Diaphragm Basics - The Analysis of Irregular Shaped Structures - page 25

shallow solid beam. This concept has been widely accepted in the engineering com-
munity. Other publications have also noted the same observation, citing that tests
were not set up in a manner to accurately record the exact distribution. Regardless of
whether the shear stresses are parabolic or uniform, the basic shear diagram is plotted
as shown in the figure, with positive shears occurring on the left side of midspan of
the diaphragm and negative shear occurring on the right side. A symbol representing
1-ft by 1-ft square pieces of sheathing, referred to as sheathing elements , has been added
to the basic shear diagram to show the direction of the unit shear forces (plf) acting on
the edges of the sheathing elements. The maximum moment occurs at the point on the
shear diagram where the shear is zero. A simple rectangular diaphragm is analyzed
as follows:

W = uniform distributed load

L = length of diaphragm

D = depth of diaphragm

X = distance from support under consideration

wL

R ===

2

V diaphragmreactionatsupport

V
==diaphragmunitshear at support

v

VRwx

x =− = shearatany distance

x from support

wL

2

M

max =

= maximummoment

8

2

wx

M

x =− =

x moment at anydistance romthe support x

2

M
D

F == chordforce

chord

Code limits the maximum length-to-width aspect ratios of flexible wood and steel
deck diaphragms. Two tables regarding allowable aspect ratios for wood diaphragms
and steel deck diaphragms have been included in Fig. 2.3 for the convenience of the
reader. These limitations were established, based on tests conducted on diaphragms by
the APA and by the Steel Deck Institute. The limits on the aspect ratios apply not only
to the diaphragm as a whole, but also to individual sections of the diaphragm (e.g., seg-
ments around an opening in the diaphragm).
Code requires that all boundaries of a diaphragm be supported by struts, shear
walls, or other vertical lateral-force-resisting elements. When the structure shown in
Fig. 2.1 is loaded in the transverse direction by wind or seismic loads, as shown in
Figs. 2.5 and 2.6, respectively, both longitudinal walls at grid lines 1 and 2 act as dia-
phragm chords that serve as boundary members or elements. In accordance with the
definitions included in Section 11.2 of ASCE 7, 2 boundary members and boundary ele-
ments are basically the same thing. The full-length shear wall at grid line B is a bound-
ary member which serves to support one end of the diaphragm. The wall receives
uniformly distributed diaphragm shears along the entire depth of the diaphragm. The
partial-length shear wall at grid line A also serves as a boundary member and provides
the support for the other end of the diaphragm. Since the wall extends only a short
distance across the depth of the diaphragm, a boundary member known as a drag strut