masonry wall with out of plane bending

[NS4U]

I was recently checking a 8" reinforced masonry wall for out of plane loading. It is partially grouted with 1-#5 in every other cell. For this wall, the applied moment exceeds the allowable moment based on unreinforced masonry allowable stresses. So, I designed it as a reinforced masonry wall.

As I was working through the calcs, it turned out that the design moment strength of the reinforced section was less than the cracking moment of the section. I know that this condition is not desirable because it implies non-ductile failure and that ACI 318 min. reinf. provisions prevent this from happening. But, 530 does not have similar provisions. I suspect that it is because similar provisions would require an impractical amount of reinforcement?

I also find it odd that for this condition, 530 allows me to use the gross MOI when checking out of plane deflections (because technically the section doesn't crack), even though for strength I am relying on the section to crack and strain the reinforcement bar to resist the applied loads. I guess this is OK because deflection check are at service-level?

This is just an odd condition that I hadn't thought about before and has put my brain in a bit of a pretzel.

Can anyone offer any other insight or thoughts on this design rationale?

 

[JLNJ]

I haven't run your numbers but at first glance this surprises me. Masonry tensile (allowable) stresses are quite low so your cracking moment would be quite low. # Additionally, 5s at 16" is not exactly lightly reinforced. For non-seismic applications #5s at 32" or even 48" is often adequate. Usually it's a fight to keep the masonry compressive stresses under control in the reinforced section.

 

[Bagman2524]

Agree with JLNJ. Doesn't seem like the strength of the reinforced section should be less than the cracking strength. How are you calculating them?

 

[TehMightyEngineer]

Two questions that would also be pertinent is which edition of the MSJC (ACI 530) are you designing to and are you using SD (strength design) or ASD?

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

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[Celt83]

"..I also find it odd that for this condition, 530 allows me to use the gross MOI when checking out of plane deflections (because technically the section doesn't crack), even though for strength I am relying on the section to crack and strain the reinforcement bar to resist the applied loads. I guess this is OK because deflection check are at service-level? ..."

This may not be an accurate statement especially for slender walls, you will need to verify the wall remains uncracked at service level loads to use gross section properties.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[XR250]

I honestly don't think I have ever checked masonry wall deflections. Seems they are pretty flimsy strength-wise even when reinforced and I just assumed that would control.