Existing RC Beam Shear & The IEBC

[MacGruber22]

IEBC requires strengthening/replacement for a structural element whose gravity demand is greater than 5-percent (using current codes). Easy enough.

However, I am checking an existing RC beam for shear (new addition loads) and find that Vu is over 75% of phiVc. With (1/2)*phiVc limit in mind, is my beam "overstressed"? Would the IEBC 5% allowance permit the shear check to be (0.05*50+50)*phiVc (i.e. 53% of phiVc)?

If it helps, I have put my eyes on these beams and they seem to be in tip-top shape.

"It is imperative Cunth doesn't get his hands on those codes."

 

[MacGruber22]

bump

"It is imperative Cunth doesn't get his hands on those codes."

 

[JAE]

Are you suggesting that the beams have Vs = 0 (i.e. no stirrups)?

If so, then I've usually treated the ΦVc/2 limit (which requires minimum stirrups) with a little looseness.

If you do have some stirrups, but they are less than the minimum, I still treat ΦVc/2 as a "strong suggestion" but, as it is an existing building, don't apply a 5% rule to the minimum stirrup limit.

There is also things like:
1. Verifying the actual f'c with cores.
2. Looking very closely at all the loads, live load reductions, analysis assumptions in an attempt to minimize Vu to the most accurate point possible.
3. Verifying actual beam sizes (bw and d) and also possibly including a bit wider bw if the beam is a pan-formed shape with tapered sizes.

Older structures many times were designed by engineers who didn't have a lot of code or prescriptive guidance on shear design so it isn't rare to find older beams working great in flexure but with questionable shear capacities. That always poses a dilemma since shear failures tend to be quick and sudden and taking liberties with things like a ΦVc/2 limit are perhaps OK but not to be too much abused.



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