Wind Load on Exterior Stair

[TracyXP]

1st time posting, but we can't come to a consensus.

A coworker is studying for the PE and our department head has him doing hand calculations to help him practice. We've hit a bit of confusion on the wind load on an exterior stair on the side of a building. The building is already existing and it's going to be a simple stair - ground supported at bottom and cantilever beam from the existing building at the top to support it.

Now we can't agree on how to load the wind. If it was just a stair tower, we'd use open building, but since it's attached to an enclosed building, do we follow design guide 34 and use open lattice? If so, how is Cf, mainly the gross area calculated? Do you use the height of the stair stringer and guardrail * the length of the stair? Or the length of the stair * the height of the stair?

 

[milkshakelake]

A question like that won't be in the PE exam. I think this is one of those cases where you don't need a consensus, because if you give it to 10 different engineers, they'll all calculate it differently.

I'm not sure what design guide 34 is. Anyway, I would design the wind load as an open structure with pressure coefficient of 0, and apply MWFRS forces to the surface area of the steel. It's difficult to calculate wind load when it comes to things like open guardrails, so yes, "height of guardrail X length of stair stringer" is a conservative way to do it. There can be other ways to do it like treating it as a nonbuilding structure, which I think you were alluding to. Any of those ways is fine, and just be conservative with it in case it's not fine and a building official questions it, so you have wiggle room. I don't spend a lot of time on these things.

Honestly, what I explained is more detail than I normally give; I'd just apply wind loads to the whole face of the stairs as if it were enclosed, because the tributary area thing is easier to do. And it doesn't make all that much difference in the grand scheme of things. Maybe one of the members becomes W12x40 instead of W12x35, or if it's concrete, you add an extra #7; not a big deal.

Last thing I'd mention is that these cases rarely come up for me. It's usually a tiny portion of the structure. If it comes up repeatedly and needs a more rigorous analysis, then throw everything I just said out the window.

 

[XR250]

Most likely, if it is designed properly for occupancy loading, wind will not likely be a controlling load case.

 

[EngDM]

Most likely, if it is designed properly for occupancy loading, wind will not likely be a controlling load case.

Is there anything codeified for lateral loads due to occupancy? All I've seen is this document.

 

[XR250]

That's usually what I use for decks. I guess my point was that most commercial exterior stairs are designed for 100 PSF. In order to support that, there is usually enough structure, connections and bracing to resist wind loads.

 

[TracyXP]

Design guide 34 is referring to AISC Design Guide 34 - Steel-Framed Stairway Design. Per section 3.2.4.1:

"Wind loads should be based on the requirements of ASCE/SEI 7, chapter 26. Stairways that are exposed to the elements will likely fall under the provisions for "Design Wind Load: Other Structures" in ASCE/SEI 7, Section 29.4."

And this stair is being supported off of the main building, but the stringers still need to not fail under the lateral wind load. And they are spanning close to 35ft between supports.

We used open building wind loads on the stair, but are looking for others opinions to see what they would do.

 

[XR250]

35 ft. span stringers would be massive

 

[phamENG]

If the stair tower is open, then yes - open building would be appropriate. The fact that the main building is enclosed doesn't somehow change the way air pressure variations work on some exterior feature.

Have you studied what the GC[sub]pi[/sub] factor means? Not just how to select it, but what it is telling you about the behavior of the wind and the source of the loading?

 

[TRAK.Structural]

I would use the provisions in ASCE Chp 29 for open signs/lattice structures. Not sure how different the pressure calculations would end up being vs. the open building provisions of Chp 27, but you are designing the stairs not the building so I think Chp 29 is more appropriate.