Deflection of CMU beam
Deflection of CMU beam
(OP)
I need to some help with checking the deflection of a 14ft long CMU 12" beam. My total DL is 446 plf. LL (roof) is 566 plf. I am limited to 24 IN depth. I have seen l/600 as a limit, but I believe this is for the design of a member supporting brick or unreinforced CMU. What is a reasonable deflection and what Moment of Inertia do I use with that deflection limit? Gross? Yield? Cracked?
Is this length to depth unreasonable? Other than completely changing materials, what options do I have to stiffen up the beam?
Thanks!
Is this length to depth unreasonable? Other than completely changing materials, what options do I have to stiffen up the beam?
Thanks!






RE: Deflection of CMU beam
RE: Deflection of CMU beam
Kaiser
RE: Deflection of CMU beam
ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION."
RE: Deflection of CMU beam
I don't think I agree with that. Strictly speaking, steel is FAR stiffer than both concrete and masonry. It's not as big of a difference when you consider EI and not just E, but by the time you include cracking, shrinkage, and creep.............. I just don't think that's a fair statement.
The only time I neglect deflections in concrete is if I meet the minimum span to depth ratios in ACI, but even then I check sometimes - if I have a heavy DL, for example.
It's not uncommon to have wide narrow beams that work very easily for strength, but are well under span/depth requirements to neglect deflection.
RE: Deflection of CMU beam
You should use Ieff for your deflection calculations. I use 50% of Ig as an approximation, unless the deflections are close to the limit.
As far as a deflection limit to observe, l/600 is always a safe bet with masonry, but you are right that it is meant for members supporting unreinforced masonry. You could use something less stringent for a reinforced masonry beam supporting roof load, but I would still advice l/480 (D+L), because I don't want to see any cracks in the masonry.
RE: Deflection of CMU beam
BA
RE: Deflection of CMU beam