Masonry Wall Maximum reinforcement limits

[reezyf]

When designing an 8" CMU bearing wall for out of plane wind loading I have found that the maximum reinforcing ratio (per MSJC 2011) only allows #5's at 16" max - therefor no reinforcing every cell...

I am refering to the maximum reinforcing ratio as discussed in MSJC '11 section 3.3.3.5 commentary; when i plug in the typical material values -without any axial load- i get a max reinforcing ratio of .007 which only goes down when axial load is considered. The .007 ratio only allows placing #5's up to 16" OC, and once axial loading is considered this .007 value drops off even lower.

Many of the older engineers in my office often say to just put #5's (or higher) @ 8" for beefy walls but this seems to be over-reinforced no matter what so i feel i must be reading something wrong.

Has anyone run across this maximum reinforcing issue? Is it still typical to call for #5's or higher at every cell, and if so how is this addressed vs the code requirements?

Thanks~

 

[DaveAtkins]

I would not follow MSJC. I would follow ACI 530. You can use ultimate strength design, which should allow you to provide bars in every core.

DaveAtkins

 

[JAE]

I think MSJC 2011 is really meant as TMS 402-11 (formerly ACI 530 which doesn't exist anymore).
So the same thing.

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[DaveAtkins]

OK, I can't keep up with changing code names

So I looked at the Commentary section referenced, and sure enough, rho max is 0.00714 by my calculations (I assumed f'm = 1,500 psi).

One option is to take advantage of the axial load on the wall, including the selfweight of the wall. This will reduce the amount of steel required.

Another option is to use a higher f'm.

A third option is to use rebar at BOTH faces of the CMU. For an 8" wall, this will increase d from 3.81" to 5.5" (for a #6 bar).

DaveAtkins

 

[masonrygeek]

I guess my question would be - why are you trying to put reinforcing in every cell? If the design doesn't call for it, then it isn't needed. We see a lot of masonry walls that are overdesigned due to lack of familiarity with masonry design. Using documents such as TMS's Masonry Designers Guide or NCMA TEK series can help.

As mentioned above, using an f'm higher than the default is helpful. In newer versions of TMS 402 (2013 and 2016 versions), the default value for f'm=1500 psi is gone. New research has shown that the minimum f'm is 2000 psi and can be even higher if the block strength is more than the minimum value for the unit. So considering an f'm of 2000 or 25000 or even 3000 psi may be appropriate, depending on the block strength. That may make your designs more efficient.

 

[EZBuilding]

I recall looking into this and finding that the ASD provisions do not have a maximum reinforcement requirement, which would allow you to reinforce at 8 inches off center. ASD also does not have provision for second order effects. Masonry design has flip flopped in the past few years as to which methodology is more efficient. Prior to the allowable stress increase in tension (was that in the 08 code?) I think LRFD was the more efficient provision. Now with the maximum reinforcement limitation, ASD has gained ground. I can't think of any other examples where the ASD/LRFD methodologies provide such a difference in results...