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Re: Unbraced Length for Stair Stringers?

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I disagree with both unbraced lengths (0 and 13 ft.).

I'm assuming that you're using a concrete-filled metal pan tread system
where the tread pans are welded to the stringers.  Even though the treads
are attached to the web of the stringer and the flanges are free, the treads
will act as a diaphragm between the stringers and prevent lateral torsional
buckling of the stringers.  There are thousands of stairs in existence with
nothing more than flat plates as stringers.

The compression unbraced length is approximately the sloped distance between
the forward points of the treads, or about 13 inches.  You'll need to verify
and/or specify that the pan plates are welded to the stringers sufficiently
to handle this.

The main issue with a MC12x10.6 is that the flange is only 1.5" wide.  The
favorite guardrail system of the architects and stair designers I work with
(for non-decorative situations) is 1.25" standard pipe welded to the top
flange of the stringers.  The pipe itself *will* handle standard loads if
analyzed as a 3D system, and precautions are taken with post spacing and at
the ends.  Unfortunately the outside diameter is 1.660", which is bigger
than 1.500", which means you can't get a full weld around the perimeter at
the base.  Assuming a 3/16" weld with the pipe centered on the flange, you
actually only get slightly more than 1/2 of the perimeter fully welded.  The
only solution I've been able to come up with is to weld a 2" square plate to
the flange then weld the guardrail post to the plate.


----
Jason Kilgore
Leigh & O'Kane, L.L.C.
jkilgore(--nospam--at)leok.com
816-444-3144
816-444-9655 (FAX)
----- Original Message ----- 
From: "Dan Boltz" <dboltz(--nospam--at)mckinleyassoc.com>
To: "SEAINT" <seaint(--nospam--at)seaint.org>
Sent: Friday, November 07, 2003 7:38 AM
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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