bottom loaded wide flange 3

[JTPE]

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

 

[jec67]

I do not understand your situation. You stat that the bottom flange is in compression. Is the wall pushing upwards on the steel beam?

 

[jec67]

(continuation of previous post)

If the wall is hanging from the beam, then would not the top flange be in compression (i'm assuming it is simply supported)?

 

[JTPE]

Sorry, top flange in compression. For the beam to buckel out of plane the load is resisting the instability...right?

sorry for the bum description.

 

[JedClampett]

I wouldn't count on it (the load resisting the top flange buckling). If you can't brace the top flange laterally, use a section where the bending stress is acceptable for that unbraced length.

 

[ChipB]

The bracing parameters are to prevent Lateral Torsional Buckling of the member. Loading the bottom flange will not keep the member from twisting under the load.

To be conservative, use AISC's formula F1-8 :
Fb=1200(Cb)/(L(d/Af)) <= 0.6Fy

 

[denoid]

Perhaps another option might be to sufficiently stiffen the top flange with a channel for lateral bending due to lateral torsional effects, if the span is short enough. (See AISC 9th edition, p.1-83 for descriptive picture).

 

[TFL]

JTPE,

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.

 

[haynewp]

For bottom flange loading, the beam top flange will buckle at a higher load than for a top flange loaded beam for the same unbraced length.

It does not mean that it won't laterally buckle just because it is loaded at the bottom flange.

 

[TFL]

the higher the load the higher the bracing force???

 

[MotorCity]

You cannot brace the compression flange if you do not connect to the compression flange.

 

[UcfSE]

You might want to check that the force required to brace that member can be equaled or exceeded if you decide to consider the load as "bracing" the top flange by countering the twisted produced by buckling. There are not only strength considerations but also stiffness considerations and both must be met by the bracing. Check AISC 3rd Ed LRFD section C3 for some guidelines. My advice is not to use that and just go with the unbraced length you have from supporting beams or whatnot. You can use a channel or plate welded to the top flange to help increase your Sx and rT if you need to.

 

[haynewp]

Loading the bottom flange does not brace the top 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/editi...eria-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.

 

[AlohaBob]

It seems to me when we design the cranerails, the top flange is the critical issue. The unbraced length of the cranerail would be it's length between supports. Often, a simple quick reinforcement is to add the channel to the top flange to increase the area for the reduced stress. The allowable stress in the bottom flange is higher being in tension, and the weights will stabilize it naturally from incidental out of plane load.

 

[JAE]

I believe that haynewp has the right answer here - that by loading the bottom flange, the load itself would tend to prohibit the rotation of the beam due to LTB.

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.

 

[jhardy1]

Most steel design codes have a capacity modification factor allowing for the height of application of the load. Beams which are loaded at or below their shear centre are considered to be relatively stable, because the load tends to stabilise the beam against lateral torsional buckling - BUT the beam can still buckle due to top flange compression!

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.

 

[jhardy1]

PS - It is true that bottom flange loading is more stable again than shear centre loading, so in theory, the effective length for a bottom flange loaded beam would be LESS than the spacing between the compression flange lateral restraints, and the collapse load would be correspondingly higher. Detailed text books will give a description and calculation method for this behaviour, but the basic beam design rules in the Australian Standard don't reflect this behaviour. In the Australian Standard, beams with bottom flange loading are assumed to be equivalent to beams with shear centre loading (which is slightly conservative). Beams with top flange loading are treated as having an increased effective length, and have a reduced capacity as a result.

 

[JTPE]

Thanks for your responses. In this case, the span is large (about 41 ft) and the load is small (about 7 kips located anywhere along the span). I think I will stay conservative and find a way to brace the beam. I'll have a deep beam anyway to control deflection in the worst case, so I should only have to brace it once or twice.
Thanks

 

[VoyageofDiscovery]

Hi JTPE,

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

 

[TFL]

Voyage of discovery,

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