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NBSIR 81-2238

NBS Studies of Mobile Home

Foundations

Felix Y. Yokel

Riley M. Chung

Charles W. C. Yancey

Geotechnical Engineering Group

Structures and Materials Division

Center for Building Technology

National Engineering Laboratory

U.S. Department of Commerce

National Bureau of Standards

Washington, DC 20234

March 1981

Prepared for: Division of Energy, Building Technology and Standards, Office of

Policy Development and Research

Department of Housing and Urban Development

Washington, DC 20410

< HX—

-i on leral Insurance Administration

1UU shington, DC 20410

. U56

81-2238

1981

c. 2

NATIONAL BUREAU

OF STANDARDS

LIBRARY

NBSIR 81-2238

• • * NBS STUDIES OF MOBILE HOME

FOUNDATIONS

APR 1 5 1981

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CzFelix Y. Yokel

Riley M. Chung

Charles W. C. Yancey

Geotechnical Engineering Group

Structures and Materials Division

Center for Building Technology

National Engineering Laboratory

U S. Department of Commerce

National Bureau of Standards

Washington, DC 20234

March 1981

Prepared for: Division of Energy, Building Technology and Standards, Office of

Policy Development and Research

Department of Housing and Urban Development

Washington, DC 20410

and

Federal Insurance Administration

Washington, DC 20410

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
.

.
NBS Studies of Mobile Home Foundations

by

Felix Y. Yokel, Riley M. Chung

and Charles W. C. Yancey

Abstract

Two papers are presented which discuss the results of tests on soil

anchors used to secure mobile homes and of an analytical study of

wind and flood loads on soil anchors.

Key Words: Flood loads; mobile home foundations; mobile home

standards; soil anchors; soil testing; flood loads

iii
Table of Contents

Page

ABSTRACT ill

PREFACE 1

PERFORMANCE OF SOIL ANCHORS FOR MOBILE HOMES 3

WIND AND FLOOD LOADS ON SOIL ANCHORS 21

iv
Preface

This report contains two technical papers which were presented at the Third

Mobile Home/Manufactured Housing Engineering Conference in Austin, Texas,

January 29-30, 1980. The first paper conveys some of the test results of

an NBS field study of the performance of soil anchors used to secure mobile

homes against environmental forces. Preliminary conclusions are presented

based on the results from tests in silty and sandy soil sites. The second

paper conveys some of the results obtained from an analytical study of wind

and flood loads on soil anchors. The results of the load calculations are

in the form of graphs which present tie forces and pier reactions for several

loading cases applied to a 14-ft wide mobile home. The complete set of

results from these two studies will be reported in future (early 1981)

National Bureau of Standards publications.

1

PERFORMANCE OF SOIL ANCHORS FOR MOBILE HOMES

by

FELIX Y. YOKEL and RILEY M. CHUNG

Loading requirements for soil anchors used to tie down mobile homes against

wind and flood loads are compared with the results of anchor tests in sandy

and silty soils. The conclusion is drawn that present anchoring techniques

as used in the field do not provide the necessary support, and that anchors

could perform adequately if installation techniques were modified to include

pre-loading to 1.2o times the design load.

Key Words: Flood loads; mobile home foundations; mobile home standards;

soil anchors; soil testing; wind loads.

3
PERFORMANCE OF SOIL ANCHORS FOR MOBILE HOMES .

by

FELIX Y. YOKEL

RILEY M. CHUNG

NATIONAL BUREAU OF STANDARDS

1. INTRODUCTION

The information conveyed herein was developed in

a study which is sponsored by the Department of Housing

and Urban Development. In the first part of this study

field measurements were made in order to determine the

wind forces acting on mobile homes and were published

in Reference [1], Subsequently, an analysis was made

of the forces on mobile home foundations resulting from

wind and flood loads. The results of that analysis,

which were not yet published, are discussed in a separate presentation in this conference. This presentation conveys the results of a study of the performance

of soil anchors which is still in progress. In the

first stage information was compiled on existing knowledge and test data and was published in Building Science

Series Report 107 [2]. In the second stage anchors are tested in three types of soil: silts, sands, and clays.

It is hoped that this work will provide the background

information needed to assure adequate mobile home foundation performance.

2. LOADS ACTING ON SOIL ANCHORS

Figure 1 shows two typical mobile home tiedown

methods. "Near-tie" and "far-tie" connections are defined in the figure. Both types of connections

are used in practice. The dimensions shown, which

are typical for a 14 ft. wide mobile home, could be

different in any particular case. For instance, the

clearance above ground could be greater, the distance

between the chassis beams could be different, or the

diagonal ties could be attached to the chassis beam

at a different point. Table 1 shows strap forces

caused by various environmental loads which were calculated for a 14 ft. wide mobile home with the strap

attachments shown in Figure 1.

4
TABLE 1: Strap Forces Induced by Environmental Loads

Strap Forces, lb/ft

Loading Near

Vertical

-Tie

Diagonal

Far-Tie

Vertical Diagonal

HUD Hurricane 0 220 80 205

NBS Hurricane 120 280 195 220

Buoyancy, 1 ft Head 250 80 300 50

Buoyancy, 1/2 ft Head + 5 ft/sec velocity

flow*

95 50 100 40

Buoyancy, 1/2 ft Head

+ HUD Standard Wind

160 130 200 110

* It is assumed that the water level is 1 ft above the underside

of the mobile home.

The "HUD Hurricane" is the hurricane load stipulated in the

Federal Standard [3]. The "NBS Hurricane" is the loading recommended in Reference [1]. The buoyancy is calculated for the stipulated differential head, which is the difference between the water

levels inside and outside the mobile home. The "HUD Standard Wind"

is calculated in accordance with the Federal Standard [3].

