Effect of wind loads on beams

[cucu101]

Hello Fellas!

I'm sorry I'm asking this kind of question but I'm new to STRL engineering field. The thing is, if we are designing, and wind load controls once I factored my loads how is the wind load changed from a lateral to a vertical load?? or what is it that happens during the factoring process?? because I have never seen a concrete or steel beam design taking the lateral load solely into consideration, or applying that load in the out of paper axis. All I always see is that they take the factored result (1.2DL+1.6WL) and apply it as a distributed load along the beam span and then determine the Mu. but truth is that the wind load it's affecting other axis of the beam. what happens to that wind load that is perpendicular to the beam?. Is it that because lateral loads are transferred to the structure diaphragm (slab, roof, floor, etc.) they don't take it into account for the beam design? if so, how are then beams able to take and withstand such high loads sometimes caused by the the reactions of the C&C attached to it?? I'm super confused on this.

I do really appreciate any help.

Thanks,
Hec

 

[JedClampett]

If you have a vertically braced system, the wind loads resolve into a lateral load, a compression load on one side and a tension load on the other.
If you depend on textbooks only, they don't always cover all the real life cases a structural engineer has to deal with. But if you're designing exterior open framing, for example, you have to apply the lateral wind load to the directly to the member and check it as weak axis bending (and shear).
Is this always done? Probably not. But wind loads are not that big, plus the design cases (90 mph) are very unlikely to ever occur in the worst direction. Add to that the combination of full LL, plus the wind load, times load factors, and there's a built in extra capacity. BUT IGNORE THESE LOADS AT YOUR OWN RISK. Just when you get cocky, a wind load will fail a beam.

 

[jayrod12]

A lot of it depends on your framing technique for the outside walls.

Do your studs run past the beam, or just to the underside? Is your beam braced by some mechanism, i.e. perpendicular beams framing into it, bottom chord braces etc.

This is stuff that the books don't tell you, you have to learn by experience. A wise man once said to me "Follow the loads, they all must find a way to the ground somehow"

 

[KootK]

Is it that because lateral loads are transferred to the structure diaphragm (slab, roof, floor, etc.) they don't take it into account for the beam design?

Mostly this for enclosed buildings. The diaphragms are often very stiff in plane relative to weak axis beam bending.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[cucu101]

the thing is that the C&C or wall studs are attached directly to the underside of the beam and they have a reaction of 384# @ 16 O.C. which I consider it's kind of high for a 25' span beam and I was wondering what kind of method I should use in order to analyze the beam for failure by lateral loads because it's not like analyzing the whole structure where I can probably use the portal frame method to analyze the ability of the structure to withstand the lateral loads. truly I wouldn't know what to do to analyze that, should I check for weak axis bending then as "JedClampett" said?. my other question that I don't still comprehend pretty well is, assuming that a beam is being designed for flexural bending and that the LC that controls is 1.2DL+1.6WL is the lateral load(WL) being applied as a vertical load when factored?

thank you all very much for taking from your time to clarify my questions.

thanks again,
Hec

 

[jayrod12]

For the stud loads my quick and dirty way of checking the beam for out of plane bending is using the moment resistance calculated with 1/2 of the weak-axis elastic modulus. And then provide bottom chord braces to limit the span until the applied moment is within this resistance.

 

[KootK]

If the top of the beam is restrained by a competent diaphragm, as is often the case, then it's really beam torsion that is the issue rather than weak axis flexure. In that case, I treat it as jayrod has described and the method is usually coined the bi-moment method.

It's important to recognize that the load cases only specify the magnitudes of the various actions on a member. They say nothing about the directions of those actions. Vertical and horizontal load effects can, and are, combined in the same load case.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.