PROTECTING MANUFACTURED HOMES FROM FLOODS AND OTHER HAZARDS
A Multi-Hazard Foundation and Installation Guide
FEMA P-85, Second Edition, November 2009
Appendix 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 are shown 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). Use ASCE 7 to calculate loads.
[Begin figure]
Figure 1 is an illustration of manufactured home dimensions from ASCE 7-
05.
[End figure]
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 may
produce greater loads for certain cases and must be evaluated during final
design.
Step 1: Determine Design Criteria
NORMAL LOADS (ASCE 7-05 Table 4-1)
Dead Load (D)
D = 20 psf Given in the example statement
Live Load (L)
L is based on one- and two-family dwellings
L = 40 psf
Roof Live Load (L subscript r)
L subscript r = 20R subscript 1 R subscript 2 =20(1)(0.85)=17 psf
R subscript 1 = 1 for A subscript t 200 ft squared
A subscript t = 2(9.2 ft)(16 in)(1 ft/12in)=24.5 ft squared
F = number of inches of rise per foot
F = 1ft (12 in/1ft)tan 30 degrees = 7 in
Note that the roof live load falls between the limits given:
12 is less than or equal to L subscript r is less than or equal to 20
ENVIRONMENTAL LOADS (ASCE 7-05)
Wind Loading (Section 6.2)
Structure is a regular shape, located in a windborne debris region with
terrain classification of Exposure C and surrounded by flat terrain.
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
resisting system (MWFRS).
Velocity Pressures (Section 6.5)
Velocity pressures are determined using
Method 2: Analytical Procedure
(A simplified alternative is to use ASCE 7, Section 6.4, Method 1. Wind
pressures are tabulated for basic conditions. The wind pressure must be
adjusted for mean roof height and exposure category.)
Velocity Pressure Coefficient (q subscript z)
q subscript z = 0.00256K subscript z K subscript zt K subscript d V
squared I
Velocity pressure exposure coefficient evaluated at height z (the height
above ground level in feet) (K subscript z)
K subscript z= 0.85
Topographical factor (K subscript zt)
K subscript zt=1 (assume a flat surface)
Wind directionality factor (K subscript d)
K subscript d = 0.85
Basic Wind Speed (V)
V = 110 mph (3-second gust)
I = 1 (Category II building: Table 1-1 (ASCE 7))
Therefore, q subscript z = 0.00256(0.85)(1)(0.85)(110)2(1)
= 23 psf
Design Pressures for MWFRS
Internal Pressure Coefficient (GCpi)
GC subscript pi = ± 0.18
External 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
Table F-1 shows the External Pressure Coefficients calculated for the
windward, leeward and side walls. Computations of the External Pressure
Coefficients for the windward and leeward roof are shown Table F-2.
[Begin table]
Table F-1. External Wall Pressure Coefficients
Surface: Windward Wall
Wind Direction: n/a
L/B: n/a
C subscript p: 0.8
Surface: Leeward Wall
Wind Direction: Perpendicular to roof ridge
L/B: 16 ft/60 ft=0.27
C subscript p: -0.5
Surface: Side Wall
Wind Direction: n/a
L/B: n/a
C subscript p: -0.7
[End Table]
[Begin table]
Table F-2. External Roof Pressure Coefficients
Surface: Windward Roof
Wind Direction: Perpendicular to roof ridge
h/L: 15 ft/16 ft/= 0.94
C subscript p =-0.30.2
Leeward Roof
Wind Direction: Perpendicular to roof ridge
h/L: 15 ft/16 ft/= 0.94
C subscript p -0.6
[End Table]
Foundation systems are considered rigid, therefore, G = 0.85.
Design Pressure (p)
The basic pressure equation (ASCE 7 6-17), which includes the internal
pressure coefficient is as follows:
p = qGC subscript p q subscript i(GC subscript pi)
However, this would only be used if designing individual components whose
effective tributary area is equal to or greater than 700 sf (ASCE 7-05
6.5.12.1.3 and IBC 2006 1607.11.2.1). When determining loads on the global
structure (i.e., shear walls or foundation design), the internal pressure
components will act in equal and opposite directions on the roof/floor and
the leeward/windward walls, thereby algebraically canceling each other.
