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RE: Unbraced Length for Stair Stringers?

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This is an interesting question, one that I haven't fully resolved myself.
I have two thoughts that may be helpful.  First, we usually use the C12x20.7
simply because the MC is bouncy.  I know it works by calcs for both
deflection and strength, but it doesn't feel as good as the C12.  Secondly,
the new LRFD spec has much more specific definitions on bracing.  You might
review those and see if there is more help there in defining braced and
unbraced.

For what its worth, when there are a series of bent 12 gauge or 1/8" plates
forming a closed tread, I usually treat it as braced.  Verify there is
enough welding between treads to form a diaprhagm.  There should be
additional welding between treads beyond the channel connection.

Part of this question should also be handrails.  The MC12 has a 1.5" top
flange.  You can't get an all around fillet weld to the handrail unless you
use a smaller slug inside the post.  Also, where does the torsion from the
handrail go?  You ultimately rely on the treads to transfer the force to the
opposite stringer.

Probably not much help, but I tried.

Jake Watson, P.E.
Salt Lake City, UT

-----Original Message-----
From: Dan Boltz [mailto:dboltz(--nospam--at)mckinleyassoc.com]
Sent: Friday, November 07, 2003 6:39 AM
To: SEAINT
Subject: Unbraced Length for Stair Stringers?


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
stringers.

What is the correct procedure for designing stair stringers?

Any suggestions are appreciated.
Dan


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