Flange brace force
Flange brace force
(OP)
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.






RE: Flange brace force
RE: Flange brace force
RE: Flange brace force
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.
RE: Flange brace force
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.
RE: Flange brace force
RE: Flange brace force
RE: Flange brace force
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.
RE: Flange brace force
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.
RE: Flange brace force
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.
RE: Flange brace force
RE: Flange brace force
RE: Flange brace force
I just try to imagine, however, a floor of beams braced by a concrete diaphragm, and supporting ONLY vertical gravity loads, with no sideway in the overall system due to rigid braces on the sides of the diaphragm. If you take this sytem (i.e. a free body diagram of the floor) and calculate ΣF in the horizontal direction how would you get any net lateral reaction to occur?. I think it would have to be zero.
RE: Flange brace force
I also think that the relative system is quite idealized because the typical floor system has beams with connections that are as a minimum 1/2 the T dimension of the beam. Because of this they provide significant torsional restraint to buckling (significant in terms of the applied buckling loads). I think that in 99.99% of the cases this is not something to worry about. I guess I just feel that you have to be aware of the possibility of it being an issue and design accordingly.
RE: Flange brace force
DaveAtkins
RE: Flange brace force
RE: Flange brace force
RE: Flange brace force
Assuming you have a diaphragm, I suggest you get the ASCE "Lateral Bracing of Beams and Columns" article by George Winter published in 1960. Essentially it says virtually any metal roof deck has enough stiffness to prevent the beam from buckling. There are column tests even done out of cardboard strips to verify that 2% is a typical conservative bracing load. Depending on your uniform load even the friction force between the deck and beam could be enough. The everyday typical elastic structural analysis engineers perform is not the basis of these formulas. Marvin Larson even uses hydrualic jacks to simulate the shear forces transmitted to the deck diaphragm. His typical 30 ft by 12 ft bay has no reaction forces shown at his columns but he does give the critical shear-deflection formula you would need. His deck also does slightly deflects. Without a deck diaphragm a different analysis would be reqiured. Nice thread. Good Luck.
RE: Flange brace force
The connection between the rafter and the top flange is what will carry the top flange force of you carry beam. I believe you should definitely account for this given the magnitude of your load. How that load is taken care of by the rafter depends on the rest of your framing and the load path for that buckling force.
To what code are you designing? If you are using the AISC steel code, there is a section concerning the design of bracing for strength and stiffness concernes.
RE: Flange brace force
I have a rafter (moment frame) that spans 207' with 18' bays, 12 bays total. To help reduce thrust, there is a carry beam located about 70' in from one sidewall. This carry beam is 90' long and supports 4 rafters, each imparting about an 80 kip point load on the carry beam. The carry beam is about 6' deep, so the flange force = .02*4500/6 = 15 kips. Unfortunatley I am thinking that this lateral load would have to be added to the moment frame rafter along with the vertical snow load.
I am designing to the 2003 IBC with 9th edition ASD. I have seen the bracing strength and stiffness deisng in the 13th edition, but I do not recall seeing it in the 9th edition. I guess I could design the rafter for the 16 kip load, and make sure that it has the stiffness per the 13th edition code. It seems to me that this type of brace would qualify as a relative brace, and not a nodal brace. Or with the carry beam bottom flange brace back up to the rafter, would it qualify more like a torsional brace, and therfore just design the rafter for a concentrated moment as opposed to a concentrated force. Probably 6 to one, 1/2 dozen to the other?
RE: Flange brace force
From Yura's stability conference, when beams are tied together and they all laterally-torsionally buckle at the same time (they are all loaded to this point and LTB is the limit state), they will ALWAYS all go the same direction. If you have 9 beams and five of them want to go one way and 4 of them want to go the other, there will be a slight tilt towards the direction of the 5 and the 4 will "change their minds" and go along with the rest.
RE: Flange brace force
RE: Flange brace force
RE: Flange brace force