Deflection of CMU beam

[Schneidly]

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!

 

[ishvaaag]

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.

 

[kaisersoze]

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

 

[rowingengineer]

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.”

 

[Lion06]

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.

 

[gte447f]

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.

 

[BAretired]

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

 

[gte447f]

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.