Loads are given for a 1 ft length of mobile home and have

to be multiplied by the anchor spacing if strap loads are calculated.

Note that all wind loads induce a substantial force in the

diagonal straps. The HUD Hurricane induces no vertical strap

force when there is a near-tie connection and a very small vertical strap force for far-tie connections. Vertical strap forces

are greater for the NBS Hurricane load; however, they are still

substantially less than the diagonal forces. Flood loads on the

other hand are primarily resisted by vertical-strap forces.

Even though it was conservatively assumed that the piers do

not resist any lateral loads, it can be concluded from an examination of Table 1 that, to be effective against wind, anchors must

be capable of resisting loads with a substantial component in the

horizontal direction.

5
3. PERFORMANCE OF SOIL ANCHORS

Since the soil anchor tests are still in progress

a conclusive discussion of the results cannot be pre-r

sented at this time. This section therefore concentrates

on qualitative aspects of anchor perf ormance

.

Tests were conducted on two sites:

Site 1 is located on the grounds of the National

Bureau of Standards in Gaithersburg, Maryland. It is a site of residual silty soils. Test borings indicate

generally about 2 feet of man-made fill consisting of

local material, overlying residual clayey and sandy

silts derived from the in-place weathering of the underlying quartz rich schist rock of the Wissahickon formation. No groundwater was observed in any of the borings

during drilling and up to 5 hours after drilling. Long

term water observations were not made. The standard

penetration resistance, or N values (ASTM D-1586), range

from 9 to 18 and 7 to 16 blows per foot for the fill and

the residual soils, respectively. Laboratory test results

indicated that the natural soils had moisture contents

ranging from 12.5 to 30.8 percent and consisted of 62 to

83 percent by weight of material passing No. 200 sieve. The

liquid limit and plastic limit were at 39 and 27 percent,

respectively. The natural soils may be described as clayey silt with some fine sand, or ML according to ASTM

D-2487. Strength tests indicate an unconfined compressive strength of about 4000 psf on one specimen, while a direct shear test yielded an angle of shearing resistance

(^) of 30° and a cohesive strength (c) of 200 psf. The

direct shear test results may have been influenced by the

presence of quartz fragments in the tested soil specimen.

Site 2 is in Odenton, Maryland and is sandy. Test

borings indicate that part of the site is overlain by a thin

but very dense crust of silty sand with gravel fill, which

has a depth of 0.5 to 1 foot and a recorded range of standard penetration resistance from 20 to 40 blows per foot.

Underlying the crust is a 6 to 9 foot thick fine to medium

sand layer which rests on sandy silty clay soils. The

standard penetration resistance of this sandy soil was recorded from 10 to 21 blows per foot. There was no water

in the test borings to the depth where cave (collapse of

the borehole) occured which ranged from 5 to 6.5 feet. More

than half of the tests on Site 2 were conducted in a location

which was not covered by the dense crust. The natural soil

deposits at the Odenton site belong to the Potomac group of

Cretaceous Age which is generally characterized by interbedded

sand and silty clay materials. The sandy soils in this group

are generally in the medium to dense state due to the over- consolidated nature of the Cretaceous-Age deposits. Soil

laboratory test results are presently not available.

6
Figure 2 shows a vertical pullout test on a 6-inch

single helix anchor in sand (Site 2) installed vertically

to its full 4 foot depth. Vertical displacements of the

anchor head in inches are plotted against loads in kip.

Two cycles of unloading and re-loading were conducted at

1 kip intervals in order to assess the characteristics of

pre-loaded anchors. Note that the initial load-displacement

curve is rather steep and there is a break (change in slope)

at point A at about 1 kip load. This trend is characteristic

for most tests and is frequently more pronounced than in this

figure. The re-loading curves are generally much steeper

than the initial ’’virgin" loading curve, indicating substantial strain-hardening effects. The characteristics

of the curve are interpreted as follows: Whenever load

is applied, the soil is consolidated, and up to this load,

its load-displacement characteristics are modified. As soon as the applied load exceeds the pre-load, the loaddisplacement curve follows the virgin curve which would

be obtained in monotonic loading. The initial break in

the curve at point "A" can be attributed to pre-consolidation of the soil which is about equivalent to a 1 kip

anchor pull.

Figure 3 shows the load-displacement curve for an approximately co-axial pull on an anchor which was installed at an angle of 45° to the horizontal. Note that this curve is similar to the one shown in Figure 2, except that

the load capacity is much lower because of the reduced

anchor depth due to the 45° installation angle. The break

in the virgin curve occurs at about 0.5 kip.

Figure 4 shows the load-displacement curve for a vertically installed anchor pulled at an angle of 40° to

the horizontal. The initial stiffness of this anchor is

very low (only 1.2 kip capacity at a 4 inch displacement),

since the 3/8 inch thick shaft provides very little lateral soil resistance. However, as the shaft is bent in the

direction of the pull the soil resistance increases and

the ultimate pullout resistance exceeds that for a vertical pull. The initial slope of the re-loading curves is

attributable to the elastic displacement of the anchor

shaft which occurs before the soil resistance is engaged.

Otherwise the re-loading curves show characteristics

similar to those in the previously discussed tests.

