calculated

as follows: (4 x No. Ln 43 or 44). Based on the size required from Line 30, a unit

cost ($/lag screw) should be determined and multiplied by the total number of

posts/piles to obtain total cost.

50. CROSS BRACING BOLTS - The total number of cross bracing bolts is calculated as

follows: (No. Ln 33 x No. Ln 27 x 2). Based on the size required from Line 33, a

unit cost ($/bolt) should be determined and multiplied by the total number of cross

bracing bolts to obtain total cost.

51. END BRACING BOLTS - The total number of end bracing bolts is calculated as

follows: (No. Ln 36 x 4). Based on the size required from Line 36, a unit cost

($/bolt) should be determined and multiplied by the total number of end bracing

bolts to obtain total cost.

52. JACKING - Based on the manufactured home size, desired elevation above grade

and foundation design, determine the total cost for jacking and placement of the

manufactured home on the elevated foundation. Table 5.8 in Chapter V can be

consulted as necessary.

53. UTILITIES - Determine any utility work which must be performed in conjunction

with the elevated foundation design and obtain total cost.

54. STEPS - Based on the height above grade and number of exterior doors, determine

the cost for exterior steps and landings.

55. SITE IMPROVEMENT - List any additional site improvements and their total cost.

56.- MISCELLANEOUS - List any additional items associated with the manufactured

59. home installation not addressed in Lines 37 - 55.

60. TOTAL ESTIMATED MATERIAL COST - Add the total cost figures for Lines 37 - 59.

61. LABOR - Some of the items in Lines 37 - 59 may not include labor costs. For those

line items, determine the man-hours required and multiply by the labor rate ($/hour)

to obtain total labor cost.

62. TOTAL ESTIMATED COST - Add Lines 60 and 61.

109
PART III

Based on the information in Parts I and 1I, Figure F.2 can be completed to yield a drawing

of the actual design. Where appropriate, spaces have been provided to enter dimensions

and material descriptions.

71 5-252

*U.S. Government Printing Office: 2000- 520-486194794 110