The
resulting simplified form of the pressure equation is:
p = q x GC subscript p
Table F-3 summarizes the design pressures calculated using this simplified
wind design pressure equation. Figure F-2 shows the application of these
design pressures on the structure. For foundation design, internal
pressures
need not be considered since internal pressure on windward walls, leeward
walls, floors, and roofs cancel each other. For example, internal
pressures
acting on a windward wall are equal and opposite to those acting on a
leeward
wall and the net force on the foundation from internal pressures is zero.
While internal pressures cancel, internal pressures for a partially
enclosed
building have been included in the example. This is to provide an example
of
more general wind load calculations.
[Begin table]
Table F-3. Design Pressures for Wind Perpendicular to the Roof Ridge
Surface: Windward Wall
Design Wind Pressure Calculations: p = 23 psf(0.85)(0.8)
pressure(psf): 15.7
Surface: Leeward Wall
Design Wind Pressure Calculations: p = 23 psf(0.85)(-0.5)
pressure(psf): -9.8
Surface: Side Walls
Design Wind Pressure Calculations: p = 23 psf(0.85)(-0.7)
pressure(psf): -13.7
Surface: Windward Roof
Design Wind Pressure Calculations: p = 23 psf(0.85)(-0.3)
pressure(psf): -5.9
Design Wind Pressure Calculations: p = 23 psf(0.85)(0.2)
pressure(psf): 4.0
Surface: Leeward Roof
Design Wind Pressure Calculations: p = 23 psf(0.85)(-0.6)
pressure(psf): -11.8
[End table]
MWFRS Roof Overhang Pressures
ASCE 7 only addresses the windward overhang, specifying the use of a
positive
pressure coefficient of C subscript p = 0.8. Acting on the bottom surface
of
the overhang in combination with pressures acting on the top surface. For
the
leeward overhang, the coefficient for the leeward wall (C subscript p = -
0.5)
could be used, but the coefficient has been conservatively taken as zero.
p = 23 psf(0.85)(0.8)
= 15.7 psf
p subscript OH = -5.9 psf 15.7 psf = -21.6 psf
A conservative simplification is to use the wind pressure acting away from
the roof case for uplift roof pressure simultaneously with the wind
pressure
toward the roof for the lateral roof pressure.
SNOW LOADING
Ground Snow Load (p subscript g)
p subscript g = 20 psf
Flat Roof Snow Load (p subscript f)
p subscript f = 0.7C subscript e C subscript t Ip subscript g
p subscript f = 0.7(1.0)(1.0)(1.0)(20)
= 14 psf
But not less than p subscript f =(I)p subscript g = 20 psf
Exposure Coefficient (C subscript e)
C subscript e = 1.0 (partially exposed roof)
Thermal Factor (C subscript t)
C subscript t = 1.0
Importance Factor (I)
I = 1.0 (Category II building: Table 7-4 (ASCE 7))
Sloped Roof Snow Load (p subscript s)
p subscript s = C subscript s p subscript f
= (1.0)(20 psf)
= 20 psf
Warm Roof Slope Factor (C subscript s)
C subscript s= 1.0 (asphalt shingle not slippery)
Unbalanced Roof Snow Load (p subscript u)
Since the roofs eave to ridge distance less than or equal to 20 ft,
unbalanced uniform snow loads shall be applied as follows:
P subscript windward = 0.3 p subscript s = 6 psf
P subscript leeward.1 = p subscript s = 20 psf
P subscript leeward.2= (h subscript d) (gamma)/square root of(S)
= (1.44 ft)(16.6 pcf)/ square root of (1.73)
= 18.2 psf
From the ridge toward the leeward eave a distance of:
x = (8/3)(h subscript d)square root of(S)
= 5.1 ft
h subscript d = 1.44 ft
gamma = 0.13 p subscript g + 14 less than or equal to 30 pcf
= 16.6 pcf
FLOOD LOADING
Hydrostatic Load (F subscript h)
If the manufactured home is elevated above the BFE on an enclosed
foundation,
venting must be provided in all manufactured homes placed in a SFHA; the
hydrostatic forces on either side of the foundation wall will cancel.
However, the hydrostatic load is calculated because it is used in the
hydrodynamic load calculation.
