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Masonry Shear Wall Reinf. Ratio

Masonry Shear Wall Reinf. Ratio

Masonry Shear Wall Reinf. Ratio

(OP)
I'm designing a partially grouted masonry shear wall controlled by wind in SDC B. For the tension reinforcement resisting overturning (if the wall is subject to flexural tension), I'm taking the reinforcement ratio based on the NET area of the masonry. In this example, using an 8" block with grouted cells at 48" OC, the Anet = 41 in2/ft. So for a 8' long wall, the total Anet = 8' x 41in2/ft = 328 in2.

This is opposed to using the gross area which would be 8' x 12"/ft x 7.625" = 732 in2.

The masonry design textbook I'm using states that the reinf. ratio is As/b*d, so it uses the gross area, but this would only apply to a fully grouted masonry section, in my opinion. Section

Do you agree that Anet is the appropriate property to use for a shear wall reinforcement ratio? Are you aware of a good resource for the design of partially grouted shear walls? The NCMA tek manuals are great but do not seem to cover this specific topic.

RE: Masonry Shear Wall Reinf. Ratio

What code provision are you dealing with when checking the reinforcing ratio KB? That might make it easier to figure this out. If it's a check to determine whether the section is tension controlled, then I'd tend to agree with your assessment of things unless the neutral axis falls within a solidly grouted boundary element.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Masonry Shear Wall Reinf. Ratio

(OP)
I don't think the code directly addresses what the reinf. ratio needs to be for calculating flexure. I'm just using the masonry section of the SE Reference Manual, which goes through the process of calculating stresses based on stress/strain relationship for a given reinforcement ratio, but assumes that this section is fully grouted.

I did have a conversation about this topic with someone from TMS. It sounds like the correct way to do this is to calculate the area of the "U-shaped" compression block created by the fully grouted end cell and part of the face shells as determined by a stress/strain analysis. Not the easiest calculation to perform, especially when you have multiple layers of tension reinforcement. However, for design purposes I think you could conservatively assume that the area of masonry is only made up of the 2 face shells (like an ungrouted section) and calculate your masonry and steel stresses using the standard textbook method.

RE: Masonry Shear Wall Reinf. Ratio

It seems to me that you are going about this in reverse. You need to determine your tension reinforcement first, and this depends on the loading and effective depth only. Then you need to check the compression on the blockwork.

I would never attempt to distinguish grouted from ungrouted in a shear wall, as I always insist on grouting them solid.

RE: Masonry Shear Wall Reinf. Ratio

(OP)
Hokie - the effective depth will depend on the depth of your compression block, will it not? Certainly, you could make a guess as to what this will be for your initial rebar trial size, but you would have to verify that you didn't underestimate the depth of the block which would result in a larger distance between your tension/compression couple.

RE: Masonry Shear Wall Reinf. Ratio

Yes, that is correct, and the way I do it. Speaking in WSD, you can use As = M/.85fd, where f is your working steel stress, and d is from the steel centroid to the extreme compression fibre. I prefer to say "estimate" rather than "guess".

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