CHAPTER 4 - DESIGN LOADS FOR\
PERMANENT FOUNDATIONS

**400. GENERAL.** Design and construction must insure that the load
bearing portion of the home\'s foundation will remain stable and
maintain its capacity to transmit all imposed loads to the ground.

**400-1. FOUNDATION DESIGNER.** The foundation designer must be aware of
the structural limitations of the home to accommodate differential
foundation movement. This is especially important with differential soil
settlement or movement of problem soils.

**400-2. REFERENCED STANDARDS.** All structural design shall be based on
generally accepted engineering practice. All loads shall be in
accordance with ASCE 7‑93, except as shown otherwise in this manual.
Local codes must be reviewed for requirements that may be more stringent
than ASCE 7‑93.

**400-3. DESIGN STANDARDS.** Foundation design criteria is based on
foundation criteria for conventional housing as defined in the *Minimum
Property Standards*, and is not based on the *Manufactured Home
Construction and Safety Standards* (Part 3280). Foundation Design Load
Tables, Appendix B, were developed based on average ASCE *Minimum Design
Dead Loads*. See Table 4-1 below. (See Derivation of Foundation Design
Load Tables, Appendix D.)

**401. BUILDING STRUCTURE AND SIZE.** Information must be provided by
the manufacturer to assist in determining the suitability of a
manufactured home for a particular site and foundation system. The
inspector shall do a preliminary check to verify that all information
has been prepared by the manufacturer. (The Manufacturer\'s Worksheet
can be found in Appendix E, page E-3.)

**402. DESIGN LOADS**

**402-1. DEAD LOADS**

**A. Computation of Forces.** Two design dead load values are used in
this guide. The values are based on typical materials used in
construction of homes.

1\. The lightest combination of loads is used for computation of
horizontal and vertical anchorage forces for wind related overturning
and sliding stability.

2\. The heaviest combination of loads is used for computation of: (1)
footing bearing area and (2) equivalent lateral inertia forces applied
at roof and floor levels for seismic related overturning and sliding
stability.

**B. Dead Load Values.** The design light and heavy dead load values are
shown in Table 4-1 for manufactured home type and nominal unit width.

**C. Distributed Weight Calculation.** The manufacturer shall provide
the total weight (W) and the length (L) of the manufactured housing
unit, including mechanical equipment. These values are used to convert
the weight (W) into the distributed value of pounds per lineal foot (w).
Use the following formula to make this conversion:

$$w = \frac{W}{L}$$

Where: L = length of home (Mfr. Wksht. #3)

W = total weight (Mfr. Wksht. #8)

**D. Distributed Weight Comparison.** The distributed home weight (w)
shall be compared with the average calculated values in Table 4-1.

1\. If the manufacturer\'s distributed value (w) is less than the light
load or greater than the heavy load, the structural engineer will be
required to design the foundation system and anchoring system. Proceed
no further until an approved system, certified by a licensed structural
engineer, has been provided. DO NOT USE THE TABLES. The tables are based
on estimated conditions. Once outside those limits, the results will not
be valid.

2\. If the manufacturer\'s value falls within the light and heavy load
limits (± 5%), USE THE TABLES IN THIS MANUAL and proceed with the
verification process.

**E. Other Dead Loads.** Manufactured home partitions and other known
loads caused by special installations such as stationary equipment, i.e.
water heater, furnace, etc., shall be included to arrive at applicable
dead loads.

**402-2. SNOW LOAD**

**A. General.** Ground snow loads are based on values from ASCE 7‑93.
The Ground Snow Load map on pages H‑11, H‑12, H‑13, shall be used to
determine a ground snow load value (Pg) for the manufactured home
location. For areas where ground snow load values are not shown, consult
local weather data or governing code authority. Ground snow loads (Pg)
are converted to roof design snow loads (Ps) by multiplication on 0.7 ×
Pg. See Appendix D for derivation. The tables in Appendix B use Pg
values from the map. Roof snow loads are assumed to be horizontally
projected over the roof area.

**B. Heavy Snow Loads.** If the ground snow load value (Pg) exceeds 100
psf, consult a licensed structural engineer for footing design.

**C. Minimum Roof Live Load.** Roofs shall be designed for a minimum
horizontally projected live load in accordance with *MPS* HUD Document
4910.1, Appendix K, art.200.926e. The load magnitude is related to roof
slope as follows: greater than 3 in 12: 15 psf; less than or equal to 3
in 12: 20 psf. The larger magnitude, between the design roof snow load
and the minimum roof live load, shall be used for design. Note that a 20
psf ground snow load (Pg) corresponds closely to a 15 psf minimum roof
live load (i.e. 0.7 × 20 = 14 psf rounded to 15 psf) and a 30 psf ground
snow load corresponds closely to a 20 psf minimum roof live load (i.e.
0.7 × 30 = 21 psf rounded to 20 psf in the Foundation Design Load
Tables).

