Need a book? Engineering books recommendations...

# Re: Plywood rigid diaphragms

• To: seaint(--nospam--at)seaint.org
• Subject: Re: Plywood rigid diaphragms
• From: ErnieNSE(--nospam--at)aol.com
• Date: Tue, 1 Sep 1998 23:42:35 EDT

```I am not done with this thread yet.

1997 UBC Chap 16 Div IV 1630.6 Horizontal Distribution of Shear Third

"Diaphragms shall be considered flexible for the purposes of distribution of
story shear and torsional moments  when the maximum lateral deformation of the
diaphragm  is more than two times the average story drift of the associated
story. This may be determined by comparing the computed midpoint in-plane
deflection of the diaphragm itself under lateral load with the story drift of
adjoining vertical resisting elements under equivalent tributary lateral

"The design story shear..........shall be distributed to the various elements
of the vertical lateral resisting system in proportion to their rigidities,
considering the rigidity of the diaphragm."

Chap 23 Div II 2315.1 reads:

".........wood structural panel horizontal and vertical diaphragms may be used
to resist horizontal forces in the horizontal and vertical distributing or
resisting elements, provided the deflection in the plane of the diaphragm, as
determined by calculations, tests or analogies drawn therefrom, does not
exceed the permissible deflection of attached distributing or resisting
elements. See UBC Standard 23-2 for a method of calculating the deflection of
a blocked  wood structural panel diaphragm.

Permissible deflection shall be that deflection up to which the diaphragm and
any attached distributing or resisting element will maintain its structural
intergrity under assumed load conditions, i.e., continue to support assumed
loads without danger to occupants of the structure."

I tried calculating diaphragm and shear wall deflections based on formulas on
UBC Standard 23-2 for a 21 ft by 21 ft wood framed garage with flat roof, 8 ft
plate ht, with two 2'-6" plywood shear walls in front and a 21 ft plywood
shear wall at the rear. Seismic zone 4, 70 MPH wind, exposure C, the following
are my deflections:

Diaphragm(1/2" plywood with 8d@6"oc EN, blocked and 2-2x4 chord, no
splice) Deflection = .025"

21 ft long shear wall (3/8" plywood, 8d@6"oc EN) Deflection = .038"

2'-6" long shear wall (3/8" plywood, 8d@6"oc EN, with deflection due to
holddowns and wood crushing non existent) Deflection = .180"

Average shear wall deflection = .109"

Allowable deflection for 8 ft high shear wall = 0.005H=.48" > .025" OK

2 X (.025") < .109"  Diaphragm is not flexible.

If my calculations and assumptions are correct, and if I interpret the code
correctly, the diaphragm is not flexible and the lateral loads should be
distributed based on shear wall rigidities.

I'm still reading my UBC97 and the proposed code changes to the IBC so I'll be
back with more later.