The re-constructed virgin curves for the tests shown

in Figure 2, 3 and 4 are plotted in Figure 5. The figure

illustrates the difference in the performance characteristics. Note the large displacement required to develop

load resistance in a vertically installed anchor subjected to diagonal load, the most commonly encountered situation associated with present mobile home anchoring

technolgoy

.
Figure 6 shows the results of cyclic tests on the

sandy site (Site 2). The upper curve is for cyclic tests

at 0.75 of ultimate pullout capacity on an anchor which

was not pre-loaded. Note that up to about 60 cycles there

is a gradual decrease in the displacement per cycle. Above

60 cycles the displacement is constant at about 1-inch per

100 cycles. This performance is consistent with the observation of volume changes in loose to medium sands subjected

to cyclic shear stresses. Note that the performance can be

substantially improved by a single monotonic pre-loading

cycle (see the lower curve).

Up to this point the discussion of performance characteristics of anchors was confined to the sandy site

(Site 2), where test results were more consistent and

repeatable. Figures 7 and 8 illustrate qualitatively

the difference between anchor performance characteristics

in the sandy and the silty sites.

In Figure 7 virgin loading curves in sand and silt

are compared. Note that the vertical pullout tests have

similar initial performance characteristics and identical

ultimate loads (a test with a similar ultimate load from

the silt site was selected to facilitate comparison).

However, the anchor on the silt site maintained its load

up to an 11-inch displacement (not the entire range is

shown) while the anchor in sand failed at a 2-1/4 inch

displacement. This can be explained in part by the fact

that the shear strength of the silt deposit did not

decrease as rapidly with decreasing depth as that of

the sand deposit. Probably for the same reason the difference in strength between the vertical and the inclined

anchor was greater in the sandy site. Figure 7 does not

show the entire range of the tests on the silty site.

In all the three tests shown, the anchors in the silty

site maintained their strength over a large range of displacements.

Figure 8 shows a comparison between cyclic tests

in the sandy and the silty site. Note that in the silty

site there was no significant displacement after 80 load

cycles (tests were continued for 300 load cycles).

Figure 9 shows a comparison between soil test

probel/ readings and anchor capacity. Since in the sandy

site test probe readings increased rapidly with depth

(see Figure 2), the reading at 3 ft depth was selected

for the sand as well as the silt data. Note that while

there is a definite correlation, there is also considerable scatter of data.

1/ The soil test probe is a commercial in-situ measurement

device developed by the anchor industry.

8
4. CONCLUSIONS

Any conclusion drawn from the previously discussed

data is preliminary, since testing is still in progress.

However, the data at hand are reasonably consistent and

their trends can be clearly recognized. Two important

conclusions can be drawn:

(1) Present anchoring techniques as used in the field

do not provide the necessary support for the diagonal ties

This conclusion does not necessarily imply that the

ultimate pullout load is too low, but rather that the

horizontal displacement necessary to develop the needed

load capacity is excessive. The situation is further

aggravated by the fact that, particularly in the silty

site, the virgin loading curves of various tests are

not very consistent. Thus some anchors will develop

their load resistance sooner than others. Since a mobile

home is rather stiff, this situation will cause certain

straps to be overloaded and fail, before other straps

are loaded to capacity.

(2) Anchors could perform adequately if installation

techniques would be modified to include pre-loading to

about 1.25 times the design load in the direction of

the anticipated reaction force .

Pre-loading would not only decrease displacements

by an order of magnitude, it would also insure that

adequate load capacity is available.

In addition to the above discussed performance

characteristics there is one more observation that

should be mentioned. Anchors are supplied with a coat

of paint which is supposed to provide protection against

corrosion. In the NBS tests , anchors were inserted and

extracted. In almost all instances this operation stripped the paint. Unless better corrosion protection

techniques are developed, anchors should not be expected

to maintain their load capacity for extended periods of

time

.

9
5. REFERENCES

[1] Marshall, R. D., (1977), "The Measurement of Wind Loads

on a Full-Scale Mobile Home," NBSIR 77-1289, Center for Building Technology, National Bureau of Standards, Washington, D.C.,

Sept. 1977, 120 pp.

[2] Kovacs, W. D. and Yokel, F. Y., (1979), "Soil and Rock

Anchors for Mobile Homes - A State-of-the-Art Report," NBS

Building Science Series 107, Center for Building Technology,

National Bureau of Standards, Washington, D.C., Oct. 1979,

147 pp.

[3] Mobile Home Construction and Safety Standards. Department of Housing and Urban Development, Federal Reporter - Part II, Dec. 1975.

10
6. S. I. CONVERSION

The following conversion factors should be used to

convert dimensions

S. I. Units:

and forces given in this report to

To Convert To Multiply by

in mm 25.40

ft m 0.3048

lb N 4.448

kip KN 4.448

psf Pa 47.88

11
FAR TIE CONNECTION

Figure 1. Typical tie down arrangements for

a 14-foot wide mobile home.

12
j

VERTICAL DISPLACEMENT, in

Figure 2. Vertical pullout test on vertically installed

6- inch single helix anchor in sandy soil.

13
4

Figure 3. Pullout test load applied at an angle of

40° to the horizontal to a 6-inch single

helix anchor installed at 45° to the

horizontal in sandy soil.

14
6

DISPLACEMENT, in

Figure 4. Pullout test load applied at 40° to the

horizontal to a vertically installed

6-inch single helix anchor in sandy soil.

15
DISPLACEMENT, in

Figure 5. Comparison of the performance of 6-inch single

helix anchors in sandy soil under various

loading conditions.
CUMULATIVE

DISPLACEMENT,

in

NO. OF CYCLES

Figure 6. Results of cyclic tests on vertically

installed 6—inch single helix anchors

in sands subjected to vertical load.

17
DISPLACEMENT, in

Figure 7. Comparison of monotonic (virgin) loading curves

for 6-inch single helix anchors in sand and silt.

