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Flexible Diaphragm Assumption, was RE: Simplified base shear formula in 97 UBC

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Thanks for the reply. 


For those of you using the 2006 IBC, have you noticed section 1613.6.1, if so what do you think?


"1613.6.1 Assumption of flexible diaphragm. Add the following text at the end of Section of ASCE 7:

Diaphragms constructed of wood structural panels or untopped steel decking shall also be permitted to be idealized as flexible, provided all of the following conditions are met:

1. Toppings of concrete or similar materials are not placed over wood structural panel diaphragms except for nonstructural toppings no greater than 11/2 inches (38 mm) thick.

2. Each line of vertical elements of the lateral-force-resisting system complies with the allowable story drift of Table 12.12-1.

3. Vertical elements of the lateral-force-resisting system are light-framed walls sheathed with wood structural panels rated for shear resistance or steel sheets.

4. Portions of wood structural panel diaphragms that cantilever beyond the vertical elements of the lateral-force-resisting system are designed in accordance with Section 2305.2.5 of the International

Building Code."


I am not positive, but I don't remember this assumption in earlier versions of the IBC, I may be wrong though.



Jason Christensen, P.E.


Project Engineer

WCA Structural Engineering Inc.

442 North Main Street

Bountiful, Utah 84010

PH. (801)298-1118

E-MAIL: jason.christensen(--nospam--at)




-----Original Message-----
From: Alexander (Sasha) Itsekson [mailto:sasha(--nospam--at)]
Sent: Tuesday, March 13, 2007 11:10 AM
To: seaint(--nospam--at)
Subject: Simplified base shear formula in 97 UBC


As we are still using UBC97 based code in California for fairly simple

wood building one can use the simplified base shear eqaution

V=3.0*Ca*I*W/R.  It gives you base shears of 20% higher than the general

base shear eqautions(or may be slightly less if Na=1.3 is used near a

seismic fault).  By using this approach, one can assume a uniform

vertical shear distribution within a building and can use a flexible

diaphragm assumption.





Alexander (Sasha) Itsekson, SE

Enginious Structures, Inc.

Oakland, CA



Subject: RE: shear wall program

From: "Jason Christensen" <jason.christensen(--nospam--at)>

To: <seaint(--nospam--at)>


"The 20% increase in base shear is a good investment as far as I am

concerned anyway for buildings in high seismic areas."


What increase is this?




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