slender column

[nitnelion]

I am using a W27x146 for a tall column. It's web is slender for compression. However, when I go through section E7 of the AISC manual, I find that there is no reduction (Qa=1). E7.2 is kind of confusing since they don't tell you what to do when b/t is less than 1.49x(E/f)^0.5. Am I missing something here? What do you do when b/t is less than 1.49x(E/f)^0.5?

 

[civilperson]

Assuming 50 Ksi yield, I get effective width of web of 22.1", (from E7-17). This gives a A effective less than A and thus Qa less than one.

 

[jnadeau]

My interpretation is that if b/t is less than 1.49*sqrt(E/f) then the element will not buckle locally prior to the column buckling globally and thus Qa=1.0. Note that if f=Fy then the above limit corresponds to the slenderness limit in Table B4.1.

 

[jnadeau]

I agree with civilperson that the effective web width is 22.1" if you conservatively take f=Fy=50ksi. However, for a pin-pinned column over about 15 ft for this Q27x146 then effective web width is the true web width and Qa=1.

 

[SteelPE]

If you assume f=50ksi (as described above) you end up with Q<1.0. However, this is incorrect, according to E7-2, f is to be taken as Fcr (with Q=1.0).

This is how I rationalized it. For a really short column, local buckling of the individual plate sections that make up the column controls. Since the column is short, you end up with a high Fcr and therefore Q<1.0. For a longer column global buckling of the section becomes more critical than local buckling and Q=1.0.

Unfortunately you have to do the calculation in order to find out what is governing the design of your column.