18
CUMULATIVE

DISPLACEMENT,

in

0 40 80 120 160

NO. OF CYCLES

Figure 8. Comparison of cyclic tests on 6-inch single

helix anchors installed vertically in sand

and silt and subjected to vertical load cycles

of 75 percent of ultimate pullout load.

19
*I o

<

o

o

O

3 2

A

AVERAGE FOR MAX. LOAD LOWER BOUND FOR

MAX. LOAD

Slower bound for load

AT 2" DISPLACEMENT

RAMARK

AVERAGE FOR LOAD

AT 2" DISPLACEMENT

. I i

AT 2" DISPLACEMENT

MAXIMUM

AT 2

1 DISPLACEMENT

MAXIMUM

AT 2" DISPLACEMENT

MAXIMUM

1

!

SITE SUBMERGED

ii ii

100 200 300 400

TEST PROBE READING In-lb

Figure 9. Correlation between test probe readings and

6-inch single helix anchor capacity.

20
WIND AND FLOOD LOADS ON SOIL ANCHORS

by

CHARLES W. C. YANCEY

Analyses were made of the forces transmitted to typical foundation

systems as the result of wind and/or flood loads acting on 12-ft and 14-

ft single-wide mobile homes. The objectives of the analytical study

were to derive unit force information from which the required number of

vertical and diagonal ties and pier supports can be determined and to

compare the tie-down requirements dictated by hurricane wind pressures

with those necessitated by flood loads. The results of the load calculations have been summarized as a series of computer-generated graphs.

The graphs present the vertical and diagonal tie forces, pier reactions

and total resultant anchor forces caused by hurricane wind pressures and

flood forces. This paper discusses the results from 8 of the total 32

conditions included in the study. It was found that hurricane loads

will induce relatively high tensile forces in the diagonal ties, thereby

suggesting that anchor systems must be designed to resist a substantial

horizontal load component. It was also found that relatively small

differential water levels may cause potentially critical forces in the

vertical ties.

21
WIND AND FLOOD LOADS ON SOIL ANCHORS

by

CHARLES W. C. YANCEY

NATIONAL BUREAU OF STANDARDS

1. INTRODUCTION

The information conveyed in this presentation was developed in one stage of a project which is being sponsored by the U.S. Department of

Housing and Urban Development. The principal objectives of the project

are: to determine the distribution and magnitude of wind forces on mobile homes, to determine the performance characteristics of soil

anchors, to quantify the forces transmitted to mobile home foundation

systems and to derive a standardized performance test method for soil

anchors. In the initial stage of the study, existing information and

test data were compiled and were published in NBS Building Science

Series 107 [1]. A concurrent activity involved making field measurements

of actual wind pressures to determine the wind forces acting on a typical

mobile home, the results of which were published in Reference [2].

Then, an analysis was made of the forces transmitted to typical foundation systems as the result of wind and/or flood loads acting on 12-ft

and 14-ft single-wide mobile homes. This presentation conveys some of

the results obtained from the analytical study of foundation loads. The

complete set of results will be reported in a future National Bureau of

Standards publication. In another stage of the study, commerciallyavailable soil anchors are being tested for their pull-out characteristics

in three types of soil: silts, sands and clays. The results obtained

from these anchor tests are discussed in a separate presentation at this

conference.

The physical characteristics of typical foundation systems were

used to generate input data for the loads analysis. Figure 1 shows

two common foundation systems, consisting of pairs of piers located

beneath the two chassis beams and soil anchors to which vertical and

diagonal metal "ties" are attached. In order to insure the adequacy

of these mobile home foundations, it is necessary to determine the

forces that would be transmitted to their components (i.e. ties, piers

and anchors) as a result of extreme winds and floods.

2. WIND AND FLOOD LOADS

Wind Loads

Three different cases of wind load application were used: wind

pressures specified in the HUD Mobile Home Construction and Safety

Standards [3] for (1) the "Standard Wind Zone" and (2) the "Hurricane

Zone" and (3) hurricane wind pressures recommended by the National

Bureau of Standards [2], Table 1 shows the pressure magnitudes applied

to the walls and roof for the three cases. The results from the two

hurricane pressure cases will be discussed in this presentation.

Figures 2 and 3 show the resultant drag and uplift forces derived from

the HUD and NBS Hurricanes (assuming 90 mph wind speeds) respectively.

22
It should be noted that the drag and uplift forces for the NBS Hurricane

were derived from idealized uniform pressures. Furthermore, only the

portion of the pressure acting on the sidewall was considered in the

calculations based on the assumption that the horizontal force on the

skirt would not be transmitted to the mobile home. As shown in figure

3, the uplift force for the NBS Hurricane is acting at a distance of

0.4B from the windward wall, where B is the width of the mobile home and

the drag force is applied at 0.6H above the ground, where H is the

height from the ground to the top of the mobile home. In the case of

the sidewalls, the uniform pressure shown in figure 3 resulted from

averaging from top to bottom and from end to end.

Table 1 - Design Wind Pressures (psf)

Wind Case Horizontal

Pressure

on Walls

Uplift Pressure

on Roof

HUD STANDARD ZONE (I) 15 9

HUD HURRICANE ZONE (II) 25 15

* * NBS HURRICANE 26.5 28

ftAveAage. Valuer

Flood Loads

Mobile homes located in flood plains may be subjected to the effects

of buoyancy and/or velocity flow depending upon the nature of the flooding and the air tightness of the home. Thus two types of flood forces

were considered in the calculations: buoyancy forces and drag forces.

