Applicablility of ASCE 7-16's Seismic Diaphragm Requirements to Distributing Wind Loads 1

[jochav52802]

Hello All,

ASCE 7-16 describes the requirements for Flexible, Semi-Flexible and Rigid Diaphragms within Chapter-12, which is titled "SEISMIC DESIGN REQUIREMENTS FOR BUILDING STRUCTURES".

My question is whether those requirements and definitions apply to designing a structure for Wind Loads too, which I believe it does, yet I don't see anything in ASCE 7-16 that clarifies this. Maybe I've overlooked something, but if not, it seems strange that ASCE 7-16's Wind Load Chapter don't cover this.

I'm reviewing a wind load distribution calculation and the Engineer assumed a flexible diaphragm, which allows them to distribute the shear loads to the Vertical Force Resisting System, (VFRS) simply via tributary areas, (the major benefit of "Flexible Diaphragms" is that you don't have to concern yourself with the stiffness of the VFRS for force distribution, which also eliminates the need to consider torsional forces that can develop.)

An interesting note that my colleague and I noticed; Section 12.3.1.1 of ASCE 7-16 indicates that when your VFRS is composed of moment frames, you generally won't have sufficient stiffness in your VFRS to classify your diaphragms as "Flexible". This is because Section 12.3.1.3 of ASCE 7-16 defines a "Flexible" diaphragm as one in which the ratio of the deflection of the diaphragm to the average deflection of the VFRS is greater than 2. That said, if your VFRS isn't stiff enough, as generally is the case for Moment Frames, even a weak diaphragm could be considered a semi-rigid diaphragm.

Many thanks in advance for your help!

jochav52802

 

[birneys]

It's my understanding that the seismic criteria for diaphragms does not apply when you are considering wind loads. I don't know the exact section, but the ASCE 7-10 commentary discusses this somewhere. I think it even goes so far as to say what types of roof deck/diaphragm can be assumed flexible for wind vs. seismic. I would read through the relevant commentary sections and see if you find any clarifications there. It might be in the serviceability appendix commentary, not sure without looking.

 

[Shu Jiang PEng SE]

I think that diaphragm design shall apply for wind load because the diaphragm works as “intermediate element” transferring the lateral forces (no matter wind or seismic) to the lateral force resistance members. Otherwise the load transfer path is “broken” if diaphragm fails work properly.

 

[sandman21]

Diaphragms for wind loading is described in 26.2 definitions; untopped steel deck, concrete-filled deck and concrete with span-to-depth ratio of 2 ir less are rigid wood panels can be idealized as flexible.