flexible vs rigid diaphragms 1

[Skicat]

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.

 

[msquared48]

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

Mike McCann
MMC Engineering

 

[JAE]

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

 

[frv]

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).

 

[KBVT]

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

 

[whyun]

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.

 

[Skicat]

KBVT,

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

 

[OldPaperMaker]

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.