Allowable Stress Design of Concrete Masonry

[Deadblow]

Hello All,
I am working on the design of a 6'-0" tall shipping dock wall. It will be 12" cmu grouted solid. I referred to NCMA TEK 14-7B and was able to analyze the wall utilizing the rectangular beam analysis. This analysis proved that the wall needs #5 bar at 40" o/c for flexure. My question is in regards to shear and the tables provided in NCMA TEK 14-19A (attached). The allowable shear stress, Fv, is equal to sq. root of f'm of 1500psi, 50psi maximum. So Fv for my wall = 38.7psi. If you look at the eighth bullet point on the first page, it states "walls are grouted only at cores containing reinforcement". So when I look at the table for 12" thick cmu with d=5.813, I see that almost all of options have a resisting shear capacity of 2,701 lb/ft. How do they arrive at this number? Why is it the same for walls grouted at 8" o/c through 72" o/c?

I know that 2701/(12x5.813)= 38.7psi. So it appears to me that these tables are accounting for shear strength from cores that are not filled with grout and are just air. Thoughts?

Lastly, why does the shear capacity change for bars spaced more than 72" apart?

Thanks

 

[mike20793]

Without getting too in depth of ACI 530, the actual shear stress is a function of the effective width of the reinforced cell, which is the least of the center to center bar spacing, 6 times the nominal wall width, and 72 inches. Once you exceed 72" bar spacing for 12" CMU, your effective width is limited to the 72" and not the center to center bar spacing, so the capacity goes down. If you look at the 8 inch tables, you will see the same is true once it exceeds 48 inch spacing (6 times the nominal wall thickness). See ACI 530 for more information regarding CMU design. Also, it is important to note that 1500 psi CMU is actually kind of hard to find now, 2000 psi is pretty much the standard so take advantage of the higher strength. Also, the allowable stresses of the steel have been increased in the ACI 530-11 to recalibrate it to closer match LRFD design. All in all, that table is probably a bit conservative.

 

[Deadblow]

Thanks Mike,

Looks like I will be purchasing ACI 530-11 or 530-13.

 

[Deadblow]

I am now looking at TEK 14-19B (attached) which is based on the 2012 IBC and I am still unable to confirm the Vr values listed in the tables. Can someone show me how to recreate the shear capacity of masonry numbers? Thanks!

 

[hokie66]

If it is a shipping dock wall, I wouldn't just depend on minimum requirements of a code, but rather build it for robustness. So I would have bars in both directions at not more than 16", and as you indicated, would fill all cores.

 

[bookowski]

Deadblow - No, they're not counting the hollow cells. It looks like they're using the formula for reinforced masonry, Fvm (the masonry component only) x the next area.