Roof Fundations Design

F. Example Calculations
Design a CMU pier and ground anchor foundation for a manufactured home to be placed in
an SFHA Zone AE having a flood velocity of 2 fps. The BFE is 9 feet and existing ground elevation is approximately 7 feet. The flood depth is 2 feet and the freeboard is 1 foot, which yields
a DFE depth of 3 feet. The manufactured home dimensions for these example calculations areshown in Figure F-1. The manufactured home is a single unit, 16 feet wide and 60 feet long
with a 30-degree gable roof with a 1-foot overhang. Roofing members are spaced 16 inches on
center (o.c.). The manufactured home weighs 20 psf. Assume an NFPA 5000 soil classification
of soft, sandy clay, or clay (allowable bearing pressure qa =1,000 psf ; ultimate bearing pressure
qu = 2,000 psf). UseASCE 7 to calculate loads.
Foundation loads selected for this example of a manufactured home in an SFHA differ from
those that may be found in HUD standard 24 CFR 3280. Design loads in this example are in accordance with ASCE 7-05 and other standards.
These example calculations assume transverse wind loads produce the controlling loading.
Wind in the direction parallel to the roof ridge mayproduce greater loads for certain cases and
must be evaluated during final design.

Figure F-1. Manufactured
home dimensions.

ASCE 7-05

P ROTECTING MaNufaCTuREd HOMEs fROM flOOds aNd OTHER HazaRds
A Multi-Hazard Foundation a nd Installation Guide

F-1

F

E XAMPLE CALCULATIONS

Step 1: Determine Design Criteria
NORMAL LOADS
Dead Load (D)
D = 20 psf

ASCE 7-05
Table 4-1Given in the example statement

Live Load (L)
L is based on one- and two-family dwellings
L = 40 psf

Roof Live Load (Lr)
Lr = 20R1R2 =20(1)(0.85)=17 psf
R1 = 1 for At ≤ 200 ft2

()

1 ft
=24.5 ft2
12 in
F = number of inches of rise per foot
12 in
F = 1ft
tan 30˚ = 7 in
1 ft

At = 2(9.2 ft)(16 in)

()

Note that the roof live load falls between the limits given:
12 ≤ Lr≤ 20
ASCE 7-05

ENVIRONMENTAL LOADS
Wind Loading
Structure is a regular shape, located in a windborne debris region with
terrain classification of Exposure C and surrounded by flat terrain.

Section 6.2

Mean roof height (h)
h = 3 ft + 10 ft + 0.5(4 ft)
= 15 ft
h < 16 ft (least horizontal dimension)
Calculations are for a foundation system, which is a main wind force resistingsystem (MWFRS).
Section 6.5

Velocity Pressures
Velocity pressures are determined using
Method 2: Analytical Procedure
F-2

PROTECTING MaNufaCTuREd HOMEs fROM flOOds aNd OTHER HazaRds
A Multi-Hazard Foundation a nd Installation Guide

E XAMPLE CALCULATIONS

F

(A simplified alternative is to use ASCE 7, Section 6.4, Method 1. Wind
pressures are tabulated for basic conditions. The windpressure must be
adjusted for mean roof height and exposure category.)
Velocity Pressure Coefficient (qz)
qz = 0.00256KzKztKdV2I
Velocity pressure exposure coefficient evaluated at height z (the
height above ground level in feet) (Kz)
Kz= 0.85
Topographical factor (Kzt)
Kzt=1 (assume a flat surface)
Wind directionality factor (Kd)

Section 6.5.10
Eq. 6-15
Section 6.5.6
Table 6-3Section 6.5.6
Section 6.5.4.4
Section 6.5.5
Table 6-1
Section 6.5.4

Kd = 0.85
Basic Wind Speed (V)

Section 6.5.11.1
Figure 6-5

V = 110 mph (3-second gust)
I = 1 (Category II building: Table 1-1 (ASCE 7))

Section 6.5.11.2.1
Figure 6-6

Therefore, qz = 0.00256(0.85)(1)(0.85)(110)2(1)
= 23 psf

Design Pressures for MWFRS
Internal Pressure Coefficient (GCpi)
GCpi = ± 0.18External Pressure Coefficient (Cp)
h

= Mean roof height, in feet

L

= Horizontal dimension of building, in feet, measured parallel to wind direction

B

= Horizontal dimension of building, in feet, measured normal to wind direction

Figure 6-6

Table F-1 shows the External Pressure Coefficients calculated for the
windward, leeward and side walls. Computations of the External...