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RE: Wood diaphragm allowables per IBC

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Refer to the Manual for Engineered Wood Construction (1997 NDS), Allowable
Stress Design, Supplement for Structural-Use Panel Shear Wall and Diaphragm.

Section 3.1 reads in part:

"Table 3.1A and 3.1B [these are the tables for "Recommended Shear for
Horizontal Structural-Use Panel Diaphragms..." for wind and seismic loading,
respectively] present the tabulated values for...diaphragms. Some model
building codes have adopted a 40% increase for allowable diaphragm
resistance when subjected to wind loading.... THE DESIGNER SHOULD CONFIRM
THAT THE INCREASE IS APPLICABLE UNDER THE LOCAL CODE; IF NOT APPLICABLE, USE
TABLE 3.1B. The basis for the increase is a change in design philosophy for
estimating wind forces. In addition, the increase helps to account for
better understanding of wind loads and the historical excellent performance
of diaphragms subjected to high wind events." [Emphasis in the original]

The wording in the table you mentioned is probably misleading. It should
show a note that reminds you that the values may be increased by 40% for
wind design applications. IIRC UBC 1997 and SBC 1999 do not allow the
increase for wind design. Perhaps a mixup occurred in preparation for
publication of IBC due to the fact that the other model code (NBC) didn't
include the table, but did allow the 40% (deferring to NDS).

That's my theory anyway. All feel free to poke holes.

William L. Polhemus, Jr., P.E.
Polhemus Engineering Company
Katy, Texas
Phone 281-492-2251
Fax 281-492-8203


-----Original Message-----
From: Sherman, William [mailto:ShermanWC(--nospam--at)cdm.com]
Sent: Wednesday, May 16, 2001 11:49 AM
To: SeaInt Listserver (E-mail)
Subject: Wood diaphragm allowables per IBC


IBC 2000, Section 2306.3.1 states that shear capacities in Table 2306.3.1
are to be increased by 40 percent for wind design (similarly for shear walls
in Section 2306.4.1). Table 2306.3.1 states that the tabulated shear values
are "for wind or seismic loading". Don't the tabulated values already
account for an increase in allowable stress as allowed for wind and seismic?
Why are the values used for seismic 40 percent lower than the values used
for wind?

William C. Sherman, PE
Camp Dresser & McKee, Inc.
Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com


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