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McEngr

ASCE 7-05 Figure 6-9

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In my standard of practice, we typically design regular shaped buildings. We rarely design buildings that do not have irregularly shaped diaphragms, etc. Because of this (right or wrong), we have ignored the CASE 2 and CASE 4 load cases because we design almost exclusively flexible diaphragms.

Do these diaphragm torsions only get introduced for rigid diaphragms? I could see it having practical considerations on a rod/cable braced roof of a metal building for example, but I'm not totally clear on flexible diaphragms.

Thanks anyone who has an opinion or has a better grasp.

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McEngr: Torsions are induced in the diaphragms due to RIGID BODY MOTION of the diaphragms. For the rigid body motion to occur, the diaphragms have to be relatively stiff. Flexible diaphragms have almost negligible rigid body motion. That being said, in a multi-storeyed building, the floor diaphragms could be considered as rigid and roof can be considered as flexible.

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EQ: I understand rigid diaphragm analysis as well as accidental and extreme irregularities, etc. My question is how one would even apply case 2 and 4 for a flexible diaphragm. Can one ignore it on figure 6-9? Afterall, ASCE 7 allows residential and commercial building wood-framed diaphragms to be flexible provided that MDD>2ADVE.

Thanks for commenting by the way. :)

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McEngr: Torsionality does not exist with Flexible diaphragms. Hence ignore Case 2 and 4 for flexible diaphragms.

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Great! Thanks EQ.

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