Yes, all the provisions in 11.7 are applied to members classified as deep beams. The commentary says the limit was based on a re-examination of test data that showed beams failed primarily due to crushing of the support regions.
I vote no. Shear and concrete crushing are explicitly checked using strut and node checks in the strut and tie design procedures.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
Yes. 11.7 applies to deep beams and deep beams are analyzed using strut and tie. The upper limit on shear strength specified in 11.7 is just that - the upper limit. The strut strength, node strengths, etc. still have to be checked per Appendix "A". The upper limit in 11.7 is pretty high. If you need more strength than that, then that is an indication that your beam is too small.
That's a good point, Koot. I wonder if this falls into the realm of shear limits that no one really understands and the researchers are no longer around to ask them to clarify.
I agree with Cliff, in that if you are exceeding that limit your beam is just too damn small. I love a nice strut and tie now and again, but those max limits are there in part to smack you in the face and say you need something bigger.
- I agree that a literal read of 11.7 would lead one to apply 11.7.3.
- I agree that 11.7.3 seems like a reasonable upper limit and I would be surprised to see it govern in any properly STM designed members.
- my suspicion is that 11.7.3 exists for the benefit of deep beam design procedures other than STM.
- 11.7.2 language kind of implies that 11.7.3 could be skipped for STM designed members.
- 11.7.3 commentary contends that we're guarding against diagonal compression failure. But then the form of the equation suggests that we're checking concrete tensile stresses. Does concrete tensile stress have a major role to play in STM designed members? Maybe it is getting at cross strut tensile splitting stresses? But then I thought that the whole idea was to allow diagonal tension cracking and keep the crack width reasonable using side face reinforcement?
In answer to OP's original question, I guess that I'd put it in functional terms:
1) in a new, minimally constrained design, would I ensure that 11.7.3 was satisfied? You bet.
2) If sasisfying 11.7.3 required a post-tender adjustment or potentially agravating a client, would I let it slide? Probably.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
