Diaphragm Basics - The Analysis of Irregular Shaped Structures - page 80

The transfer diaphragm functions as a propped cantilever beam. The upper diaphragm
shown in Fig. 2.45 also has its supporting walls located along grid lines A and B and has
the section between grid lines A and C from 2 to 4 supported by the strut along line C.
The drag force at grid line C must be transferred across a transfer area bounded by grid
lines 2 to 3 from B to C into the collector coming off of the shear wall at grid line B. The
offset of the strut and collector causes a couple which must be resisted by the offset
chord at grid line 2 and the collector at grid line 3, which oppose the couple forces. The
collector at grid line 3 transfers the opposing force into the shear wall at that line. The
disrupted chord at grid line 2 is embedded into a transfer diaphragm that has its sup-
ports located at grid lines 2 and 3.
The lower diaphragm in Fig. 2.45 has supporting walls along grid lines A, B, and C.
The wall lines at lines B and C are assumed to act in the same line of resistance. For that
to happen, the shared shear wall forces that are transferred across the offset produce a
couple that is resisted by the collectors of the transfer diaphragm. The direction of the
framing in the lower diaphragm of Fig. 2.45 causes the transfer diaphragm to be ori-
ented parallel to the applied load and could extend in width from grid line B to A due
to the depth of the diaphragm. While the framing examples presented cover most of the
conditions encountered in today's structures, other layouts and load paths are possible.
The ability to visualize how forces are applied and transferred through areas of discon-
tinuity is key to solving the load path problems. These examples will be fully developed
in later chapters.
Problem 2.1: Load Distribution into a Diaphragm (See Fig. P2.1)
Given: The diaphragm shown is a one-story structure with shear walls placed at the
exterior walls only. Wind loads are shown acting in the transverse and longitudinal
directions.
Find:
1. Break the wind loads shown acting in the longitudinal direction into strip loads
w 1 through w 9 . Determine their magnitude.
2. Determine seismic strip loads w 1 through w 9 . Walls occur at the exterior wall
lines only.
3. After reviewing Chaps. 3 through 8, solve for all strut, collector, and chord
forces in the transverse and longitudinal directions.
DL roof = 20 psf
DL walls = 10 psf
Wall height = 15 ft
Wind load to diaphragm = 200 plf as shown
Occupancy Category II
I e = 1.0, I w = 1.0
C s = 0.112 (ASD) ▲