Flexural capacity of W-Beam with plate welded on both flange?

[RobertEIT]

I am checking the flexural capacity of a W12x65 beam, its un-braced length Lb<Lp, because its flange is noncompact, so I welded a 10"-width by 0.25"-thick A36 plate on both flanges. My question is with this welded plate, can I now take the beam as compact I-shape member and use AISC-13th Part16, Section F2(page 16.1-47) to calculate its flexural capacity? (without these two welded plate, I have to use section F3. to do calculation.)

Attached is my calculation. Can anybody tell me if my calculation is correct (or if it meets AISC-13th code)?

Thanks a lot!

 

[Bagman2524]

You have to recalculate the Moment of Inertia of the combined section and check compactness per Table B4.1. You should also check to make sure the web is not to thin.

 

[Lion06]

compact flanges have nothing to do with unbraced lengths. The section compactness affects local buckling. Unbraced lengths affect LTB of the section, not local buckling of a plate element of the section.

 

[RobertEIT]

To Bagman2524: That's what I want to know if I can take the welded W-beam as a solid one piece, if I can, then both flange and web is compact based on Table-B4.1. With Ld<Lp, I just need check limit state of yielding using section F2. That is where my calculation procedure comes from.

To StructuralEIT: Yes I know compact flanges has nothing to do with unbraced length. But with Ld<Lp, I don't need check LTB, and I can just check yielding use plastic section modulus Z. My question is for those non-compact or slender W-beam, after we welded a plate on its non-compact or slender element, can we take it as if it were a solid compact member with the combined thickness as its flange or web thickness?

 

[JAE]

The b/t ratios provided in Table B4.1 in AISC are based on specific buckling characteristics. When you put a cover plate over the flange, you don't have an equivalent solid plate.

You also don't have two individual plates either as the stitch welds at the ends of the cover plate do force the two to act somewhat together in terms of second order buckling under flexural compression.

So my response would be that you have "something in between" a fully combined plate and two separate plates.

Since you don't know for sure where "between" you are - you should tend towards the conservative and assume that the original flange plate is still the same b/t.

The cover plate would follow section B4.2.c where it says that the distance b would be the distance between the stitch welds. For your 1/4" cover plate this is even worse than your original b/t of the W12 flange alone.

The cover plate can buckle away from the flange plate per the attached sketch (with stitch welds). If you had continuous welds you could still buckle the top plate away from the top flange but in a spherical (hump) shape. How you would develop a b/t for that and utilize the given b/t limits I wouldn't know.

Again - where you don't have solid direction in the specs/codes you should tend toward the conservative assumption.

 

[kslee1000]

Never encounter this type of problem before, but I think you can. Because the mean focus on "compact vs non-compact" section is out of concerns over local buckling. If you do not have problem with torsion, the build-up flanges shall perform stisfactorily in that regard.

 

[civilperson]

Compact is a function of yield strength. (50 ksi vs 36 ksi in a mixture use the most stringent rule for 50 ksi).

 

[haynewp]

I think the same as JAE, there are equations given by AISC for cover plate local buckling alone. Even if the plate is continuously welded to the flange it is not the same as a solid piece of steel.

 

[kslee1000]

I don't have any thing handy to handout, but suggest to look for reference on design of built-up columns/beams. It will provide insight to the problem you are facing.