Steel Beam - Fcr - unbraced length

[vandede427]

I'm trying to calculate Fcr (equation F2-4 on page 16.1-47 of the 13th AISC) for W-shape beams with long unbraced lengths. I'm getting some outrageous numbers that don't make sense.

Has anybody had trouble with this equation before? Is there any erata that corrects the terms of the equation?

 

[JAE]

We've prepared a spreadsheet to calculate this and gotten reasonable numbers.

Here's some numbers we get from some various sizes:

A992, Grade 50 steel
W18x35, Cb = 1

For Lb = 2 ft, Fcr = 1145 ksi
For Lb = 4 ft, Fcr = 290.5 ksi
For Lb = 8 ft, Fcr = 76.7 ksi
For Lb = 12 ft, Fcr = 36.9 ksi

For the Lb = 8 ft, here are some values of the formula's variables:

c = 1 (wide flange)
r_ts = 1.514
Lp = 51.7 in.
Lr = 148.6 in.
J = 0.506 in^4
Cw = 1140 in^6
Sx = 57.6 in^3
ho = 17.28 in

Hope this helps a bit.

 

[vandede427]

Thanks for the help.

I was using the shapes spreadsheet that you can download from AISC. They've got a J term in the spreadsheet but it's nothing like what's listed in the book. Once I corrected that, the numbers worked.

By the way JAE, do you reply to every structural thread on this site? I see your handle on almost every one.

 

[JAE]

I have a disease.
eng-tips-itus

 

[swearingen]

He's got a mild case. I fear Kenat will actually die of it soon...



If you "heard" it on the internet, it's guilty until proven innocent. - DCS