Diaphragm Basics - The Analysis of Irregular Shaped Structures - page 79

across the discontinuities. The upper plan in Fig. 2.42 is loaded in the transverse direc-
tion with supporting shear walls located at grid lines 1 and 4. The framing is oriented
parallel to the applied load. The horizontal offset at grid line B creates a discontinuity
in the diaphragm chord at grid line 2B. The disrupted chord is extended into the trans-
fer diaphragm located between grid lines 2 and 3 by a collector, where its force is then
transferred out to the transfer diaphragm chords located at grid lines A and C. When
that same diaphragm is loaded in the longitudinal direction, as shown in the lower plan
of Fig. 2.42, the load path through the discontinuity is the same as the transverse load-
ing. For this case, the diaphragm can be visualized as two separate diaphragms, which
consist of the section between grid lines 1 and 2 from A to B (section A) and the section
between grid lines 2 and 4 from A to C. Section A is supported by the shear wall at grid
line A and by the strut located at grid line B. Since there is no vertical lateral-force-
resisting element along grid line B, the support of the strut must be provided by the
transfer diaphragm. The strut force is transferred out of the transfer diaphragm into the
supports located at grid lines A and C.
The upper diaphragm shown in Fig. 2.43 has the framing oriented 90° from the
previous examples. Assume that the interior shear wall shown has not yet been installed.
The supporting walls for the diaphragm would then be located at grid lines 1 and 4. The
offset of the chord from C to B causes a disruption of the chord at grid line 2B, which
creates a notched diaphragm. Since the framing is oriented parallel to the chord, the
transfer diaphragm must also be oriented in that direction. Without the interior shear
wall, the transfer diaphragm must extend between grid lines 1 and 4. It should be
remembered that all transfer diaphragms or sections being used as individual dia-
phragms must meet code-allowable aspect ratios and that collectors that are embedded
into transfer diaphragms must extend the entire depth of the transfer diaphragm. Main-
taining the required aspect ratio for the transfer diaphragm could cause the depth of the
transfer diaphragm to extend from grid line A to B, making it impractical. An alternate
approach would be to analyze the diaphragm as a simple span diaphragm that occurs
between grid lines 1 to 4 from A and B. This assumes that that the main section complies
with the allowable aspect ratio. The section between grid lines 2 and 4 from B to C
would be supported off of the main diaphragm. If an optional interior shear wall is
installed as shown in the figure, two separate diaphragms are created. The diaphragm
on the left has a horizontal offset in the bottom chord at grid line 2B, and the diaphragm
on the right is a simple span. The diaphragm between grid lines 1 and 3 is analyzed as
a notched diaphragm with the transfer diaphragm oriented horizontally. The right sup-
port for the transfer diaphragm is the collector coming off of the interior shear wall. The
installation of the interior shear wall allows a shorter and shallower transfer diaphragm,
making it more economical and easier to construct. The diaphragm at the bottom of the
Fig. 2.43 shows the same diaphragm loaded in the longitudinal direction. The dia-
phragm support walls are located along grid lines A and C. The top chord occurs along
grid line 1 which is offset to grid line 2, requiring the diaphragm to be designed as a
notched diaphragm. The transfer diaphragm is the same configuration as the previous
example.
All the diaphragms in Figs. 2.44 and 2.45 are loaded in the longitudinal direction.
The upper diaphragm in Fig. 2.44 has supporting walls along grid lines A and B only.
Since there are no vertical supporting elements along grid line C, the diaphragm sec-
tion between grid lines A and C from 3 to 4 must be supported by a strut along grid
line C, which is embedded into and supported off of the end of the transfer diaphragm.