Civil Engineering Reference
In-Depth Information
Optimizing the transfer diaphragm and collector system requires careful consider-
ation. Issues critical to the layout and design of a transfer diaphragm are
1. Aspect ratios
2. The magnitude of the resulting shears—capacity vs. demand
3. Direction of the framing
4. Splitting issues
5. Collector length
6. Diaphragm blocking requirements
For transfer forces, the redundancy factor rho (ρ shall be the same as that used for
the structure in accordance with ASCE 7-05 Section 12.10.1.1. Special attention should
also be given to ASCE 7-05 Section 12.10.2.1 regarding the required use of the over-
strength factor, Ω
o
.
3.3
Diaphragm Deflection
The method of calculating the deflection of a simple rectangular diaphragm has been
established and refined over the past few decades by extensive testing and research
conducted by the American Plywood Association (APA).
5
The building code sections
and AF&PA SDPWS
6
sections addressing diaphragm deflection require that deflection
be checked to verify that the diaphragm and any attaching element will maintain struc-
tural integrity under the design loads and continue to support the design loads without
danger to the occupants of the building. It is interesting to note that even though that
requirement has been in the code since the 1940s, it is rarely checked by the design pro-
fessional and seldom requested by building officials. ATC 7 noted that in this regard,
there is an inconsistency between the code requirement and the standard of practice. It
has become common to assume that diaphragms, regardless of their shape, still act as
simple rectangular diaphragms. It is often believed that as long as the allowable aspect
ratios are maintained, deflections will be within acceptable limits and calculations are
not necessary. For regular diaphragms this may be a reasonable assumption; however,
it would be prudent to verify the deflection of irregular-shaped diaphragms due to
their nonuniform shape and the demand placed on their performance.
Simple Rectangular Diaphragm Deflection
The method of calculating deflection for simple rectangular diaphragms is covered in
the code and nearly every publication regarding diaphragm design. The four-term
equation found in the IBC
7
and several APA publications is based on simple span rect-
angular diaphragm tests of constant depth that are uniformly loaded and have uniform
nailing with all edges blocked throughout the entire length:
∑
3
vL
EAb
(
∆
X
)
5
8
vL
G
C
∆=
++
0 188
.
Le
+
IBC Eq. 23-1
4
n
2
b
t
where
A
= area of chord, in
3
b
= diaphragm width/depth, ft
E
= elastic modulus of chords, psi
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