Drift As Per ASCE7-05

[Ganesh Persaud]

Hello Everyone,

My question is when calculating drift for steel portal frame steel structure, does the Ridge/Apex level need to be checked also or is it only at Levels with diaphragms?

Thank you

 

[WARose]

Generally you check story levels....or whatever could hit an adjacent structure. A ridge is not something I'd check without a reason.

 

[JAE]

We use various drift limits (for wind) such as H/300, H/400, and H/500 depending on the type of exterior cladding - and use the levels at the floor diaphragms.
Have never used H (height) as to the top of the ridge of a sloped roof....perhaps to the mean roof height but not the ridge.

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[Ganesh Persaud]

I spoke to a couple local engineers, they had different responses. See image below for frame cross section. I was told that the drift would have to be calculated at ridge level because it’s a flexible diaphragm, and another said it’s not necessary.

 

[Ceinostuv]

A ridge is not something I'd check without a reason.

Here's a potential reason - Do you have deferred submittal/delegated design for non-structural framing? Will they need to isolate interior framing for drift and will there be any framing to the ridge/deflections large enough for you to be concerned? Perhaps a non-issue, but maybe worth a minute of consideration.

Judgement-In-Training

 

[WARose]

Here's a potential reason - Do you have deferred submittal/delegated design for non-structural framing?

Typically the answer to that is: no. But even if I didn't, I've never run across a situation like you mentioned (assuming I understand what you mean) where that would control.

 

[Ganesh Persaud]

Thanks, I would definitely have a look at the design for none structural members.

Another question as it pertains to a portal frame, when checking type 1a and 1b vertical irregularity checks, according to asce7-05, 12.3.2.2, this check is not required for a one-story building, in sdc b,c or d.

The sdc for this frame is D.

My question is, according to asce, is a portal frame considered a one story structure?