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A. Gravity Load Considerations for the Type C Single-Section Unit. 1.
A. Gravity Load Considerations for the
Type C Single-Section Unit.
1.
General: The foundation to support
the superstructure gravity loads is
provided only by the spaced piers
under the chassis beams.
2.
Superstructure load to a pier: As
shown in Figure D-8A the snow
load, the attic live load and the roof
dead load are transferred equally to
each exterior wall. The exterior
walls in turn transfer the roof loads
to the floor framing. The floor live
and dead load combine with the
roof and wall load to reach the chas-
sis beam, where the foundation
piers receive the total concentrated
superstructure load (Rp) in propor-
tion to the pier spacing.
(
)
(
)
Rp
Pf
Wt
DL
spacing
=
+
+
×
+





×
40 10
2
2

3.
Typical chassis beam pier founda-
tion weight: The typical pier as-
sumed for the calculations is based
on a pier composed of four
8x8x16 concrete masonry units
grouted solid with a 2 foot square
footing that is 8 inches deep. Thus
the assumed pier weight is as fol-
lows:
pier: 2.67
× 1.33 × 84 psf =
298.0 lbs.
footing: 150 pcf
× 2 × 2 × .67 = 402.0 lbs.


total = 700.0 lbs.
4.
Required chassis beam Pier Footing
size: The footing area (Aftg) must
be large enough so that the net al-
lowable soil bearing pressure (Pso)
is not exceeded under the full grav-
ity dead, live and snow loads. Note
that the pier and footing weight be-
come additional dead load.

Aftg
Rp
Pso
=
+ 700

B. Gravity Load Considerations for the
Type E and I Single-Section Unit.
1.
General: Support of the superstruc-
ture gravity loads is shared by the
exterior longitudinal walls and the
spaced interior piers under the chas-
sis beams, which together comprise
the foundation.
2.
Superstructure load to the exterior
longitudinal foundation walls: As
shown in Figure D-8B, the snow
load, the attic live load and the
heavy roof dead load are transferred
equally to each exterior wall. The
exterior wall weight is added, and
both loads transfer directly to the
exterior foundation walls. A por-
tion (dc/2) of the floor live and
heavy dead load also goes to the ex-
terior foundation walls. The total
superstructure gravity load (Rw)
transferred to the exterior founda-
tion wall is in units of lbs./ft. of
home length. The equation is as
follows:


(
)
[
]
(
)
Rw
Pf
Wt
dc
=
+
+
×
+
×
+
9 7 10
2
40 13
2
44 25
.
.
+


[snow + (roof DL+attic LL)]

(floor LL+DL) + (wall DL)
D - 18 3.
Superstructure load to an interior
pier: The remainder of the floor
dead and live load is equally di-
vided between the chassis beam
lines, and concentrated at the foun-
dation piers based on their spacing.
The total superstructure concen-
trated gravity load to a pier (Rp) is
as follows:
(
)
Rp
Wt dc
spacing
=
+
× +





×
40 13
2
9


(floor DL+LL) (chassis beam DL)
4.
Typical exterior longitudinal foun-
dation wall weight: The typical ex-
terior longitudinal foundation wall
is assumed to be composed of a 6
poured concrete wall, 3-8 high,
and a 6 x 24 continuous concrete
footing. Thus, the assumed weight
is as follows:
wall: 150 pcf
× 3.67 × 0.5 = 275.0 plf
footing: 150 pcf
× 2 × 0.5 = 150.0 plf

total = 425.0 plf
5.
Required Exterior Wall Footing
Width: The footing width (Wf)
must be large enough so that the net
allowable soil bearing pressure
(Pso) is not exceeded under the full
gravity dead, live and snow loads.
Note that the longitudinal founda-
tion wall and footing weight be-
come additional dead load. The re-
quired footing width:

Wf
Rw
Pso
=
+ 425
6.
Required Interior Pier Footing
Area: The footing area (Aftg) must

Gravity Loads
Figure D -8B
D - 19 be large enough so that the allow-
able soil bearing pressure (Pso) is
not exceeded under the full gravity
dead and live loads. Note that the
pier and footing weight become ad-
ditional dead load. The required
footing area:


