Flexible Diaphragm max span to depth ratio

[riveron]

Hello All,

I am designing a single storey commercial, rectangular building with 50m x 10m dimension. It has metal deck as its roof. From what I have I read, the span to depth ratio needs to be limited to 3 for flexible diaphragm. Is that more of guideline? Can I still treat the building as flexible and transfer half lateral loads on steel frames(braced) at the perimeter of the building?

 

[lexpatrie]

What code is that in?

 

[riveron]

Canadian Code. CSSBI - 13-06. I know this has been superseded by newer code (B13-19, but I am more interested in understanding the reasoning and logic that pertains to it.

 

[jayrod12]

Once you exceed those ratios, deflection of the diaphragm would start to govern I'd bet.

 

[human909]

but I am more interested in understanding the reasoning and logic that pertains to it.

The logic is that it will cease to perform and will fail to adequately transfer loads to the perimeter walls. Failure of the diaphragm is a real possibility. The bigger the ratio the bigger the bending stresses and shear stress are in the diaphragm. You will eventually get buckling failure or shear failure of the fasteners (or another failure mode).

While in my locality we rarely use metal deck roofs as diaphragms, a 5 to 1 ratio seems awfully slender for any light diaphragm. Sure it might work, but you'll have perform analysis to check it.