Stair Lateral Loads

[SteelPE]

I have been tasked designing a set of self supporting stairs inside a building. The stair is a "switch back" stair and only goes up one story (approximately 13'-0" +/-). I have done my fair share of star design before but I am not sure I have ever been approached to design a stair quite like this.

We have proposed to the fabricator that they install bracing at the lower landing and upper landing..... and parallel to the stringers (as they run from the lower landing to the upper landing). However, we now need to provide "calculations" on these stairs and we need to make sure that they work by the numbers. What lateral loads would you use on the stair outside of those outline in chapter 13 of ASCE 7?

 

[phamENG]

I'd probably consider a traction along the stair of 10% of the vertical live load, and traction across the treads of 5%. This is roughly in line with the traction loads for grandstands in ASCE 7. Lateral load tests for tractive live loads on decks with 40psf live load ranged from 2psf to 12.5 psf of effective horizontal loading.

Alternatively, look at 5psf across the projected vertical face of the stair structure.

 

[bones206]

I haven't designed a "monumental" stair like this before, but I'd probably focus on footfall induced vibrations as a major serviceability issue. It's probably the most likely thing that could lead to a callback from the client. And the hardest thing to fix after the fact.

 

[KootK]

Is this a monumental stair? I originally thought so but, upon reading closer, came to the conclusion that it's an ordinary stair except for the fact that it's free standing & in the interior of the building -- an unusual combination. Assuming that it's the latter:

1) I like pham's recommendation.

2) In the past, I've just taken the full live and dead load and applied that to the top assuming that the whole tower is out of plumb by 5%, like an AISC notional load. Unless you're using licorice for your bracing, I imagine that this just comes down to OT at the base and satisfying plan check.

 

[bones206]

I guess I misunderstood. I was picturing a wraparound stair supported only at the first step and at the top landing.

For strength considerations I agree with the traction load, 5 psf pressure, or or notional load approaches. I feel those methods adequately capture the lateral loads generated by a group of people careening down the stairs in an emergency.

I guess my point was that the critical design consideration may be vibration, which is not necessarily addressed with static analysis of gravity and lateral loads.

 

[phamENG]

I imagined an industrial/mill building with a new stair from the mezzanine/intermediate level to the ground floor. So another possibility if that's the case: impact from an average sized suspended object at the typical crane operating speed in the space. I think this would be an important design consideration if exit paths are limited.

 

[SteelPE]

Sorry to abandon this thread for a bit.

I know very little about this project..... only that we need to have this stair located on the mezzanine. The stair is going to be used to access a ship ladder which is about 9'-4" tall which will be used to access the roof. So no real "public access".

Now, I know what your thinking "Why are they using a free standing stair to access a ships ladder to the roof?". Well, that's above my pay grade.

 

[phamENG]

It can make good sense. I worked in a mill for a short time and these things were everywhere. But it was a big mill, so ground floor to the mezzanine was 25' in some cases, and the roof was closer to 120'.

 

[bones206]

So is it something like this?

 

[SteelPE]

bones206,

Yes, something similar to that.

 

[EZBuilding]

I would agree with phamENG's approach as a good practical starting point.

Additionally, ASCE 7 1.4.2 identifies a minimum lateral load as a proportion of dead load.

 

[bones206]

Not quite the monumental stair I was picturing originally! But my overall point still somewhat applies... you want to limit sway to whatever level will avoid a callback, even if that means adding more bracing than is needed for strength. I had a former colleague who designed a similar freestanding access platform and it was designed to code, but the client felt it was poorly designed due to the perceptible movement.

 

[SteelPE]

Well, I would like to think we have more bracing than that is shown in the picture. Perpendicular to the stringers, we have full height bracing at the upper and lower landing. Parallel to the stringer, we have a portal frame parallel to the upper stringers (we don't have anything on the lower stringers as we feel they will support themselves laterally in the direction parallel to the stringers).

One issue I have is that we are not responsible for the design of the actual mezzanine, there is an EOR for that already. This stair will not be light and we highlight in out calculations that we are not responsible for the loads once they leave our "structure". I hoe this wasn't looked over.

 

[bones206]

In my experience clients often overlook the "foundation by others" caveats when procuring pre-engineered mezzanine and access structures. I suspect some of those companies make sure that requirement is buried in the fine print, as they don't necessarily want to highlight a requirement that increases the effective cost of their product. I have a client who just purchased and installed a couple 125 psf pre-engineered mezzanines without an EOR involved. They ended up just bolted down to a 4" SOG.