bottom loaded wide flange
bottom loaded wide flange
(OP)
If a beam is loaded at the bottom flange only, does it still need to be laterally braced? In the case I am considering now, the bottom flange is equally loaded on each side of the web by a hanging wall. I realize there are several considerations such as flange bending, section modulus reductions because of connections, etc, but in regard to bracing I'm having trouble.
It seems to me that for the compression flange (the bottom flange)to buckel out of plane it has to overcome the "resisting" force which is the load itself. Does anyone have any suggestions, or perhaps a code that describes these situations.
Thanks
It seems to me that for the compression flange (the bottom flange)to buckel out of plane it has to overcome the "resisting" force which is the load itself. Does anyone have any suggestions, or perhaps a code that describes these situations.
Thanks






RE: bottom loaded wide flange
RE: bottom loaded wide flange
If the wall is hanging from the beam, then would not the top flange be in compression (i'm assuming it is simply supported)?
RE: bottom loaded wide flange
sorry for the bum description.
RE: bottom loaded wide flange
RE: bottom loaded wide flange
To be conservative, use AISC's formula F1-8 :
Fb=1200(Cb)/(L(d/Af)) <= 0.6Fy
RE: bottom loaded wide flange
RE: bottom loaded wide flange
You are absolutely right. This topic just came up in our office today. A bottom load wide flange is stable because of the stabilizing moment produced by the load. As the top flange attempts to buckle sideways it produces an eccentricity that allows the load at the bottom of the beam to "torque it" back to the upright position.
It had been so long since school that i had forgotten about it. However it was taught in my graduate class in stability years ago.
RE: bottom loaded wide flange
It does not mean that it won't laterally buckle just because it is loaded at the bottom flange.
RE: bottom loaded wide flange
RE: bottom loaded wide flange
RE: bottom loaded wide flange
RE: bottom loaded wide flange
If LTB controls, then the beam will be able to take more load if the bottom flange is loaded instead of the top flange being loaded. But it can still buckle. This book gives values in an example for what I am talking about.
http://www.directtextbook.com/editions/guide-to-stability-design-criteria-for-metal-structures-theodore-v-galambos
I would still design the beam as if the top flange is loaded and disregard any help from the load actually being at the bottom flange. I don't think this bottom flange assistance is recognized in the AISC steel codes anyway, so I wouldn't do it.
RE: bottom loaded wide flange
RE: bottom loaded wide flange
Under a typical loaded beam with LTB, the compression (top) flange wants to buckle "out of the way" of the compressive force and this lateral buckling works against the tension flange and initiates a twist or rotation in the overall cross section of the member. The AISC design equations in Chapter F provide the designer a competent way of estimating the limiting load that causes this failure.
But with a bottom loaded beam, the hanging load on the tension flange creates a resisting couple that would reduce the tendency of the top flange to buckle laterally. I say reduce, not eliminate - and this is what haynewp has stated above.
Motorcity - I don't believe you are entirely correct - you can brace the compression flange by bracing the web of the beam against twist. This has been shown in research and has been taught by Yura in many of his AISC stability seminars. The bottom flange load does sort of the same thing by creating a load couple that helps prevent the rotation, thus, reducing the tendency for LTB, thus adding capacity.
But AISC gives no guidance in the spec on how to calculate this so I ignore it and go conservative.
RE: bottom loaded wide flange
If the beam is loaded on the top flange, the load tends to exacerbate the lateral torsional buckling effect, so it will ultimately fail at a lower load. The only way to prevent lateral torsional buckling is to restrain the critical (compression) flange adequately. You can't do this just by applying the load to the bottom flange.
For example, in the Australian code (AS 4100), simply supported beams which are loaded on the shear centre or bottom flange are considered to have an effective length equal to the spacing between the lateral restraint points on the compression flange. If the beam is loaded on the top flange, the effective length is increased by 40%, and the calculated capacity will be greatly reduced as a result. I don't know the details of the American and British codes, but I would assume they would use a similar approach.
RE: bottom loaded wide flange
RE: bottom loaded wide flange
Thanks
RE: bottom loaded wide flange
If you want to look further into it, Timoshenko's "Theory of Elastic Stability" Second Ed., discusses Mcr and has a formula for simply supported and cantilever beams accounting for bottom flange loading. Derivation of the formula is also provided.
Regards
VOD
RE: bottom loaded wide flange
I don't have a copy of that can you fax me the formula. i would like to see how much gain there is
203-933-7824
RE: bottom loaded wide flange
i got the fax thanks!!!