F subscript h = ½ P subscript h H
Hydrostatic Pressure (P subscript h)
P subscript h = H
Specific Weight of Fresh Water (omega)
= 62.4 pcf
Floodproofing Design Depth (H)
H = 2 ft (base flood depth) + 1 ft
Hydrodynamic Load
The hydrodynamic load is calculated by converting it to an equivalent
hydrostatic load by increasing the flood depth. The increase in flood
depth
is referred to as d subscript h.
d subscript h = C subscript d V cubed/2g = 2.0 (2ft/s)squared/2 (32.2ft/s)
=
0.13 ft
Drag Coefficient (C subscript d)
In the above equation, a value of 2.0 was assumed for Cd. This is a
conservative estimate; the actual value for Cd could be anywhere between
1.2
and 2.0.
Acceleration Due to Gravity (g)
g = 32.2 ft/s squared
with a hydrodynamic pressure of
P subscript hydr = gamma (d subscript h) = 62.4 pcf (0.13 ft) = 8.2 psf
The equivalent hydrostatic load (F subscript h/ad) taken into
consideration
the hydrodynamic load is :
F subscript h/ad = P subscript hydr x H = 8.2 psf x3 ft = 24.6 plf
Since piers are 16 inches wide, the total hydrodynamic force on the pier
is
=24.6 lb/ft 16 in 1 ft/12 in 32 lbs per pier
CHECK SCOUR
Reference: Publication No. FHWA NHI 00-001, Evaluating Scour at Bridges,
4th
Edition, May 2001, Hydraulic Engineering Circular No. 18.
Y subscript s/Y1 = 2.0 x (K subscript 1)x (K subscript 2) x(K subscript 3)
x(K subscript 4) x (a/Y subscript 1)superscript 0.65 x F subscript r1
superscript 0.43
Where: Y = Scour depth
Y subscript 1 = Flow depth directly upstream of pier
A = Pier width (ft.)
L = Pier length (ft)
F subscript r1 = Froude number
F subscript r1= V subscript 1/(gY subscript 1)superscript 1/2
Where V subscript 1 = Mean velocity of flow directly upstream of pier
g = acceleration due to gravity (32.2 feet/sec squared)
K subscript 1 = Factor for pier nose shape. For square nose
K subscript 2 = Factor for Angle of attack. K subscript 2 = (cosine
omega + (L/a) x sine omega)superscript 0.65
K subscript 3 = Factor for bed condition/. K subscript 3 = 1.1
K subscript 4 = Factor for armoring by bed material size.
Project parameters:
Flood low = 2 fps
Flood depth = 3 ft
Assume flood angle of attack = 0Ί
So that:
K subscript 1 = 1.1 (Table 6.1)
K subscript 2 = [cosine 0Ί +(L/a)x sine 0 superscript 0] superscript
0.65 = [1.00 + (1.33/0.67) x 0] superscript 0.65 = 1.00
K subscript 3 = Factor for bed condition/. K subscript 3 = 1.1 (Table
6.3)
K subscript 4 = Factor for armoring by bed material size. K subscript 4
= 1.0 unarmored
F subscript r1 = V subscript 1/(gY subscript 1) superscript 1/2 =
2/[32.2 x 3] superscript 1/2 = 2/9.84 = 0.203
And
Y subscript s/Y subscript 1= (2) x (1.1) x (1.0) x (1.1) x (1.0) x
(0.67/2)
superscript 0.65 x (0.203) superscript 0.43
Y subscript s/Y subscript 1= (2.42) x (0.491) x (.504) = 0.6
Y subscript s = (0.6) x (Y subscript 1) = (0.6)x(3) = 1.8 ft
Scour protection or increased footing embedment required.
Step 2: Select a Design Methodology and Assess Load Combinations and
Failure
Modes
Figure F-3 illustrates the loads applied to the manufactured home. Table
F-4
lists the nomenclature of the applied loads shown in Figure F-3.
[Begin table]
Table F-4. Load Nomenclature
D = dead load
L = live load
L subscript R = roof live load
R subscript H = horizontal reaction
R subscript LV = leeward vertical reaction
R subscript WV = windward vertical reaction
S subscript B = balanced snow load
W subscript H = horizontal wall wind pressure
W subscript RH = roof horizontal wind pressure
W subscript LRV = leeward roof vertical wind pressure
W subscript WRV = windward roof vertical wind pressure
[End table]
Note that snow load governs over roof live load and wind downward load,
and
wind lateral load governs over earthquake lateral load. Load combinations
for
non-governing cases are not shown.
