AICS 9th Cover Plate

[volunteera]

Cover plate is welded to the top flange of an I-Beam in full length. The beam is simply supported and top flange is in compression.

AISC 9th Manual Table B5.1 row 4th - Outstanding legs of pairs of angle in continues contact; angles or plates projecting from rolled beams or columns; stiffeners on plate girders accepts no compact sections - which means that all beams with cover plates are whether noncompact or slender. Is this correct? If so, why? Say, w-shape which has compact flanges becomes non-compact once a cover plate welded on?

AISC 9th Manual F1.2 - Allowable stress for non compact sections is calculated as per Fb-Fy(0.79-(0.002*bf*F^(1/2))/2tf). What is tf and bf standing for? Are they parameters of cover plate or a rolled beam flange the cover plate welded on.

Any help is appreciated.

 

[TehMightyEngineer]

In the 13th & 14th edition; Table B4.1 lists case 12: "uniform compression in flanges of rectangular box and hollow structural sections of uniform thickness subject to bending or compression; flange cover plates and diaphragm plates between lines of fasteners or welds". This is similar to the 9th edition, Table B5.1, row 7.

A beam may become non-compact with a cover plate welded on if the cover plate is thin, wide, and welded only at the ends. Remember non-compact means that it cannot reach the plastic moment capacity prior to an element going into a buckling failure mode under compression. Thus, a tension only cover plate wont matter; a thick flange cover plate wont matter; and the original flange likely wont matter (assuming it's a modern wide flange shape with fy <= 50 ksi). If all these are compact and the cover plate meets Table B4.1 (B5.1 if you insist on using a 26 year old specification) then the section will be compact for flexure.

As for the F1.2 equation, that's a good question with no good answer I've found. I personally simplify it by ignoring whatever extends past the other (flange or cover plate) that way bf = flange width = cover plate width. For tf, I believe you can use this as the combined thickness of the flange and cover plate. Of course, this all assumes you have correctly connection the cover plate to the beam flange with sufficient welding for the shear flow.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[volunteera]

TehMightyEngineer,

Thanks for repling back.

Table B5.1, row7. applied to .....between lines of fasteners or welds. In my case, the cover plate projecting out from the egde of flange where the two paralel stream of welds are. I think i have to stick with row 4. What do you think?

Concerning with your second statement, I am in full agreement with you. However as you underlined, the cover plate might be designed such that it can compose a compact section which is not an available option in row 4.

I see it as a good conservatine approach to ignore the projecting parts and combine the thicknesses. However, in my particulat example PNA is almost between the beam flange and cover plate. On the other hand, as the section is not considered compact it will not defelop a full plastication.

 

[straub46]

How I understand it is the "between lines of fasteners or welds" refers to the width to take when comparing the width/thickness ratio for the plate. The welds would be on the toes of the flange, so you would use the full flange width for "b" and then the thickness of the plate to determine whether it's compact, noncompact, or slender. This seems to be the intent with the graphical representation shown in Table B4.1 of the 14th edition.

As for the initial part of your question, reading B5.1 4th row indicates the elements are either slender or non-slender. The more recent versions of the AISC code clarify this, and have different capacity calculations for members in compression for slender and non-slender, as opposed to compact, noncompact, and slender for flexure.

Go Bucks!

 

[TehMightyEngineer]

volunteera: how far is your cover plate projecting from the weld? It seems like it would have to be really far for the cover plate to be considered slender.

See what straub posted regarding the clarifications the 13th and 14th editions have on this.

If you actually need to consider the true dimensions of the flange width then I'd design it as a singly symmetrical built-up section.



Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com