Code Sections and Analysis - The Analysis of Irregular Shaped Structures - page 12

There have been many debates on the necessity of full-depth blocking at exterior
wall lines, especially in areas of low to moderate seismicity. Some stakeholders are
pushing for half-height blocking or to eliminate the blocking entirely. These efforts are
not consistent with the goal of providing a complete load path and should be scruti-
nized for rationality and substantiation. Attempting to transfer diaphragm shears
through the truss top chord would put the truss top chord in cross-grain bending at the
truss heel joint if partial-height blocking were used and could potentially pull off the gang-
nail plate. This type of failure has been observed in the field. In the case where blocking is
eliminated, the truss would have to transfer the shears into the wall top plate by roll
over action. The APA has conducted tests on sloped mobile home roof diaphragms. An
interesting mode of failure was that the gang-nail plates at the ridge line joint of the
trusses were pulled apart by opposing shear forces in the diaphragm sheathing because
blocking was not provided for the sheathing at the ridge joint. Substantial testing for
gravity plus torsion (rollover) or gravity plus cross-grain forces would have to be con-
ducted on gang-nail trusses before serious consideration could be given to reducing the
full-depth blocking requirements. Also, trusses would have to include these rotational
forces in their design. Configuration B is the same framing condition, but the truss is a
deep heel configuration. Under this condition, a prefabricated shear panel consisting of
2× members with plywood sheathing replaces the solid blocking. The load path is the
same regardless of the blocking material installed.
Figure 1.4 shows the condition where low roofs frame into the walls at midheight of
the studs. Configuration A shows the condition where the ledger is attached directly
to the wall studs. The exterior wall sheathing is terminated at the low roof elevation,
and the shear wall sheathing is continued on the inside of the wall. The upper wall
shears are transferred into the double blocking, through the nailing connecting the dou-
ble blocking together and then into the inside wall sheathing. The lower roof shears are
transferred from the diaphragm sheathing into the ledger, into the double wall blocking,
and then into the inside wall sheathing. A more direct load path would be to continue
the exterior wall sheathing full height of the wall and attach the ledger on the outside
of the sheathing. Configuration B shows the condition where the wall sheathing is dis-
rupted at the interface of the wall and ledger. The shear from the upper wall sheathing
is transferred into the blocking, into the ledger, back into the lower blocking, and then
back into the wall sheathing. The low roof shears are transferred into the ledger, then
into the lower blocking and wall sheathing.
Figure 1.5 shows two floor framing sections. Joints in the wall sheathing can occur
at many locations in the floor framing area. There are no guarantees where these joints
will occur unless the joint locations are specifically detailed in the drawings. The nail-
ing required to establish a complete load path should be based on the worst-case sce-
nario, assuming that the joints will fall at the locations shown in configuration B and
that the sheathing will not be lapped onto the rim joist. Configuration A is somewhat
unconventional because the floor joists are hangered off of the wall instead of being in
direct bearing, as shown in configuration B. The wall shear is transferred from the
upper wall into the wall double top plate below and then back into the outer sheath-
ing. The floor shears are transferred from the floor sheathing into the double top plate,
then into the wall sheathing. In both cases, the shears are then transferred into the
lower floor or roof through the blocking that is nailed to the edge joist. The low floor
or roof sheathing is nailed to the edge joist, which completes the load path. Configura-
tion B represents the common method of framing a floor onto a bearing wall. Since