collector design
collector design
(OP)
I have a steel framed building with braced frames. On the frame lines, I have wide flange girders supporting joists spaced at 6' o.c. The wide flange girders are acting as collectors and support an axial force. Looking at the weak axis of these wide flange collectors in compression, what should I use as their unbraced length... the length of the beam or the spacing of the joists? The joists are only attached at the top flange. Do they indirectly also brace the whole beam to some extent. Is it too conservative to say that the beam is totally unbraced in it's weak axis because the joists only brace the top flange?






RE: collector design
By Yura's info. on LTB, a reduced effective length can be calculated. As you suspect, the welded joist seats have a tendency to hold the top flange in such a way as to reduce the overall slenderness ratio of the beam.
RE: collector design
RE: collector design
RE: collector design
RE: collector design
Alternately, you can always increase the beam size!
RE: collector design
RE: collector design
Now it makes sense to me. You have these collector beams at the frame lines, which I think is a correct way to approach the problem.
RE: collector design
The lateral load gets transfered thru the diaphragm until it gets "collected" by the collector beam on the (braced) column lines.
RE: collector design
Anyone else learn it this way?
RE: collector design
I think that might be reasonable. You are assuming that the collector load stays in the top flange.
But each collector beam would need to be designed for the moment induced by the eccentricity between the top flange of the beam and the mid depth of the end connection. The collector load can't just "jump" from top flange to top flange--it has to go through the beam end connections.
DaveAtkins