Buoyancy forces result when there is a difference between the level

of the floodwater outside the mobile home and that on the inside. The

magnitude of the buoyancy force is dependent on the rate of entry of the

water into a closed mobile home relative to the rate of rise of the

floodwater. Buoyancy forces were calculated for differential water

levels of 0.5 - 2.0 ft in increments of 6 in. These values were selected on the basis of some limited field observations made by NBS. It was

assumed that these uplift forces were uniformly distributed across the

underside of the mobile home floor area.

A drag force would be applied to the sidewall of a mobile home if

the floodwater has horizontal velocity. The drag forces were calculated by the following equation.

23
where

Fp = Drag force

Cp = Drag coefficient

A = Projected vertical submerged area

p = Hass density of water

V = Average flow velocity

A flow velocity of 5 ft/sec was used in computing the forces. This

velocity was selected on the basis of the assumption that it would be

undesirable to locate a mobile home in a zone where larger flow velocities

are expected. The drag coefficient was assumed to be of the order of

that used for a typical barge, which was determined to be approximately

1 according to reference [4], Thus, was assumed equal to 1. The

projected vertical submerged area was determined for a 1-ft wide strip

of the mobile home by varying the submerged depth, h . Values of 0, 1.0

s and 2.0 ft were used for h . The bottom of the mobile home floor was

s taken as the datum for values of h

g and the depth of the chassis beam

(10 in) was then added to h . Hence h equal to zero (0) corresponds to

s s the case where only the chassis beams are submerged.

The resultant drag and buoyant forces are shown schematically in

figure 4. The drag force, F^, was assumed to act at a distance of 1/2

the submerged depth (based on a uniform velocity profile) above the

bottom of the chassis beams. As in the case of wind forces, it was assumed that the drag force on the skirt would have a negligible effect on the mobile home. The buoyant force was also combined with the HUD

Standard Wind (Zone I) but the results of that case study will not be

discussed herein.

3. VARIABLES CONSIDERED IN THE ANALYSES

3.1 Physical Characteristics of Mobile Homes

Realizing that there is a wide range of features of mobile homes as they are positioned on their supports that would affect the forces

transmitted to the foundation systems, it was necessary to identify the

most important features and to set limits on their magnitudes. The

characteristics considered as variables in the calculations are:

(1) width of mobile home, (2) weight of mobile home, (3) horizontal

eccentricity of the center of gravity with respect to the vertical

geometric center, (4) center to center spacing of chassis beams, (5)

height of the mobile home above the ground (y^ in figure 1), (6) the

horizontal distance from the edge of the mobile home to the diagonal tie

attachment point (x^_ in figure 1), and (7) tie-down arrangement.

24
Two widths of mobile homes were used in the analysis, 12 ft and 14

ft. Double-wide units were not specifically considered. The height of

the mobile home was assumed to be 8 ft in all cases. Several sources were consulted in establishing a range of weights for the above-mentioned

widths of single-wide units. It was found that the unit weight (i.e.

per lineal foot) varies from 180 to 300 lb for 12-ft wide models and

from 240 to 560 lb for 14-ft wide mobile homes. Typical weights were selected on the basis of information obtained from several producers and

industry sources. The weight values used in the calculations are shown

in table 2.

Because of the layout of the furnishings and major appliances in

mobile homes, their weights generally do not act through the geometric

center. Rather, there is an inherent eccentricity in the transverse

direction. To determine reasonable values of the eccentricity for 12-ft

wide and 14-ft wide units, a study was made of several spatial layouts

and weights of the contents of mobile homes as shipped from the manufacturer. The eccentricities (e) that were selected for the analyses

are shown in table 2. When considering overturning about the leeward

pier, it was assumed that the weight was eccentric on the downwind side

of the centerline. Thus, a lower bound resisting moment (contributed by

the weight of the mobile home) was used in the calculations.

The spacing between the main chassis beams can vary from about 5 ft

to 8 ft. Measurements were made on several 1960- and 1970-vintage

mobile homes located on the NBS grounds and 6.46 ft was found to be a typical spacing. This, cent er-to-c enter spacing was used for the calculat ions.

The height of a mobile home above the ground is dependent on the

height of the piers or other load-bearing support system and could vary

over a considerable range. For example, the bottom of the chassis beams

may rest virtually on the ground. On the other hand, a mobile home in a flood plain locale may have to be elevated 5 feet or more above the

ground to satisfy code requirements. A typical value for y is 2.5 ft,

which can be derived by assuming a 10-in chassis beam, a 2-in thick cap,

two 8-in high concrete blocks and a 4-in thick footing and that the

bottom of the footing is 2 in below ground level. The range of y£ values used was from 1 - 8 ft (table 2).

Dimension x

t is dependent on the chassis beam spacing. It was assumed that the frame ties are wrapped around the chassis beams,

resulting in the tiedown configurations shown in figure 1. The physical

characteristics used in the analyses are summarized in table 2.

•k Table 2 - Physical Characteristics Used in the Loads Analyses

B, ft W, lb/ft e, ft x

t (Near Tie), ft xt (Far Tie), ft yt

» ft

12 230 0.55 2.62 9.08 f—

i 00 14 250 0.75 3.62 10.08 1— 1 00

1

25
B = Nominal width of mobile home

W = Unite weight of mobile home

e = Horizontal eccentricity of the center of gravity with respect

to the vertical centroidal axis of a cross-section •' x = Horizontal distance from the point beneath an exterior wall at

which the vertical and diagonal ties intersect to the point

where the diagonal tie attaches to the underframe of the mobile

home

y

= Vertical distance from the point of intersection of the ties

with the anchor head to the underside of the floor of the

mobile home.

Refer to figure 1 for a pictorial description of these symbols.

