huangyhg

flange brace force
i have a question regarding the flange brace force on a beam. lets say i have a beam in bending due to gravity loading, the flange brace force on the compression flange is 1% to 2% of the flange force (moment/depth). so i have applied a gravity load to a beam and now have a lateral load to resist, and since we all know that we cannot apply a gravity load and get a net lateral load, where does the equal and opposite force to the flange brace force come from? is it taken by the flange brace back up to a lateral beam that is attached to the tension flange, and is it the tension flange that then provides the equal and opposite force? thank you for any and all responses.

the lateral force that resists the lateral buckling force in your beam is usually taken into the floor/deck diaphragm system..it is outside of your beam system.
in a multiple beam system, would the flange force be cumulative then? this could be quite a large force. if it is cumulative and it winds up in the deck diaphragm, it would then have to be braced out, is this correct.
note that there is no lateral force until the beam actually buckles. once it does buckle or tries to buckle, whatever bracing is available will provide the reaction and transmit the force to the foundation.
the buckling force is cumulative when the framing consists of multiple beams. while it sounds like a lot, it usually is quite small compared to the lfrs forces. thats why we normally see braces that tie into or kick back to the diaphragm or some other part of the lfrs. the diaphragm acts somewhat like a "load sink" for these brace forces.
i don't think so - the force is resisted by the "interior" stiffness of the diaphragm. there is no net lateral force developed in the whole floor.
an analogy:
its like taking a beam and having a point load of p at midspan and two upward point loads (-p/2) on either side of the concentrated load, 2 feet away. the reactions are cancelled out as far as the beam end reactions go. but the beam feels a shear within its length.
so the diaphragm has interior shears and stresses, but no exterior reactions.
in large span beams with large moments, the cumulative brace force could far exceed the lfrs forces, which is the case i have. since the lateral force is generated from lateral movement of the flange during buckling, and the direction that it buckles could depend on materal inconsistencies, internal stresses, fabrication tolerences, and loads slightly eccentric, it would seen to make sense that the forces would cancel internally. it would seem to be highly unlikely that all of the beams would buckle in the same direction.
jae, what supplies the equivalent of the two upward point loads from the example? it seems like the diaphragm should resist the buckling load internally, not supply another external force to balance it.
ucfse - first off, my statement "i don't think so" was directed at the earlier question by ajnweb...just wanted you to know that - you popped in your response before i did and it sounds like i'm disagreeing with you.
as far as your question - my analogy was weak - i admit. but what appears to me is that the lateral force from the buckled beam is lost in the "load sink" (love that term) of the diaphram. i small beam in the large field of the diaphragm is simply trying to distort a small portion of the diaphragm internally and i just can't see that the cummulation of all the floor beams on that level would add up to some large lateral force that would have to be taken out of the structure via braces to the ground.
i concur with jae that the individual beam bracing forces will tend to cancel out within a given diaphragm or subdiaphragm.
the cumulative bracing force is best considered in terms of the total horizontal column bracing forces required at that story for the sum of the gravity forces at that story. this is the "stiffness" and "strength" necessary to call a frame system "braced" as opposed to "unbraced" (sway).
ucfse correctly makes the point that this strength and stiffness demand (for stability) is usually quite small in relation to the building's overall lateral requirements for wind and/or seismic forces.
it looks like we're saying about the same thing. i've never worried about the diaphragm takign buckling load or tried to figure it up to see what happens. i guess i was taking the academic point of view that a 20-pound buckling force should find its way to the ground, though that force would never be seen anywhere nor matter with anything.
what made me think of that is the design of bracing for light-gauge stud walls. when you're bracing with very small straps and angles, that cumulative force can add up to be something you need to consider, though it's seldom very large in the grand scheme of things. along that same note, i would expect a floor system to behave in a similar manner. brace forces may add up and may need to be considered though in the big picture they still aren't much.
so would it be safe to say that the lateral brace forces are somewhat localized, and tend to cancel themselves out internally, as opposed to seismic bracing forces that have to be added together and braced out directly to the foundation? in my example i am dealing with a 5 kip brace force at 3 points along a beam, with 11 total beams. if added together this would produce a 165 kip total force, which is over twice the wind force. but, if these brace forces tend to cancel out, then i would only have to consider the local effects of the brace forces, and maybe compare a 5 or 10 kip brace load to the cummulative wind load at any given flange brace location.

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