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Deflection of CMU beam

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!
 

RE: Deflection of CMU beam

Is your beam a reinforced concrete one that just happens to have some U masonry pieces as formwork? If so some RC approach would be the best, following the recommendations for the case as RC.

RE: Deflection of CMU beam

Please indicate what reinf. you have in the beam. Typically, if a masonry or concrete beam works for bending and shear, I wouldn't worry about deflection. Masonry and concrete is a lot stiffer then steel, to put it in layman's terms.

Kaiser

RE: Deflection of CMU beam

agree with ishvaaag and remember that you can take into account the top steel to figure out your Ieff.  

ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION."
 

RE: Deflection of CMU beam

kaiser-
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

I agree with Lion06.  I don't think your span/depth ratio is unreasonable in general, but you should definitely check the deflections for the particular loading conditions.

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

At 24" depth (say 20" effective depth), the beam is under-reinforced as a simple span, so I would not bother to check deflection.  2-#5 T&B should be adequate to carry the moment.  If desired, the top bars can be extended 3' beyond each edge of the opening to develop some continuity, further reducing deflection.

BA

RE: Deflection of CMU beam

Sorry, I misread the original post... I thought it was a 12" deep beam.  At 24" deep, BA is right... deflections probably aren't an issue.  Regardless, if you want to check them, I still suggest Ieff and a more stringent than normal deflection limit.

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