3.2 Resistance of Pier Supports to Lateral Load

As mobile home chassis beams are generally not positively connected

to the vertical support members (i.e. piers), the amount of lateral load

resistance provided by the pier supports is dependent on the magnitude

of the resulting friction force. The appropriate coefficient of friction to be used in assessing the lateral load resistance would be a function of many variables even when the specific materials are defined.

It was observed by the author that the current ANSI/NFPA Standard 501A

1 51 has no explicit requirement that piers be built to resist lateral

loads. In the light of the absence of an explicit code or standard

requirement, it was conservatively assumed that the soil anchors must

resist the entire horizontal load. In addition, the effect on the anchor

loads of assuming lateral friction factors between 0 and 40% (i.e. percent

of the piers' vertical reaction), was considered in a parametric study.

3.3 Load Transfer by Over-the-Roof Ties

An over-the-roof tie is a continuous strap or cable which is

attached in varying degrees to a mobile home. The amount of tensile

force transferred from one side of the mobile home to the other depends

on the magnitude of friction between the tie and the unit. The two

extreme cases are when (1) no load is transferred from one side to

another and (2) all the load is transferred (i.e. a frictionless attachment of the tie to the mobile home). The first condition was assigned a load transfer coefficient, a = 0, while a = 1 represents the second

condition. Intermediate load transfer cases were considered in a parametric study.

4. RESULTS FROM LOAD CALCULATIONS

The results of the load calculations have been summarized as a series of computer-generated graphs. The graphs present the resulting

tie forces, pier reactions and total resultant anchor forces for the

loading cases described in section 2. A total of 32 conditions have

been summarized in graphical form and will be presented in a future

NBS publication. The results of several of the cases will be discussed

in the following paragraphs of this section. Primarily the tie and pier

force results will be discussed in this presentation.

26
A.l Wind Loads

Figure 5 shows the free-body diagrams for the mobile home when

subjected to the idealized wind drag and uplift forces. The resulting

forces for both near tie and far tie underframe connections are shown.

Figures 6A and 6B compare the tie and pier forces as a function of the

slope of the diagonal ties for the HUD Hurricane and NBS Hurricane

respectively. The two graphs are the results of analyzing a lA-ft wide

mobile home with a near tie configuration. To the left of the vertical

dashed line overturning is impending and the windward vertical ties are loaded (T^ in fig. 5A) , while to the right of the line sliding is

impending and the upwind vertical piers are loaded (PLin fig. 5A)

.

The boundary line represents the point at which overturning is just

balanced by the diagonal frame tie without any resistance provided by

either the windward vertical tie or the windward pier. The symbol a on the graphs indicates the load transfer coefficient as discussed in

section 3.3. It is of interest to compare the two graphs for the typical

case of yt = 2.5 ft and - 3.62 ft, which results in a diagonal slope

of 0.69. From figure 6 it is seen that for yt

/*

t =0.7 the windward

vertical tie is not loaded as a result of the HUD Hurricane pressures.

On the other hand, a force as high as 130 lb/ft could be induced in the

windward vertical tie by the NBS Hurricane pressures (refer to figure 7, a * 1) , due to the higher overturning moment. It is also noted that the

windward pier force is significantly higher for the HUD Hurricane,

because sliding of the mobile home would be the response mode over a wider range of values of the diagonal tie slope.

Figures 7A and 7B compare the tie and pier forces, resulting from

the HUD and NBS Hurricanes respectively, for the far tie arrangement.

The slopes shown along the abscissa reflect a range of yt values from 1

to 8 ft. It is noted that on these two graphs there is no vertical

boundary line because overturning is the mode of failure over the entire

range of underframe tie slopes. As a corrollary to this condition, the

windward vertical tie and the leeward pier are always loaded.

Since graphs such as the ones discussed above show the tie and pier

forces for a unit length of the mobile home, they can be used to determine the numbers of vertical and diagonal ties and pier supports that are required for a given total length.

Thus, using specified minimum tie and pier support capacities, one

can compute the minimum number of these support elements required per

side of single-wide mobile home. For example, given a 70-ft long mobile

home and using the minimum working load of 3150 lb as specified in

ANSI/NFPA 501 A [5] the number of diagonal ties required is derived as follows. Refering to figure 6A (HUD Hurricane) for a diagonal tie slope

of 0.7, a windward diagonal tie force of 2A0 lb/ft is scaled.

Total Force = 2A0 x 70 = 16800 lb

27
No. of Diagonal Ties Required = 16800

3150

= 5.4, say 6

Table 4. 2.2.1 (a) of ANSI/NFPA 501A also indicates that 6 diagonal

ties are required for a 70-ft long mobile home located in a hurricane

zone. It was found that the numbers of ties required according to figures

6A and 7 A were in reasonably good agreement with those specified in the

ANSI/NFPA 501A table, in the range of diagonal tie slopes permitted by

Standard 501A. For slopes steeper than those permitted by the ANSI/NFPA

Standard, the numbers of diagonal ties required for the near tie arrangement (according to figure 6A) exceed the 501A requirements. It should

be noted the above-mentioned comparisons of numbers of ties required are based on the assumption that all ties are loaded to 3150 lb. As is

suggested by the vertical tie force ordinates in figure 7B, the NBS

Hurricane would dictate the use of more vertical ties than are required

according to ANSI/NFPA 501A.

4.2 Flood Loads

4.2.1 Buoyancy Forces

Figures 8A and 8B show the diagonal and vertical tie forces resulting from buoyancy pressure acting on the underside of a 14-ft wide

mobile home, without flow velocity. Figure 8A shows the tie forces for

the near tie connection, while figure 8B shows the forces for the far

tie connection. Differential heads (Ah) of 0.5, 1.0, 1.5 and 2.0 ft

were used in the calculations. It was assumed that the line of action

of the weight of the mobile home was concurrent with uplift force vector.