**402-3. WIND LOAD**

**A. General.** Wind loads must be based on values from ASCE 7‑93. The
Basic wind speed map on page H‑14 must be used to determine the basic
wind speed (v) for the manufactured home location. Refer to Appendix D
for factors influencing wind load. Map values below 80 mph shall conform
to the minimum wind speed of 80 mph in accordance with *MPS* HUD
Document 4910.1, Appendix K, art. 200.926e.

NOTE: Tornadoes have not been considered in the development of the basic
wind speed map, and resistance to such conditions is not included in
this manual.

**B. Coastal or Inland Sites.** Coastal regions include any locations
within 100 miles of the Atlantic Ocean or Gulf of Mexico hurricane
coastlines. All other locations are to be considered Inland regions.
Exposure Category C has been assumed regardless of Coastal or Inland
location in accordance with *MPS* HUD Document 4910.1, Appendix K, art.
200.926e.

**C. Severe Wind and Design Pressures.** In hurricane zones, or where
severe wind pressures occur, foundations and anchoring for manufactured
homes will require special treatment.

1\. Foundations may be required to resist greater uplift and overturning
than values shown in this manual.

2\. Heavier, more deeply buried foundations may be required than values
shown in the tables. It may be necessary to provide additional
foundation shear walls and/or specially designed cantilever piers.

3\. Home-to-foundation connections must be strengthened.

4\. Refer to *Mobile Home Anchoring Systems and Related Construction and
An Engineering Analysis: Mobile Homes in Windstorms*, Institute for
Disaster Research in Lubbock, Texas.

**D. Design Verification.** The field office must verify the existence
of engineered drawings showing connection and anchorage details. The
connection details shall be engineered to resist wind speeds at the
building site.

**E. High Wind Design.** For high wind areas, foundation designs must be
those that are suited to both high wind and other site conditions, such
as seismic or soil conditions.

**402-4. SEISMIC LOADS**

**A. General.** Seismic design loads and requirements are based on
criteria and values from ASCE 7‑93, which are taken from the NEHRP
*Recommended Provisions for the Development of Seismic Regulations for
New Buildings* (NEHRP 1991). The two seismic maps on pages H‑15 and H‑16
shall be used to determine the seismic values [Aa]{.underline} and
[Av]{.underline} for the manufactured home location (county). Seismic
values of [Aa]{.underline} and [Av]{.underline} that equal or exceed 0.3
shall conform to the special requirements of seismic performance
category C and D (cited on page H‑17) as they apply to foundation design
and detailing. When [Av]{.underline} values from the map on page H‑16
are less than 0.15, the seismic provisions of ASCE 7‑93 need not be
considered, and anchorage design is then based on wind considerations
alone. In seismic areas where [Av]{.underline} and/or [Aa]{.underline} ≥
0.3, foundations must be designed by a professional engineer licensed in
the applicable state.

**B. Design Verification.** The design concept proposed in question 10
of the \"Manufacturer\'s Worksheet\", found in Appendix E, should be
compared with information in the Foundation Design Concept Tables
(Appendix A) to determine whether the foundation is potentially suitable
for location in a seismic zone.

**C. Characteristic Differences between Wind and Seismic Loading.**

1\. Wind loads subject the exterior building envelope to pressures and
suctions on each wall or roof surface. Thus, exposed surface area is
important. Seismic loads are generated by the ground's acceleration
being transferred to the foundation, according to the site soil
characteristics (S) and then the building's structural system
characteristics (R). This modified acceleration excites the building
mass, which generates the inertia forces (F = m × a) at each level (i.e.
floor and roof). Thus, the entire building participates in the creation
of seismic force, while only the exterior envelope participates in wind
load generation.

2\. Wind loading is usually long duration with short duration gusting
that usually creates slow stress reversals, while seismic events are of
short duration, creating accelerations that generate rapid oscillations
in all directions with sudden stress reversals.

3\. The slow structural response from wind loading permits frictional
resistance from gravity loads to be considered for sliding resistance
between superstructure and foundation. The simultaneous horizontal and
vertical acceleration during a seismic event, generally negates the
frictional resistance from gravity loads. Thus, friction is ignored as a
potential resistance between superstructure and foundation for seismic
loading. Even when wind loads exceed seismic loads, positive connections
between superstructure and foundation are required for areas with Av
equal to or greater than 0.15.

**D. Seismic / Wind Force Comparisons.** Overturning and sliding
anchorage forces found in the Foundation Design Load Tables of Appendix
B are based on the largest lateral forces from a consideration of wind
and equivalent lateral seismic inertia forces. The results were as
follows:

1\. Wind controls for single or multi- section units subjected to (1)
overturning from lateral forces in the transverse direction
(perpendicular to long dimension of unit) and (2) uplift forces in the
vertical direction. Both conditions require vertical anchorage.

2\. Wind or seismic may control for single or multi-section units
subjected to sliding in the transverse and/or longitudinal direction.
Values in the tables of Part 3 and 4 of Appendix B are grayed if seismic
controls.