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flexible vs rigid diaphragms

flexible vs rigid diaphragms

flexible vs rigid diaphragms

(OP)
Most texts and manuals I've referenced seem to indicate that traditionally wood diaphragms are idealized as being flexible.  The lateral analysis for flexible diaphragms is easier than a rigid analysis and can sometimes result in a more conservative design.  That being said, when looking through either ASCE 7 or the IBC, I can't find a passage or paragraph that definitively states which type of diaphragm assumption is correct.  I know there is a method for calculating the "actual" rigidity of the diaphragm, but I'm hoping for a passage or paragraph I can reference with some guidance for selecting either flexible or rigid.  Thank you.   

RE: flexible vs rigid diaphragms

This has been discussed many times here.  "Search" out the links.

Mike McCann
MMC Engineering

RE: flexible vs rigid diaphragms

I think in the definitions sections of the IBC and/or ASCE 7 they define flexible vs. rigid.

 

RE: flexible vs rigid diaphragms

JAE is right. It's based on the ratio of the in-plane diaphragm deflection vs. the supporting element deflection.

I think the definition is if the diaphragm deflects twice as much or more than the supporting elements (shearwalls, etc.), then your diaphragm is defined as flexible.

I believe all wood and metal deck diaphragms can be considered flexible, even though, if you actually go through the calcs, they are really quiet stiff (the metal deck, anyway; I haven't actually done the calcs for a wood diaphragm).

RE: flexible vs rigid diaphragms

There is a clause in IBC for allowing the designer to select a flexible diaphragm analysis for wood diaphragms without calculation.

RE: flexible vs rigid diaphragms

A wood panel diaphragms in a single family homes or duplexes are permitted to be idealized as a flexible diaphragm without justification per ASCE 7-05, 12.3.1.1.  

Wood diaphragms in larger buildings such as multi story apartment buildings, warehouses/supermarkets, etc shall be explicitly checked per ASCE 7-05, 12.3.1.3.

RE: flexible vs rigid diaphragms

(OP)
KBVT,

Where is the clause you are referring to?  Thank you.   

RE: flexible vs rigid diaphragms

Skicat,
In the 2006 IBC, on page 306, Section 1613.6.1 allows one to assume a diaphragm that is constructed from wood structural panels shall be permitted to be idealized as flexible if it meets the conditions noted.

ASCE 7-05 has the same allowance.

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