The effects of this assumption were considered to be negligible in view

of the relatively small eccentricity (0.75 ft) used and the fact that

imminent tilting would tend to align these forces. To derive the diagonal

and vertical tie forces, it was assumed that their magnitudes would be

proportional to their relative strains resulting from a unit extension

of the vertical tie. Thus, the force in the diagonal tie was found to

be equal to the product of the force in the vertical tie and the sine

squared of the angle of the diagonal tie with respect to the horizontal.

This approach implies that the stiffnesses of soil anchors in both the

horizontal and vertical directions are equal. While inclined pull test

results indicate that the orthogonal stiffnesses are not equivalent, it

is not felt that the tie force results are significantly affected by

the assumption. Since the relative contribution of the diagonal ties is

generally small, the direction of the resultant anchor force is going to

virtually be vertical. Note that although the vertical tie forces are significantly greater than the diagonal tie forces over most of the

range of diagonal tie slopes, there is a trend of convergence as the

slopes increase. As would be expected from the resulting flatter slopes

the differences between the vertical tie force and diagonal tie force

ordinates are even greater for the far tie connections.

By comparing the ordinates in figures 6B and 8A, it is seen that

the magnitudes of vertical tie forces caused by a 1-ft differential head are greater than those caused by the NBS Hurricane loading condition.

Since the same anchors have to resist wind as well as flood loads, it is

28
important to determine the buoyancy force resistance of anchors originally installed for resisting wind loads. For example, it could be assumed

that mobile home anchors have been installed to resist the NBS Hurricane

forces shown in figure 9. The resultant forces for both the windward

and leeward anchors and the angle of inclination of the resultant with

the horizontal are shown for the near tie arrangement. Likewise, figure

10 shows the resultant anchor forces and their angles of inclination as caused by floodwater buoyancy forces. Thus, anchors installed to resist

NBS Hurricane forces in figure 9 could be evaluated for the forces shown

in figure 10 for various assumed differential floodwater levels. Note

that graphs such as figure 10 may not be directly applicable to some actual installations since the resultant forces were calculated for

cases in which there is a diagonal and vertical tie at each anchor

location. This arrangement is not typical of many field installations.

Anchor forces can always be calculated by combining the vertical and

diagonal tie forces vectorially to obtain the magnitude and direction of

the resultant force that must be resisted by a double-headed anchor

(i.e. an anchor head to which two ties are attached).

4.2.2 Drag Forces

Vertical and diagonal tie forces caused by a floodwater flow

velocity of 5 ft/sec acting normal to the long side of a 14-ft wide

mobile home are shown in figures 11 and 12 respectively. The drag

forces are proportional to the depth to which the mobile home is submerged, h

g

. The graphs were developed for submerged depths of 0.5 to

2.0 ft. For convenience, h is measured from the underside of the

s mobile home floor and then 10 inches were added to the assigned value

of h

g to account for the exposed surface of the chassis beam. Calculations were performed for three levels of differential head (Ah), 0,

0.5 and 1 ft. Figures 11 and 12 were derived for the case Ah = 0.5 ft.

In general, the effect of the flood drag forces is less than that

of a horizontal wind force of equal magnitude, since the former does not

exert an overturning moment. Thus, the extent to which vertical tie

forces are induced is primarily dependent on the level of differential

head and not on the magnitude of the drag force.

It should be noted that the drag forces were calculated assuming

that the exposed area would be limited to the submerged portion of the

mobile home wall and chassis beam. It is acknowledged that the lateral

forces could be significantly greater should large pieces of floating

debris get lodged against the mobile home or caught by the anchoring

ties.

5. SUMMARY

As a result of the wind and flood load calculations discussed

above, the following observations have been made.

29
1. Hurricane loads, be they caused by the HUD Hurricane or the

NBS Hurricane pressures, will induce relatively high tensile

forces in the underframe diagonal ties. This relatively large

diagonal tie force suggests that anchor systems must be designed to resist a substantial horizontal load component. Furthermore, the vertical tie forces resulting from the HUD Hurricane

are of less magnitude, particularly for the near tie connection.

2. In light of the magnitudes of the diagonal tie forces caused

by the hurricane loads, conventional anchor installations - characterized by installing anchors nearly vertical - may not

be capable of resisting the horizontal force components

without excessive displacements.

3. It was found that relatively small differential heads may

cause potentially critical forces in the vertical ties. For

example, buoyancy forces accompanying differential heads of

0.5 ft and greater will cause vertical tie forces approximately equal to or greater than those caused by the HUD

Hurricane over the entire range of diagonal tie slopes used in

the calculations. In the same vein, buoyancy forces accompanying differential heads of 1.0 ft and greater will cause vertical tie forces approximately equal to or greater than those

caused by the NBS Hurricane.

4. In view of observation 3 above, it is suggested that in cases where mobile homes may be subjected to hurricane winds and

flooding, the number of vertical ties be determined by the

flood load requirements and the number of diagonal ties be

determined by the wind load requirements.

6. REFERENCES

[1] Kovacs, W.D. and Yokel, F.Y. , (1974) "Soil and Rock Anchors

for Mobile Homes - A-State-of-the-Art Report," NBS Building Science

Series 107 , National Bureau of Standards, Washington, D.C. , October

1979, 147 pp.

[2] Marshall, R.D. , (1977), "The Measurement of Wind Loads on a Full-Scale Mobile Home," NBSIR 77-1289 , National Bureau of Standards,

Washington, D.C. , September 1977, 120 pp.

