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RE: Unbraced Length for Stair Stringers?

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I have worked for a fabricator, but most of my work has been with consulting
firms.  Typically concrete filled pan stairs are designed as if they are
braced by the concrete filled pans.  It is a quasi diaphragm.  

I have load tested some of these stairs (to the design load according to ACI
318 procedure), and I am comfortable with the bracing assumption.  When you
do the load test, the actual applied load for a design load of 100 psf is
actually close to 190 psf.  Some engineers design stair stringers as metal
plates on edge.  They, too, work well as stringers for metal pan stairs.  

If the stair treads are grating or expanded metal, the assumption is that
the stringers are unbraced.  

The best thing to do is look at NAAMM's Metal Stairs Manual.
http://www.naamm.org/literature.php

Regards,
Harold O. Sprague

-----Original Message-----
From: Dan Boltz [mailto:dboltz(--nospam--at)mckinleyassoc.com] 
Sent: Friday, November 07, 2003 7:39 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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