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Re: UBC 16188.8.131.52.1.7 Bracing Load[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: UBC 16184.108.40.206.1.7 Bracing Load
- From: "Kevin Below" <kbofoz(--nospam--at)gmail.com>
- Date: Tue, 9 Oct 2007 21:04:06 -0400
Jim, it seems to me too that if the load is applied on the bottom flange then the beam is stable and cannot rotate. I had the same scenario some months ago with a small foot-bridge (30 ft span) using through-trusses. i.e ., the supporting trusses also act as the guard-rails, and the traffic surface is supported directly on the bottom chords of the trusses. The top chords have no lateral support at all, but the trusses cannot rotate.
Wesley and Jim,Thanks for the AISC link but I have always been curious about this force. The AISC design force (or the 2%, etc force) is in pounds but say, for a simple beam with only the bottom flange continuously braced, how often should the top flange be braced? At the Lu points? Lc distance? An arbitrary distance? If say I picked 2 ft. increments then is that same force required at each one? Or if I doubled that length then is it still the same force but only every 4ft.? This has always puzzled me since the force is in pounds but the application distance is in feet.I also noticed in Appendix 6 that the force is based on Mr, not the capacity of the beam. Most of my beam selections are based on deflection not stress so it stands to reason that the brace force should be based on the required stress in the beam not the capacity of the beam.One more comment. Appendix 6 requires a brace on the tension flange at the end of a cantilever beam. This seems reasonable for a beam loaded on the top flange but what if the load to the end of the cantilever is applied at the bottom flange? It seems to me that this is a condition where it would be difficult or impossible for the end of the beam to rotate. Any comments?Jim Persing
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