REG : WEAK AXIS SHEAR AREA OF I SECTION 1

[padeyedesigner]

Dear Sir/Madam,

In sacs, I have checked the Major axis shear area of " i section " while reviewing the member in postvue, I came to know software is taking only 2/3 of the Two flange areas.

But as per AISC, it should take 2 x flange areas.

If anybody answer to my question means, it will helpful to me.

Thanks.

 

[Mawaca]

I think that you are looking at the AISC provisions for shear when I-sections are in strong-axis bending. I don't have my copy on hand, but I believe it is chapter H that delves into other situations.

Essentially, AISC forces you to wipe the dust off your mechanics of materials textbook and return to first principals. So, for the pair of rectangles that you would have with shear along the weak axis, we know that the maximum stress is 1.5 times the average stress. Another way to account for this is for your software to work with average stress on an effective shear area of 2/3 A.

 

[Lion06]

mawaca-

That may be a conservative approach by the software developer, but I don't believe that's consistent with AISC.

AISC allows you to take the full web area for shear, so I don't believe that weak axis shear with the flanges would be any different.


padeye-

This seems like a completely academic exercise. Shear rarely controls strong-axis sizing of a beam, I am hard pressed to think of any scenario (outside of the most unique loading condition) in which is would control weak axis sizing of a beam.

 

[RockEngineer]

It goes back to (V*Q)/(I*t). On an I beam strong axis the large area creating low shear stress in the flanges so (V*Q)/(I*t) is very close to V/d*tweb. On the weak axis the web is at the neutral axis and is ignored. Q/(I*t) is equivalent to 1.5/(2*b*t) of the area so V/(2/3*Aflange*2). Remember you are calculating maximum shear not average shear.

 

[Lion06]

I don't think I agree with that. The maximum shear in the web of an I beam is higher (approximately 1.15 * the average shear stress in the web), but AISC doesn't account for this in the design equation for strong-axis shear, so it doesn't make sense to account for it with weak-axis shear.

VQ/(It) for a WF is about 15% higher than V/Aweb. I think that the two flanges become the webs for the shear equation.

I'm not disagreeing from a mechanics standpoint, but as far as AISC is concerned that is what I would do with the design equations. Again, I think this is academic as shear rarely controls for strong axis and will almost never control for weak axis.

 

[KootK]

I've never done the 2/3 thing myself but it makes sense. 50% is a lot more potential over stress than 15%. And, obviously, the presence of the flanges in a strong axis wide flange is the reason that it's only 15%. Can an argument not be made for going beyond yield in shear (like Zx in flexure)?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.