Diaphragms With End Horizontal Offsets - The Analysis of Irregular Shaped Structures - page 87

produce tension in the upper half of the strut and compression in the lower half, by
inspection. The strut force diagram is drawn at the left of grid line 1. At grid line 2, the
wall is located near the center of the wall line, with struts placed on the plan above and
below the wall. The direction of the shears transferred into the strut above the wall shows
that it is acting in compression. The strut below the wall is acting in tension. The strut
force diagram is drawn at the right of grid line 2. The direction of the shears transferred
into the top and bottom chord shows that they are in compression and tension, respec-
tively, in accordance with typical diaphragm behavior. From this example, it can be seen
that the sheathing elements provide a valuable visual reference of how the shears are
distributed through the diaphragm and into the boundary elements. This will become
even more apparent when we analyze a complex diaphragm.
Initially, load paths across discontinuities are typically assumed based on engineer-
ing judgment and then verified by analysis. Throughout the analysis, construction of
the collector/chord force diagrams will prove or disprove the assumed load paths and
load distributions. In some cases the force diagrams will not close to zero, which indi-
cates that an error exists in the assumptions. In that event, the engineer must evaluate
the initial results and adjust the analysis accordingly. Solving for only a few key mem-
bers will leave the design incomplete and possibly leave a major error undiscovered.
The method of analyzing a diaphragm with a horizontal offset (notch) is summa-
rized below (see Fig. 3.4). The basic shear values shown in the figure are presented only
for the purpose of describing the procedure.
Procedure
1. Solve for the diaphragm reactions.
2. Construct the basic shear diagram (unit shears, plf or klf).
3. Find the shear values at critical locations on the basic shear diagram (at all col-
lector locations and areas of discontinuities).
4. Find the chord force at the discontinuity (notch at 2B). This step is not manda-
tory because the force can be determined by using the same procedures as those
used to find the force acting on the other members.
5. Determine the additional shear applied to the transfer diaphragm from the
transfer of the disrupted chord force (transfer diaphragm shears).
6. Determine the net diaphragm shears in the transfer diaphragm area by adding
or subtracting the transfer diaphragm shears from the basic diaphragm shears.
7. Place all the sheathing elements and the direction of the shears being trans-
ferred into all the chords, struts, and collectors on the plan.
8. Determine all the strut, collector, and chord forces. The force is equal to the area of
the shear diagram. Sum the forces along the length of the collectors or line of lateral
force resistance. The force diagrams must close to zero, or else an error exists.
9. Determine the diaphragm-nailing requirements.
10. Design all struts, collectors, chords, and connections, applying the over-strength
factor as required by code.
The section of the diaphragms to the left of the discontinuity, bounded by grid
lines 1 and 2 from A to B, shown in Fig. 3.4 represents the section of the diaphragm with
a reduced diaphragm depth. The reduced depth causes a disruption in the diaphragm