Civil Engineering Reference
In-Depth Information
pressures due to the effects of earthquake motion must also be applied in accordance with
ASCE7-5 Sections 11.8.3 and 12.1.5. As an example, assume the following:
Given
:
Assume wind controls the design.
The addition of dynamic soil pressures is not required.
h
1
= 10 ft upper story plus 1 ft of projected area for the depth of the floor
h
2
= 9 ft basement
p
= 18 psfwindpressur,
q
= 65 pcf, soil equivalent fluid pressure (at rest)
Uniform load to the diaphragm from the soil and wind pressure:
wp
h
+
q h
h
()
3
h
+
q h
()
2
=
1
+
1
2
=
p
1
+
1
2
2
6
2
6
2
10
2
+
65 9
6
()
2
=
18
+
1
=
985 5
.plf
This is a significant load which would make it difficult to provide a reasonable dia-
phragm design. If the floor joists bear on top of the sill plate, three 8d common nails are
typically installed for the joist-sill plate connection in accordance with IBC Table
2304.9.1. It is obvious that this nailing alone will not transfer the shear of 986 plf times
the joist spacing from the sill into the joist. The connection design must include the load
duration factor of
C
D
= 0.9, because soil pressure is considered to be a permanent load,
in accordance with the NDS.
4,5
The connections providing a continuous load path into
the diaphragm must be completely designed. The complete load path starts with the
soil pressure being applied to the basement wall. That load is transferred into the anchor
bolt, which applies a concentrated load into the 2× or 4× sill plate. The load is trans-
ferred into the joist/plate connection and then into the joists. The force is finally trans-
ferred into the diaphragm by special nailing of the sheathing to the joists. If the joists are
oriented parallel with the wall, blocking must be installed between the joists in a suffi-
cient number of bays to effectively transfer the force into the diaphragm. The connec-
tions of the blocking to the joists and diaphragm must include the effects of the eccentric
force applied from the bottom of the blocking to the underside of the sheathing. The
shear capacity of the bolt perpendicular to the grain combined with shears applied
parallel to the wall (as they occur), the shear capacity of the sill plate with the reduced
depth to the anchor bolt as previously discussed in Chap. 1, the joist-sill plate connec-
tion, and nailing of the sheathing to the joist or blocking must all be designed. The
problem with the lack of capacity in the sill plate and the anchor bolts can be avoided if
the joists are hangered off of the sill plate with top flange hangers. In that configuration, the
lateral force at the top of the wall is transferred directly into the joists by bearing, and the
bolt spacing can be increased. Attention should also be given to detailing the sill
plate flush with the inside face of the wall to provide full bearing against the joist and
to ensure that the top flanges of the hanger are installed in accordance with the manu-
facturer's installation requirements. The shear connection along the diaphragm sup-
port walls and the diaphragm nailing also require special review. The shear per linear
foot will be very high given the calculated load in the example.
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