[3] Mobile Home Construction and Safety Standards, U.S. Department of

Housing and Urban Development, Federal Register, Vol. 40, Part II,

December 1975.

[4] Hoemer, Sighard F. , "Fluid Dynamic Drag: Practical Information on Aerodynamic Drag and Hydrodynamic Resistance." 2nd Edition, Midland Park,

N.J., 1958.

30
[5] Standard for the Installation of Mobile Homes, ANSI/NFPA 501 A, (1977) Manufactured Housing Institute, Chantilly, VA, and National

Fire Protection Association, Boston, MA.

7. ACKNOWLEDGEMENTS

The assistance of Mr. Christopher Mullen, graduate student at Rice

University and a guest worker at NBS, in performing the computer analysis

and generating the graphs used in this paper, is gratefully acknowledged.

The author also acknowledges the efforts of Mrs. Joan Hydorn in patiently

typing the drafts of this paper.

8. S.I. CONVERSION

The following conversion factors should be used to convert dimensions

and forces given in this report from U.S. Customary Units to S.I. Units:

ivert From To Multiply by

in mm 25.40

ft m 0.3048

lb N 4.448

psf Pa 47.88

mph mps 0.447

31
1A - Near Tie Connection

Figure 1 - Typical Tiedown Configuration for Mobile Homes

32
25 psf

drag force

t 0. 5 H

i

—0.5 B

roof uplift fore

HIM"1JJ 1,111

* u

.1 1*

15 psf

T T

Figure 2 - HUD Hurricane Forces

roof uplift force

33
flood water

level

inside I,

Idrag force

water level

__i

|w

z

velocity

J

1

.. r \ itm

buoyancy force

Figure 4 - Flood Forces
Figure 5 - Free Body Diagrams for Mobile Home

Subjected to Wind Forces

35
TIE

AND

PIER

FORCES,

Ib/ft

TIE

AND

PIER

FORCES,

Ib/ft

SLOPE OF DIAGONAL TES (yt/xt)

6A - HUD Hurricane Pressures

SLOPE OF DIAGONAL TIES (yt/xt)

6B - NBS Hurricane Pressures

Figure 6 - Tie and Pier Forces for Hurricane Pressures - Near Tie Connection

36
TIE

ANO

PIER

FORCES,

»/ft

TC

AND

PER

FORCES.

i/«

SLOPE OF DIAGONAL TIES (yt/it)

7A - HUD Hurricane Pressures

SLOPE OF DIAGONAL TIES (yt/xt)

7B - NBS Hurricane Pressures

Figure 7 - Tie and Pier Forces for Hurricane Pressures - Far Tie Connection

37
re

forces,

ft/»t

m

forces,

n/rt

SLOPE OF DIAGONAL TIES (yt/xt)

8B - Far Tie Connection

Figure 8 - Tie Forces for Flooding Condition

without Flow Velocity

38
ANCHOR

FORCES,

Ib/ft

ANCHOR

FORCES,

Ib/ft

I

SLOPE OF DIAGONAL TIES (yt/xt)

Figure 9 - Resultant Anchor Forces Induced

by NBS Hurricane Pressures

see

6ee

4ee

see

i - r_ -t 1

2.0

"

MU.

\ —

1.5

It \ rtsiltif \ 1

f.rc.s _

\

\

\

\ : .9 SX / 1 1 1

10 '

75* e.e e.s i.e l.s s.e s.s

SLOPE OF DIAGONAL TIES (yt/xt)

Figure 10 - Resultant Anchor Forces Induced by Floodwater

Buoyancy Forces Without Flow Velocity

39

ANGLE

WITH

HORIZONTAL

AN6LE

WITH

HORIZONTAL
VERTICAL

TIE

FORCES,

lb/11

VERTICAL

TV

FORCES,

Ib/lt

125

100

?s

56

25

0.0 6.5 1.6 1.5 2.6 2.5

SLOPE OF DIAGONAL TES (yt/xt)

11A - Near Tie Connection

11B - Far Tie Connection

Figure 11 - Vertical Tie Forces Caused by a Flow Velocity

of 5 fps and a Differential Head of 0.5 ft

40
DIAGONAL

Tit

FORCES,

ft/ft

DIAGONAL

TIE

FORCES,

ll/ft

12B - Far Tie Connection

Figure 12 - Diagonal Tie Forces Caused by a Flow Velocity

of 5 fps and a Differential Head of 0.5 ft

41
NBS-114A (rev. 2*8C

)

U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report NoJ 3. Publication Date BIBLIOGRAPHIC DATA

REPORT NO.

SHEET (See instructions) NBSIR 81-2238 March 1981

4. TITLE AND SUBTITLE

NBS Studies of Mobile Home Foundations

5, AUTH.OR(S)

Felix Y. Yokel, Riley M. Chung, Charles W. C. Yancey

! S. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

8. Type of Report & Period Covered

; 9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP

)

Department of Housing and Urban Development

Washington, D.C. 20410

10. SUPPLEMENTARY NOTES

_i Document describes a computer program; SF-185, FIPS Software Summary, is attached.

! 11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

Two papers are presented which discuss the results of tests on soil anchors used

to secure mobile homes and of an analytical study of wind and flood loads on soil

anchor s

.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names ; and separate key words by semicolons)

flood loads; mobile home foundations; mobile home standards; soil anchors; soil

testing; flood loads

13. AVAILABILITY

jjxD Unlimited

| | For Official Distribution. Do Not Release to NTIS

~] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

, Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

43

15. Price

$6.50

USCOMM-DC 8043-P80