Aftg
Rp
Pso
=
+ 700

C. Gravity Load Considerations for the
Type C Multi-Section Unit with a Continu-
ous Superstructure Marriage wall.
1.
General: The foundation to support
the superstructure gravity loads is
provided only by spaced piers under
the chassis beams and under the
continuous marriage wall.
2.
Superstructure continuous marriage
wall load to a pier: As shown in
Figure D-9A the snow load, the at-
tic live load and the roof dead load
are transferred between the mar-
riage wall and the exterior walls as
bearing walls. The marriage wall in
turn transfer the roof loads to the
floor framing. A small portion of
the floor live and dead load is as-
sumed to combine with the roof
loads and marriage wall weight to
reach the top of the foundation pier
as the total concentrated superstruc-
ture load (Rpm) in proportion to the
pier spacing.
(
)
Rpm =
Pf + 19.7
Wt +
(40 + 13) dc
spacing
52 5
.
+
×
×





×



[marr.wall+(snow+roofDL+attic LL)]

(floor
DL+LL)

3.
Superstructure load to an exterior
chassis beam pier: As shown in
Figure D-9A the snow load, the at-
2Wt
Superstructure
Free-body
Diagram
Foundation
Free-body
Diagram
dc
Pf
attic LL = 10 psf
LL = 40 psf
588
lbs.
700
lbs.
700
lbs.
700
lbs.
700
lbs.
Pso
A
ftg
Pso
Wt-2d
c
Continuous
Marriage Wall
Rpi
Rpm
Rpi
Rpe
Rpe

Type C - Multi-Section Unit w/Continuous Marriage Wall
Figure D - 9A
D - 20 tic live load and the roof dead load
are transferred equally between the
exterior wall and the marriage wall.
The exterior wall in turn transfers
the roof loads to the floor framing.
The floor live and dead load com-
bine with the roof and wall weight
to reach the chassis beam, where the
foundation piers receive the total
concentrated superstructure load
(Rpe) in proportion to the pier spac-
ing.
(
)
Rpe = Pf + 19.7 + 40 + 13
Wt / 2 +
(44.25 + 9)
spacing
×





×

[snow+roofDL+atticLL+floorDL+LL]

(ext.wall DL+chassis bm.)
4.
Superstructure load to an interior
chassis beam pier: As shown in
Figure D-9A The floor live and
dead load comprise the only load to
reach the interior chassis beam,
where the foundation piers receive
the total concentrated superstructure
load (Rpi) in proportion to the pier
spacing.
(
)
Rpi = 40 + 13
Wt - dc
spacing
×


+





×
2
9


[(floorLL+DL)+chassis
bm.]
5.
Typical Continuous Marriage Wall
Pier: The typical continuous mar-
riage wall within the superstructure
of the multi-section unit is assumed
to have a foundation pier composed
of five courses of 8x8x16 con-
crete block (ungrouted), and a con-
crete footing 2x2 by 8 deep. The
dead load of a typical continuous
marriage wall foundation pier is as
follows:
pier: 42 psf
× 3.33 × 1.33 = 186.0 lbs.
footing: 150 pcf
× 2
2
× .67 = 402.0 lbs.


total = 588.0 lbs.
6.
Required continuous marriage wall
pier footing. The footing area
(Aftg) must be large enough so that
the net allowable soil bearing pres-
sure (Pso) is not exceeded under the
full gravity dead, live and snow
loads. Note that the pier and foot-
ing weight become additional dead
load. The required footing area:

Aftg
Rpm 588
Pso
mar
=
+

7.
Required exterior chassis beam
Pier Footing Area: The footing
area (Aftg) must be large enough so
that the allowable soil bearing pres-
sure (Pso) is not exceeded under the
full gravity dead and live loads.
Note that the pier and footing
weight become additional dead
load. The required footing area:

Aftg
Rpe 700
Pso
ext
=
+

8.
Required interior chassis beam Pier
Footing Area: The footing area
(Aftg) must be large enough so that
the allowable soil bearing pressure
(Pso) is not exceeded under the full
gravity dead and live loads. Note
that the pier and footing weight be-
come additional dead load. The re-
quired footing area:
D - 21
Aftg
Rpi 700
Pso
int
=
+


D. Gravity Load Considerations for the
Type C Multi-Section Unit with a Super-
structure Marriage wall containing one
opening or two large adjacent openings.
1.
General: The foundation to support
the superstructure gravity loads, as
illustrated in Figure D-9B, is pro-
vided only by spaced piers under
the chassis beams, piers placed un-
der the posts at the ends of marriage
wall openings, and equally spaced
piers under the continuous portions
of the marriage wall.
2.
Marriage wall openings: limitations
and assumptions. Two marriage
wall opening situations were re-
viewed: (1) a single opening, as il-
lustrated in Figure D-9C, is
bounded by posts at the ends of the
opening with continuous marriage
walls extending beyond the opening
width in both directions, and (2)
two adjacent marriage wall open-
ings, as illustrated in Figure D-9D,
consisting of three posts with the
ou