switch back stair analysis/design 1

[matrixeric]

Dear All:
I am a structural engineer who has never done stair analysis/design ...for one of the projects I am currently working on the client wants us to design a "switchback" stair.The spans are such that deflections are becoming a big head ache esp when I apply the uniform live load on one flight of stairs only and nothing on the other flight or landing ...Upon searching IBC etc I got the design loads to be the worst effect of a uniform load of 100 psf or a conc. load of 300 lbs; but these do not tell me if I can assume a percentage of this 100psf load to help with the deflections esp. when I apply the load ONLY on one flight of stairs in the unbalanced load case....
Please let me know if there is a reference/code out there that addresses this issue. Any advice would greatly help me in designing this stair.

Thanks.
MatrixEric

 

[UcfSE]

I think you can move your framing around and take care of it. There will be more than one way to do it.

I would move the column to the inside beam and use that beam to support the stringers, extending to the opposite edges of the stair each side. A WF can extend out of the column to pick up the opposite side of the landing. Use the stringers only as stringers and not part of the landing. Essentially you would be building a landing and connecting stringers to it on each side.

I would not model the base of the stringer as fixed. You just blew the cost of that connection out of the water, not to mention that a pinned connection is so popular it may get installed that way anyway. You should be able to get tubes to work, probably 8-inch or maybe 6. Tubes cost more than wide flanges and present a different look so I would talk to the client first about just what he or she wants to see.

 

[matrixeric]

Thank you Jike,UcfSe and csd72 ...I will try to work on the stairs with the alternate framing suggestions you all have suggested ... I understand that having the base fixed is a big no no ...

 

[apsix]

I agree with UcfSE but would run the 2 outside stringers continuous to the outer edge of the landing (ie N16-N17-N19 and N1-N2-N3-N5 are continuous cranked stringers).

If you are set on locating the column central to the landing you could try the column supporting a welded cross piece, ie N9-N11-N14 & N8-N11-N15.

 

[DWHA]

Are the steps going to be R.C. If so, you can get a lot of stiffness out of the R.C. that should help with the deflection issue.

 

[JLNJ]

If it looks like a teeter-totter it might act like a teeter-totter. You need to be vary careful about the connections at places like N10 and N12. Do you actually have the diagonal bracing as shown or is it approximating the action of the stringer treads?

A fixed base for the stringer is not impossible (the base of the post can be fixed, yes?) just unusual. The important thing is to design the connections so that they act like you think and assumed and modeled them to act. For unusual framing schemes like stairs, it is easy to accidently model in some torsional restraint and forget to design and detail that restraint into the connection.