For the purposes of these calculations, the worst case wind load is taken
to
be perpendicular to the roof ridge for all failure modes. Wind in the
direction parallel to the roof ridge may produce greater loads for certain
failure modes.
Uplift and Downward Failure Mode
Uplift failure is a vertical force phenomenon. The loads that act
vertically
are wind, snow, dead, and live loads. Table F-5 summarizes the loads that
influence uplift and downward failure mode. Table F-6 assesses uplift and
downward failure load combinations. Note that uplift is based on MWFRS
pressures for the global foundation design. Design of the connections to
the
foundation may require components and cladding (C&C) pressures to be used.
[Begin table]
Table F-5. Vertical Load Values
Load Type Total load acting on the structure and, therefore, must be
supported by the foundation
D = [dead load per square foot][width of the manufactured home]
D = [20 psf][16 ft]
D = 320 lbs per linear ft of manufactured home length
L = [live load per square foot][width of the manufactured home]
L = [40 psf][16 ft]
L = 640 lbs per linear ft of manufactured home length
L subscript r = [roof live load per square foot][width of the manufactured
home]
L subscript r = [17 psf][18 ft]
L subscript r = 306 lbs per linear ft of manufactured home length
W = Maximum wind uplift loads occur for winds parallel to the roof ridge
at
the windward end.
W = W subscript WRV+W subscript LRV = [(vertical component roof wind
pressures)(area roof)]/manufactured home length
W = [-17.6 psf][(9 ft)(15 ft)(2)] 0 ft to 15 ft +
[-9.8 psf][(9 ft)(15 ft)(2)] 15 ft to 30 ft +
[-5.9 psf][(9 ft)(30 ft)(2)] 30 ft to 60 ft
W = -10,584 lbs/60 ft = -176 lbs per linear foot of manufactured home
length
(average)
In this case, vertical uplift loads are low and so this simplification is
acceptable. However, to account for the unbalanced uplift if wind loads
were
higher, either overturning in this direction would need to be considered,
or
the windward uplifts conservatively made symmetrical about the middle.
Maximum wind downward loads occur for wind perpendicular to the roof
ridge;
however, they are much less than, and do not govern over, roof live or
snow
loads.
S = [snow pressure][horizontal projected roof area]
S = [20 psf][(9 ft)]SW + [20 psf][(9 ft)]SL
S = 360 lbs per linear ft of manufactured home length
[End table]
[Begin table]
Table F-6. Vertical Failure Mode ASD Load Combinations
Load Combinations: 4
D + 0.75L + 0.75S
320 lbs + 0.75(640 lbs) + 0.75(360 lbs) = 1,070 lbs per linear ft of
manufactured home length
Load Combinations: 7
0.6D + W
0.6(320 lbs) - 176 lbs = 16 lbs per linear ft of manufactured home length
acting downward
[End table]
Note that, for load combination 7, the 0.6 load factor should be applied
to
the dead load that would actually be present over the whole structure.
Additions to the dead load tabulation such as mechanical and miscellaneous
or
shingles should not be included in this value as they may not be present
in
all areas or during a high-wind event and their inclusion would not be
conservative.
Sliding or Shearing Failure Mode
Sliding failure is a lateral force phenomenon. The loads that act
laterally
are wind and flood loads. Table F-7 summarizes the lateral loads and their
values. Maximum lateral wind loads occur when the wind is perpendicular to
the roof ridge. Note that lateral wind loads act on the overall structure
(i.e., foundation), whereas flood loads act on the individual piers. Table
F-
8 gives the load combinations for sliding failure. Once the number of
piers
is defined, the hydrodynamic forces on these piers are to be added to load
combination 4, and the foundation design will have to be checked to make
sure
it can resist the added hydrodynamic loads.
