Allowable Stress 1

[dannyo]

Can someone explain to me how the allowable axial stress for a W shape column (being used as a support leg ~ 24" high) is determined? I have two differnt (or maybe the same?) ways, but am not sure which to use.

AISC has a table that depends on the value of (K1*L/r). There is a table for 36 and 50 ksi yield stress steel.

I have also seen the allowable stress defined as Fa = 0.6*Fy (Fy = yeild stress).

 

[IJR]

The 0.6Fy is the upper limit in pure tensiona and compression.

When you deal with columns, you have no choice but to take slenderness kl/r into account in determining the Fa. So there is no two ways. AISC ASD cleary gives equations for Fa. When kl/r equals Cc you will see that equation ASD E2-1 will give you Fa =3/5Fy which is 0.6Fy


regs
IJR

 

[ronster]

Use the the AISC column table for 36 ksi steel and kl=2 ( kl=4 if the column is cantilevered). If your support column is subject to earthquake and/or wind loads and is not braced then you may also have a bending moment in the column which makes the analysis a little more complicated.

 

[ishvaaag]

In any case don't attempt to design any relevant column without proper guide. Columns -or compressive members- are maybe the more risky -ruin- business in construction, at least in buildings. The permissible stress varies wildly with parameters such length, boundary contitions atop and bottom and placement in high risk sesimic area or not. So if you are walking an unknown path ensure you follow exactly the guide of one that knows how, or study the subject enough not to risk important failure.