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Related Articles


Steel Stair Inter-Story Drift Design

Steel Stair Inter-Story Drift Design

Steel Stair Inter-Story Drift Design

I am designing a steel stair tower which is enclosed by CMU walls for seismic. In order to not induce any of the building inter-story drift into the stair, I have provided a slip connection for the stair stringers ascending to the floor to accommodate the inter-story drift provided by the EOR. My question is if the intermediate landings are required to accommodate that same inter-story drift or if the slip connection at the floor is all that is required. I keep getting different answers so I wanted to post on here to see what others think.

RE: Steel Stair Inter-Story Drift Design

You have to make it safe, that's all. There is no *need* to use a slip connection at all, not even at the bottom, unless you are code mandated to do so (which I doubt).

Without the slip connections, you MUST design the stairs to take the compressive load in combination with their more typical bending, shear, etc, of all loads (including seismic!). There is also a stress reversal case where tension is induced, but I have yet to see a case where that governs. Check it regardless.

With the slip connections, you are effectively hanging each from the top (if you are applying slip connections always at the bottom, this is typical) which means you have to design the stair for all gravity forces and seismic, but no longer have a lock in compression.

There you go: Both answers were correct, that's why you keep hearing them!

RE: Steel Stair Inter-Story Drift Design

For shorter structures, it is generally assumed that the sesmic drift profile is linear from top to bottom (shear mode). As such, if you are required to accommodate drift at the story levels, it would be reasonable to accommodate drift at the intermediate landings as well. The amount of drift could be taken as:

Story drift x (height_intermediate - height_lower)/ (height_upper - height_lower)

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Steel Stair Inter-Story Drift Design

Stair ways are emergency exit, and emergency access. After a seismic event, they are almost certainly the only way up or down (loss of power to the elevators is almost assured, right?)

Keep your solutions simple and safe. Don't make assumptions about long-acting loads and long effects of assumed movements over many floors if you don't absolutely have to.

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