Diaphragm Basics - The Analysis of Irregular Shaped Structures - page 63

F i g u r e 2.30 Examples of chord and strut discontinuities.
Figure 2.30 shows a typical residential structure that is a simple rectangular dia-
phragm with supporting shear walls at each end of the structure. Entry areas with dor-
mers have been placed at locations A and B. For architectural reasons, the diaphragm
chord is not allowed to continue through the openings. Frequently, one or both entry
walls are set back from the exterior wall, or the diaphragm sheathing under the dor-
mers is omitted, creating a disruption in the diaphragm chord and web. Because of the
lack of horizontal members at the diaphragm chord locations, hinges are created at the
ridge and at the bottom of the rafter to wall or main roof connections. Since the hinges
cannot transfer the diaphragm chord forces across the discontinuity, the load path is trans-
ferred through the roof diaphragm, which is the stiffer element. The diaphragms under
this framing condition would have to be designed as having an intermediate horizontal
offset or notch. A pop-out section has been included at location C. The roof joists span
across the main wall line and bear on the pop-out wall. This bearing condition causes a
vertical offset in the diaphragm, preventing the main roof diaphragm chord from pass-
ing through the pop-out area. The transfer of the disrupted chord force across the verti-
cal offset at C can be difficult. A method of transferring the disrupted chord force across
the vertical offset at location C is shown in Figs. 2.31 and 2.32. The vertical offset can
vary in height. A method of transferring the force across the offset can be seen in
Fig. 2.31. A vertical post is cantilevered above the low roof to transfer the disrupted
chord force. In the upper connection, tie straps are installed up the post and over the top
of the roof. The strap is nailed onto the blocking between the joists. For the lower con-
nection, a tie strap is wrapped around the post and is nailed into the wall top plate.