Diaphragm forces

[BAGW]

Hi,

Per ASCE7 the diaphragm need not be designed for over-strength combination for structures in SDC D. Am I reading this correctly? If that's correct, what is the reasoning behind this?

They have the over-strength stated in collector design and not diaphragm design.

Thanks

 

[driftLimiter]

The commentary in ASCE7-16 says

The provisions of 12.10.3 consider both significantly greater forces observed in near-elastic diaphragms and the anticipated over strength and deformation capacity.

Basically, they are saying that Fpx provides enough strength based on observed past performance.
In high seismic areas, and in multi story construction Fpx can be significantly higher than Fx level forces in the diaphragm.

 

[BAGW]

What about the provision 12.10.1? Still no over-strength for diaphragm forces?

 

[driftLimiter]

Yes I think the commentary is suggesting that when you calculate Fpx based on equations 12.10-1 through 10-3 you have sufficient load on your diaphragm to keep it elastic. But if you have transfer forces on the diaphragm they need to overstrength on those forces. See end of Paragraph 12.10.1.1

 

[BAGW]

You are talking about the overstregth forces for horz irregularty correct? ONly here the over-strength applies

 

[driftLimiter]

Thats how I read it. You omega the transfer forces and add that with Fpx level forces to get diaphragm shears. The collectors get the maximum of Fpx, Fpx min, Omega unless you meet the exception in 12.10.2.1.

 

[BAGW]

I am looking at an example from AISC 341 diaphragm design. They are using omega for diaphragm design. Not sure why they are doing that though.

 

[Deker]

The diaphragm itself need not be designed for overstrength unless you have transfer forces, but collectors are required to be designed for the larger of Ω₀F_x, Ω₀F_px, and F_px-min per 12.10.2.1. The collector force need not exceed F_px-max determined by Eq. 12.10-3.

 

[KootK]

I am looking at an example from AISC 341 diaphragm design.

Which edition and which example?

 

[BAGW]

AISC 341-16 page 8-14 (Design example 8.4). They have the same example in ASIC 341-10 as well.

"Diaphragm should be selected that has a shear strength greater than 2.24 kip/ft". This 2.24 kip/ft is a force with over-strength.