Stability of SS Beam w/o bracing
Stability of SS Beam w/o bracing
(OP)
I am evaluating a structure that has an existing simply supported steel wide flange girder. There are timber joists that sit on the girder, but are not rigidly connected to it. The girder sits on a bearing plate, and has no stiffeners, nor any lateral support at the ends.
Essentially the beam has no lateral bracing, but is OK- no signs of instability, it could be getting stability from the friction with the timber joists, or the section could be stiff enough all together where lateral stability does not need to be provided.
AISC pp. 2-13 says something to that effect: "beams with thick webs and relatively shallow depths could be properly designing without providing lateral stability"
How do I check this though? Appendix 6 doesn't seem to contain any thing that address if stability is required, it just seems to assume that bracing is required. I'm not an expert on appendix 6 though, so maybe I am missing something.
Thanks
Essentially the beam has no lateral bracing, but is OK- no signs of instability, it could be getting stability from the friction with the timber joists, or the section could be stiff enough all together where lateral stability does not need to be provided.
AISC pp. 2-13 says something to that effect: "beams with thick webs and relatively shallow depths could be properly designing without providing lateral stability"
How do I check this though? Appendix 6 doesn't seem to contain any thing that address if stability is required, it just seems to assume that bracing is required. I'm not an expert on appendix 6 though, so maybe I am missing something.
Thanks






RE: Stability of SS Beam w/o bracing
That being said, if you have a shallow beam with a relatively wide flange, it could be stable without the aforementioned torsional restraint. Additionally,even though you can't quantify the bracing provided by the timber joists, it is providing some.
RE: Stability of SS Beam w/o bracing
RE: Stability of SS Beam w/o bracing
RE: Stability of SS Beam w/o bracing
The preamble to chapter F states, "This chapter applies to member subject to simple bending about one principal axis. For simple bending, the member is loaded in a plane parallel to a principal axis that passes through the shear center or is restrained against twisting at load points and supports."
RE: Stability of SS Beam w/o bracing
It my review of the framing, I noticed the beam doesn't have any positive bracing. So I was wondering if I should add some bracing, but if I can show the beam is OK as is (by the code) then I won't bother.
What do you think? How would/could I approach this?
RE: Stability of SS Beam w/o bracing
RE: Stability of SS Beam w/o bracing
tg
RE: Stability of SS Beam w/o bracing
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Stability of SS Beam w/o bracing
I don't believe that bottom flange bracing is adequate to consider the section braced. It wants to be braced at or near the compression flange. The only way that bracing the bottom flange works in a bearing connection is to provide stiffeners and a positive connection to the support.
RE: Stability of SS Beam w/o bracing
I don't want to add stiffners to the beam for two reasons:
1) I don't like the idea of welding to beam that supports timber framing (fire hazard).
2) I'd have to shore the beam to weld it, which would be expensive- since I can't weld a simply supported beam while it's under load.
RE: Stability of SS Beam w/o bracing
this is really outside of my scope and budget as well as the owner's so I'm trying to get something simple done to convince myself it is ok or add a cheap (cost-wise) fix.
if I absolutely have to add bracing, I'm thinking of some sort kicker from the top flange (or possibly centroid?) back to an existing joist near the support.
RE: Stability of SS Beam w/o bracing
My biggest concern is that a column coming to the underside of a beam with no positive connection to anything is inherently unstable. How do you even just the column being braced at it's top at that point?
RE: Stability of SS Beam w/o bracing
RE: Stability of SS Beam w/o bracing
If there is no problem with web cripping (thick web), and the flange is capable of handling local bending due to rotation induced by incidental eccentricity, plus all atresses along the beam are within allowable that was derived without coniidering intermediate braces, you are good to go without end brace, nor bear stiffeners.
The requirement from the newer code could be different. Also, I could be wrong. Check it out.
RE: Stability of SS Beam w/o bracing
RE: Stability of SS Beam w/o bracing
If your joists bear directly on the steel flange with no physical connection, you could use a pair of steel straps spaced at about four feet on center nailed to the joists and bent around the beam flange to prevent lateral movement.
BA
RE: Stability of SS Beam w/o bracing
95% of which have "unstable" lally columns supporting a steel beam which could not really be considered cantilivered even though the column is buried in a few inches of concrete.
Another requirement of AISC, if I remember correctly, is that all members with "unframed ends" must have a pair of stiffners. In a home, with joists 16" o.c., I would consider the ends framed.