Diaphragm Basics - The Analysis of Irregular Shaped Structures - page 34

section. This concept can be seen in Prob. 2.1 at the end of the chapter and in Chap. 8.
For wind loading, the internal pressures are added to the windward and leeward
pressures and are then combined as a unit uniform load which is applied to the wind-
ward side of the diaphragm. At areas where diaphragms contain openings, it can be
advantageous to apply the pressures on both sides of the diaphragm opening in
accordance with their respective windward and leeward pressure distribution. Apply-
ing the separate wind pressures to both sides of the opening will reduce the shears
and nailing requirement in both sections. Calculation of the seismic unit strip loads in
the individual sections is in accordance with the depth of each section as previously
described for wind.

2.4 Diaphragm Boundary Elements

Diaphragm boundary members, drag struts, and collectors are perhaps the most mis-
understood elements of a diaphragm. Although sections of the code generally describe
where they are required, detailed information on their use and design is somewhat
lacking. A diaphragm boundary is defined as a location where shear is transferred into or
out of the diaphragm sheathing. Transfer is either to a boundary element, which is
typically a drag strut or collector, or to another lateral-force-resisting element such as a
shear wall, braced frame, or moment-resisting frame. All edges of a diaphragm are
defined as the boundaries of the diaphragm. A partial-length shear wall with a drag
strut or collector extending across the full depth of the diaphragm provides support for
the diaphragm and therefore is defined, by code, as a diaphragm boundary. This
includes the interior shear wall which supports the inside edges of both diaphragms in
the two-span diaphragm. Code also provides a definition of a drag strut as a diaphragm
boundary element parallel to the applied load that collects and transfers diaphragm
shear forces to the vertical elements of the lateral-force-resisting system. Figure 2.9
shows two diaphragm configurations, a single-span diaphragm and a two-span dia-
phragm. In both cases, the diaphragms are loaded in the transverse direction. The top
and bottom edges of both diaphragm configurations consist of partial-length shear
walls and/or a continuous boundary member, which act jointly as the diaphragm
chords. The left edge of the single-span diaphragm is supported by a full-length shear
wall. The right edge is supported by a partial-length shear wall and a drag strut. All
edges of this diaphragm are in compliance with the code. The two-span diaphragm is
assumed to be two separate simple spans in accordance with accepted engineering
practice, designated as diaphragm 1 and diaphragm 2. Diaphragm 1 is supported on
the left and right edges by partial-length shear walls SW1, and SW2, respectively. Since
neither of the walls supporting diaphragm 1 extends the full depth of the diaphragm, a
drag strut or collector must therefore be installed to support the remaining edges of the
diaphragm. Diaphragm 2 is supported on the left edge by the shear wall and collector
that support the right edge of diaphragm 1. The right edge is supported by a partial-
length shear wall and a drag strut. Both diaphragm sections of the two-span diaphragm
are in compliance with code requirements. In the longitudinal direction, both wall lines
that support the diaphragm consist of shear walls and drag struts along their entire
length. Both diaphragms are in compliance with code in both directions.
Figure 2.10 demonstrates how the collectors coming off of shear walls SW1 and
SW2 function, showing that they collect the unit shear from the diaphragm sheathing
and transfer the accumulated shear forces into the shear walls. The drag strut or