Tee Web Compression (ASD)

[visitor]

If I have a built-up tee section that is receiving a load such that the web (stem) is in compression, what is the allowable bending stress of the section if the b/t ratio is less than the noncompact limit of 127/(square root of Fy)? There is an entire appendix devoted to situation where noncompact limits are not satisfied and a reduction factor Q is applied, but what happens when the section qualifies for noncompact? It states directly in chapter F that specification F1.3 is not valid for this scenario. Would spec section F1.2 apply, even if the general heading for section F1 is "Allowable Stress: strong Axis Bending of I-Shaped Members and Channels"? What I have is a stub-beam picking up a point load, and the cantilevered configuration of the stub beam results in compression in the stem. Please help!! Thanks folks.

 

[whymrg]

For your case use Qs equal to 1, and Fb equal to 0.6*Fy

 

[visitor]

Even with, say, a span of 58' (hypothetically)!? Fb=.6Fy*(Qs=1)? This doesn't seem right unless stringent lateral bracing criteria are met for the stem (76bf/(square root of Fy), maybe?). And, well, if that's the case, why even bother looking at it as an inverted tee, just examine it as a plate. Seems awfully conservative to design as a plate bent about its strong axis, though... Anyone??? Why give guidelines for inverted tees that exceed noncompact limits, but be entirely too vague for scenarios in which noncompact limits are satisfied.

 

[visitor]

"Even with, say, a span of 58' (hypothetically)!? Fb=.6Fy*(Qs=1)? This doesn't seem right unless stringent lateral bracing criteria are met for the stem (76bf/(square root of Fy), maybe?). And, well, if that's the case, why even bother looking at it as an inverted tee, just examine it as a plate. Seems awfully conservative to design as a plate bent about its strong axis, though... Anyone??? Why give guidelines for tee stems in compression that exceed noncompact limits, but be entirely too vague for scenarios in which noncompact limits are satisfied."

The above statement refers to a simple span inverted tee beam; not exactly the original problem, but definitely related...

 

[tum]

Visitor,
I think you are probably confused with concept of local buckling and global stability (Lateral-Torsional Buckling, Torsional buckling, etc.) The criteria you mentioned (b/t < (76bf/(square root of Fy)) is only for local buckling of member.
You may want to check max. allowable bending based on global stability criteria (Lateral bracing will play important role on this criteria, see Dr.Galambos 's book for reference) and compare with max. allowable bending stress based on yielding criteria (0.6FY)...use that smaller one..say Fb.
Then check the local buckling criteria to calculate Q. The max. allowable stress that you can use is Q*Fb