Concrete T beam flange thickness minimum

[nhtr86]

ACI 318-05 8.10.4 requires that the flange of a T beam be at least half as thick as the web is wide. Does anybody know why this is? I have a T beam with h=37" bf = 60" bw=26" and tf=9". It seems crazy that you could engage the flange with a narrower web, but not with a wider web. Also, this only applies to isolated members, so if I had a double t and sliced it in half, I would suddenly lose a bunch of capacity?

I tried to find the research that went into this but the provisions appear without any explanation way back into the 70s and farther.
Thanks

 

[hokie66]

Good question, but I don't know the answer. I looked back at ACI 318-63, which has essentially the same provision. The wording is as follows:
"906(c) Isolated beams in which the T-form is used only for the purpose of providing additional compression area, shall have a flange thickness not less than one-half the width of the web and a total flange width not more than four times the width of the web." The latest code has slightly modified wording.

I looked through my college texts without finding any reference to this requirement, although one of the authors was on the ACI 318 committee.

My only thought is that the provision was a recognition that thin outstanding elements shrink faster than the thicker web, and shrinkage cracking results. That is not much of a problem in precast elements because of accelerated curing.

 

[BAretired]

What is an isolated beam in which the T-form is used only for the purpose of providing additional compression area? I don't dig that kind of lingo.

When I look at "Reinforced Concrete Fundamentals" by Phil Ferguson, I find many examples where the flange thickness is less than half of the web width. And Phil was a member of ACI 318-63. I'll bet it was a hangup of Chairman Raymond C. Reese (just kidding).

BA

 

[hokie66]

You may be right, BA. My textbook was authored by George Winter and others, and Winter was on the ACI 318-63 committee also. Interesting how many clauses from that seminal little document are unchanged/unchallenged. Surprising that this one got by T Y Lin.