How to determine b/2tf when using a cover plate.

[morriscpm]

Hello,

I have a question concerning how to determine if a flange is compact or noncompact when a cover plate is attached to the flange of a currently in-service beam. I have attached a dropbox link to a sketch showing the proposed setup. My question is, do you use the whole flange width and thickness when calculating compactness, or do you use lesser dimensions since the cover plate will only be one half the width of the current flange. I know Table B4.1 case 18 shows how to determine compactness for just the cover plate, but would I not be interested in the compactness of the flange as a whole unit?

Thank you for any help.

 

[dhengr]

Morriscpm:
Compact or non-compact has to do with the idea that a compression element might buckle locally at some compressive stress, namely Fy for compact shapes and elements. We do this by relating b/t ratios and some F or Fy at the point of buckling of the pl. element. Is the existing WF shape compact as it is? Is your cover pl. compact with longitudinal welds on both sides? Both assumed not to buckle at Fy, if compact. Then why would the combination not be compact at Fy, but in fact the flange on the WF shape will be working at less than Fy in your new configuration. The important thing is that you unload the existing beam (un-stress it) before you apply the cover pl., or the cover pl. will only pick up bending stresses from additional loads, and the existing flg. might be pushed above Fy with these new loads. Unloading the beam and fixing the cover pl., then reloading the beam, allows the whole new section to react to all of the loads.

 

[morriscpm]

dhengr,

Thank you for your help,

The existing WF flange is slender, 36ksi steel, while the cover plate will be a compact shape of A992 steel. This is why I am a bit unsure of how to tackle this. If the existing flange were compact then this wouldn't be an issue at all, but I am mixing shapes of different compactness.

 

[structSU10]

The new steel manual has this situation shown in the table for checking compactness. you check the full width of the cover plate 'b' against the thickness 't' as b/t versus 1.4*SQRT(E/Fy)

 

[structSU10]

also note you can download AISC 360-10 for free from the AISC website.

 

[morriscpm]

structSU10,

I noticed that case in Table B4.1b. That compactness check appears to disregard the existing flange. So, does that mean that the compactness criteria of the existing flange is not important once the cover plate is added?

 

[dhengr]

Morriscpm:
What size is the existing WF, and what size cover pl. are you thinking of using? You know have a hybrid beam. At what stress level was the existing beam working? Shear and moment diags. existing and new conditions? The proportions of you sketch show a pretty wide and thick WF flg. and a fairly thin cover pl. You might want to use a thicker cover pl., somewhat less wide, but still reasonably proportioned. This will allow you to take more advantage of the higher pl. Fy over its thickness, before you get to the A36 mat’l. Then, two things about the existing beam: I would allow running that somewhat above 36ksi, knowing it can’t go anyplace as long as the cover pl. isn’t fully stressed. This is in keeping with the ASD concept of allowing some yielding to take place at some highly stressed locations. Strain compatibility at the faying surface causes more stress to be shifted to the cover pl. when the WF starts yielding. Secondly, when you attach this huskier cover pl. properly it provides a new support system and a new ‘b’ in the b/t ratio for the flg. pl./tip. So likely, the existing flg. is now compact, but I gotta think on that a bit, exactly how I would justify that in my calcs. Also, I’d want to think a bit about the welds to the cover pl., they weld to the A992, will fail in shear at the throat, but then weld to A36 mat’l. But, I think it’s doable.

 

[morriscpm]

dhengr,

The existing WF is 24" wide and .4375" thick. The proposed cover plate can only be 12" wide and I have it at .75" thick. Shear is not an issue with this beam, but the maximum current moment is approximately 4100ft-kips and the propsed moment capacity will be approximately 6100ft-kips. I agree that once the cover plate is attached the existing flange will change to a compact state, but I cannot determine how to justify that with my calculations, except to say that if the existing flange were removed and replaced with just the cover plate the flange would be sufficient to carry the additional capacity, mainly because of the higher yield strength, but also because of the compact shape.

 

[SAIL3]

Assuming the existing WF is A36, then I would tend to use A36 pl which is also much more readily available.
I also would tend to assume that the addition of that pl would render the fla compact. To put it another way, even though there is still 6" of outstanding fla unreinforced I can not visualize a local buckling failure mode and would consider the existing fla to be braced(for local buckling) along a line(longitudinal) of the weld between the cover pl and the fla. I doubt if one can theoretically prove this....it's an engineering judgement ....trying to visualize a buckled mode has been an invaluable tool for me and ,if after some thought, I can not come up with any reasonable failure mode then I would consider the design ok.I would use a continous weld of the cover pl to the fla based on the above reasoning.

 

[morriscpm]

SAIL3,

Thank you for your response. I like that I am getting confirmation of my contention that the existing flange will become compact with the addition of the cover plate.

 

[wannabeSE]

Section B4.1 of AISC 360-10 states "If the width-to-thickness ratio of one or more compression elements exceeds . . ." I would consider each element (original flange, and cover plate) separately and take the worst case. By the way, I don't think there is such a thing as A992 plate. A572 Gr 50 may be a better choice. There was a recent article in Modern Steel about reinforcing beams and columns:

http://www.modernsteel.com/Uploads/Issues/January_2014/012014_steelwise.pdf

 

[morriscpm]

wannabeSE,

Thank you for the article link.