[Begin table]
Table F-7. Lateral Load Values
Total load acting on the structure and, therefore, must be supported by
the
foundation:
Load Type: W
Maximum lateral wind loads occur for winds perpendicular to the roof ridge
W = W subscript RH + W subscript H =
[lateral roof pressures][roof height] + [wall pressures][wall height]
W = [4 psf (-11.8 psf)] (4.7 ft) [(15.7 psf + 9.8 psf)(10 ft)]
W = 329.3 lbs per linear ft of manufactured home length
Load Type: F subscript a
Hydrodynamic load per pier
F subscript a = [hydrodynamic force][pier length]
F subscript a = 32.8 lbs per pier
Assume total of 9 piers x 2 rows for 1st iteration
F subscript a = (32.8 lbs per pier)(9 piers per row)(2 rows)
F subscript a = 590.4 lbs / 60 ft = 9.84 lbs per linear ft of manufactured
home length
[End table]
[Begin table]
Table F-8. Sliding Load Combinations
Load Combinations: 5
W + 1.5 F subscript a
329.3 lbs + 1.5(9.84 lbs) = 344 lbs per linear ft of manufactured home
length
[End table]
Note: The vertical gravity loads are not considered to be conservative.
Thus,
the frictional resistance of the footings under the piers has been
neglected.
This component may be used in borderline situations at the discretion of
the
engineer.
Overturning Failure Mode
Overturning failure results from loads that act on the whole structure and
pivot about the bottom of the leeward pier. Dead, live, wind, and snow
loads
can all influence the overturning moment. Table F-9 summarizes the moments
that affect overturning due to wind in this case. Table F-10 assesses the
moment load combinations. Only the portions of the roof and floor live
loads
that are over the part that cantilevers out past the leeward pier will
contribute to the overturning. Since this is the worst overturning case
for
each, only these conditions will be calculated.
[Begin table]
Table F-9. Moment Load Values
Moment Type: D
Total moment about the bottom of the leeward foundation support(positive
moment is counter clockwise):
D = [dead load per square foot][home width][moment arm]
D = [20 psf][(16 ft)(4 ft)]
D = +1,280 ft-lbs per linear ft of manufactured home length
Moment Type: L
Total moment about the bottom of the leeward foundation support(positive
moment is counter clockwise):
L subscript 1 = [live load per square foot][home width][moment arm]
L subscript 1 = [40 psf][(16 ft)(4 ft)]
L subscript 1 = 2,560 ft-lbs per linear ft of manufactured home length
L subscript 2 = [live load per square foot][cantilever width][moment arm]
L subscript 2 = [40psf][4 ft][-1 ft]
L subscript 2 = -160 ft lbs per linear ft of manufactured home length
Moment Type: L subscript r
Total moment about the bottom of the leeward foundation support(positive
moment is counter clockwise):
L subscript r = [roof live load per square foot][roof width][moment arm]
L subscript r = [17 psf][4 ft][1 ft]
L subscript r = -68 ft-lbs per linear ft of manufactured home length
Moment Type: W
Total moment about the bottom of the leeward foundation support(positive
moment is counter clockwise):
WIND PERPENDICULAR TO THE ROOF RIDGE
W subscript WRV = [vertical component roof wind pressures][roof
width][moment
arm]
W subscript WRV (-21.6 psf)(1 ft)(12.5 ft) + (-5.9 psf)(8 ft)(8 ft)
W subscript WRV = -648 ft-lbs per linear ft of manufactured home length
W subscript LRV = [vertical component roof wind pressures][roof
width][moment arm]
WL subscript RV (-11.8 psf)(9 ft)(-0.5 ft)
W subscript LRV = +53 ft-lbs per linear ft of manufactured home length
W subscript RH = [horizontal component roof wind pressures][roof
height][moment arm]
W subscript RH = [4 psf (-11.8 psf)](-4.67 ft)](15.3 ft)
W subscript RH = -1,129 ft-lbs per linear ft of manufactured home length
W subscript W+L = [windward wall pressure + leeward wall pressure][homes
height from ground to roof eave][moment arm]
W subscript W+L = [15.7 psf + 9.8 psf](10 ft)(-8 ft)
W subscript W+L = -2,040 ft-lbs per linear ft of manufactured home length
Moment Type: F subscript a
Total moment about the bottom of the leeward foundation support (positive
