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Re: Unbraced Length for Stair Stringers?

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The contractor has a very good point there. I suggest you check the unbraced length of the whole stair system also, i.e. the stringer as chord and with the pans acting as web members.

I have worked with a Korean group for a Samsung semiconductor plant here in the Philippines where they used only 16mm (5/8") thick by 300mm (12") plates for the stair stringer with metal pans as treads, though the span is only 10 feet. The stair system did perform well and their were no vibration problem either.


Wilson Ching

seaint(--nospam--at) wrote:

>Hello.  I'm new to the email list and looking forward to gaining valuable
>knowledge from fellow structural engineers.  I previously worked for a small
>structural engineering firm and now work for a full-service A/E firm.  At my
>previous job, stair stringers (typically channels) were designed for an
>unbraced length equal to the full span of the stringer unless the stringer
>was connected to the adjacent cmu wall.  When attached to the cmu wall, the
>unbraced length was equal to the distance between the connections.
>At my current job, the stair systems are typically designed by the
>fabricator and checked by myself.  The stringers are MC12x10.6 spanning 13
>feet.  The fabricator's engineer is stating that the stringer is fully
>braced by the stair pan resulting in an unbraced length of 0 feet.  Some
>stair systems are apparently designed with the stringers and pans analyzed
>as one support system rather than looking at the stringers as a single beam
>spanning from header to header.  Obviously, there is quite a difference in
>allowable flexural stress of a MC12x10.6 for an unbraced length of 0 ft vs.
>13 ft.  I guess I should just start specifying a C12x20.7 for all the stair
>What is the correct procedure for designing stair stringers?
>Any suggestions are appreciated.

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