moment is counter clockwise):
Hydrodynamic load on piers
F subscript a = [horizontal component][moment arm]
= (9.84 plf)(-3 ft/2) = -15 ft-lbs per linear foot of manufactured home
length
Moment Type: S
Total moment about the bottom of the leeward foundation support(positive
moment is counter clockwise):
S subscript B = [balanced snow pressure][horizontal projected roof
area][moment arm]
S subscript B = [20 psf][18 ft][-4 ft]
S subscript B = 1,440 ft-lbs per linear ft of manufactured home length
[End table]
[Begin table]
Table F-10. Overturning Load Combinations
Moment Load Combinations (positive moment is counter clockwise):
6
D + 0.75W + 0.75L + 0.75 L subscript r + 1.5 F subscript a (Partial live
loading to produce max OT)
(1,280 ft-lbs) + (0.75)(-648 ft-lbs + 53 ft-lbs 1,129 ft-lbs 2,040 ft-
lbs) + (0.75)(-160 ft-lbs)+ (0.75)(-68 ft-lbs) + (1.5)(-15 ft-lbs) = -
1,737
ft-lbs per linear ft of manufactured home length
D + 0.75W + 0.75 L + 0.75S + 1.5 F subscript a (Full live and snow to
produce
max downward reaction)
(1,280 ft-lbs) + (0.75) (-648 ft-lbs + 53 ft-lbs 1,129 ft-lbs 2,040
ft-
lbs) + (0.75)(-2,560 ft-lbs) + (0.75)(1,440 ft-lbs) + (1.5)(-15 ft-lbs) =
1,435 ft-lbs per linear ft of manufactured home length
Moment Load Combinations (positive moment is counter clockwise):
7
0.6D + W + 1.5 F subscript a
(0.6)(1,280 ft-lbs) + (-648 ft-lbs + 53 ft-lbs 1,129 ft-lbs 2,040 ft-
lbs)
+ (1.5)(-15 ft-lbs) = -3,019 ft-lbs per linear ft of manufactured home
length
[End table}
Table F-11 summarizes the load combinations that govern for each of the
three
failure modes. The maximum roof vertical and lateral load cases are
assumed
to act simultaneously as a conservative simplification.
[Begin table]
Table F-11. ASD Load Combinations for Example Problem (loads are in pounds
per linear foot)
Failure Modes:
Failure Modes: Uplift
Load Combinations 4: 1,070 lbs
Load Combinations 5: n/a note 1
Load Combinations 7: 15 lbs
Failure Modes: Sliding
Load Combinations 4: n/a note 1
Load Combinations 5: 313 lbs
Load Combinations 7: n/a note 1
Failure Modes: Overturning
Load Combinations 4: n/a note 1
Load Combinations 5: n/a note 1
Load Combinations 7: -979 ft-lbs
Note 1: Load combination does not govern.
Load combinations 1-3 do not govern. Load combination 6 does not comply
with
HUD 24 CFR 3280.
[End table]
Step 3: Select Foundation Type and Materials
The example statement specified a CMU pier and ground anchor foundation
type.
Since the flood velocity is 2 fps, CMU piers must have surface bonded
mortar
that meets ASTM C887-79a (2001) and ASTM C946-91 (2001) and maintain
bonding
between CMUs.
Step 4: Determine Forces at Connections and on Foundation Components
CMU piers transfer the compressive loads from the manufactured home into
footings and then into the ground. The masonry piers are not considered to
provide any lateral or uplift resistance. The governing load combination
for
downward forces is the vertical failure mode (load combination 4), which
produces a total downward force from the manufactured home equal to
(downward force)(length of manufactured home)
Therefore, (1,070 lbs)(60 ft) = 64,200 lbs
This downward force governs the number of footings and, therefore, piers
needed to transfer the downward load into the ground.
Following the braced masonry pre-engineered foundation design for flood
velocities over 2 fps specification given in Chapter 10 of the use of a
dry-
stack 16-inch by 8-inch block pier with a minimum of an 1/4-inch thick
surface bonded mortar and a 24-inch square, 10-inch deep footing,
calculate
the number of footings needed to adequately transfer the downward loads to
the ground.
Required footing area = comprehensive load/allowable soil bearing capacity
Consult the geotechnical engineer for the ultimate soil bearing capacity
value. An approximate method to calculate the ultimate soil bearing
capacity
is to multiply the allowable soil bearing capacity by a safety factor. The
maximum pressure given in the NFPA 5000 Soil Classification Table can also
be
used as the ultimate soil bearing capacity.
Required footing area =64,200 lbs/1,000 psf = 64 ft squared
Individual footing area = (24 in x 24 in) 1ft squared/144 in squared =
4 ft squared
Number of footings =total required footing area/individual footing area =
16
footings/piers
Therefore, provide 8 piers per side of the manufactured home
Pier spacing = manufactured home length/(number piers per side-1)= 60
ft/8-1
= 8.6 ft
The maximum spacing of the piers is set to 8 feet to provide effective
floodborne debris protection. To protect against floodborne debris, it is
assumed that 1 pier will be lost due to floodborne debris.
Minimum number of piers = 60 ft squared/8 ft +1= 8.5 piers, say 9 piers
per
side (i.e., 8 spaces at 7.5 feet). Therefore, the home will be supported
by a
total of 18 piers (9 piers on each side) spaced at 7.5 feet.
Lateral wind loads are resisted by the strapping and ground anchors. The
final number of piers equals the initial guess; therefore, the lateral
load
on the piers does not have to be updated.
Calculate the number of anchors needed to resist sliding failure.
The recommended design stiffness of the anchors in Table 7-5 in this guide
is
given for 5-foot anchors installed at 45 degrees and axially loaded is
1,200
lb/in (Figure 7-4). The horizontal component of the ground anchors
strength
is equal to
(1,200 lbs/in)(cos 45°) = 848 lb/in
The manufactured home industry gives an allowable lateral manufactured
home
movement of 3 inches. So the total lateral strength of a ground anchor is
(3
in)(848 lbs) = 2,544 lbs.
Number of ground anchors needed = lateral load/anchors lateral capacity
Number of ground anchors needed = (313 lb)(60 ft)/2.544 lbs = 8 anchors
per
side
Calculate ground anchor spacing 60 ft/(8-1)= 8.5 ft
The anchor strapping should attach into a wall stud; therefore, anchor
spacing must be adjusted to 16-inch increments.
Both uplift and overturning failure modes are resisted by the vertical
strength of ground anchors. The uplift forces will be resisted by all the
ground anchors and the overturning moment will be resisted only by the
windward ground anchors.
For the worst uplift of the vertical failure mode, load combination 7
(refer
to Table F-6) governs. However, the maximum net uplift is 16 plf downward,
which means that overturning will govern the uplift requirements.
For the overturning failure mode, load combination 7 (refer to Table F-10)
governs for wind perpendicular to the roof ridge. Overturning moment is
only
resisted by the windward anchors. Therefore, the total vertical load each
anchor will have to resist is
(overturning moment)(length fo Home)/moment arm/number of anchors per side
=
(979 ft-lbs)(60 ft)/12 ft/8 anchors = 612 lbs per anchor
The vertical component of the anchor stiffness equals
(1,200 lbs/in)(cos 45) = 848 lbs/in
The manufactured home industry gives an allowable vertical movement of 2
inches. This results in a vertical strength per anchor equal to
(2 in)(848 lbs/in) = 1,697 lbs per anchor
This is more than the strength needed by each anchor to resist the
overturning moment.
The anchor strapping should attach into a wall stud and, therefore, anchor
spacing must be adjusted to 16-inch increments. Place anchors at each end
of
the home and space at 72 inches.
For the overturning case, the connection of the straps to the stud and the
ground anchor embedment is based on MWFRS pressures. However, although it
would likely not govern, to be thorough, the uplift only condition using
C&C
pressures should be checked for these two anchorages (straps to studs and
anchors in ground).
Step 5: Specify Connections and Framing Methods Along with Component
Dimensions to Satisfy Load Conditions
The CMU pier and ground anchor foundation will consist of 16 dry-stack,
16-
inch by 8-inch block piers with a minimum of a 1/4-inch thick surface
bonded
mortar and 24-inch square, 10-inch deep footings. Ground anchors will be
placed at 45-degree angles at each end of the manufactured home and spaced
at
72 inches.
Step 6: Note All Design Assumptions and Details on Drawings
Refer to pre-engineering foundation design drawings contained in Appendix
H
and specifications presented in Chapter 10 herein as to how to adequately
document assumptions and detail drawings.