In Plane Shear Strength of Unreinforced Masonry Wall

[Jake927]

Hi all,
I am working on a renovation of a 1920s house that is wood framed over an 8" unreinforced brick masonry 1st story. They are looking to open up the 8" wide 1st story wall as much as they can. The part of the design I am having trouble with is the lateral resistance of the brick wall when we create new window and door openings. So my question boils down to this: how do I calculate the in-plane shear capacity of an unreinforced brick masonry wall? I've intentionally not included specific numbers as I don't think they are relevant. I am more so looking for the appropriate equations / references. Thanks and please let me know if I can provide anything else.

 

[tolchijb]

P/a & Mc/I of the new brick piers is how I would approach it. I would be OK using URBM to resist wind loads, but I would recommend against using it for seismic resistance. The load reversals will break the wall apart after just a few cycles.

 

[Luceid]

OP is looking for capacity. I'm not sure where I would look, since URM isn't permitted in CA.

 

[milkshakelake]

This is a problem I deal with on a daily basis. Technically, unreinforced brick walls have some lateral value, but I don't think it's published anywhere. Furthermore, we've seen earthquakes around the world where brick masonry fails spectacularly and dangerously during a seismic event. However, we have to assume that it has some lateral value, because there are 6 story brick buildings everywhere and they're still standing.

The approach we take in NYC is that old brick buildings don't need to be reinforced. As soon as you remove any of the lateral system or add more than 10% seismic/wind force to it (like an extension), you need to design the entire building in that orthogonal direction (non-orthogonal in some cases) per latest codes. The reason I say entire building is because lateral stiffness of one side can affect the other side, but some judgment is needed for that. In your case, when making the opening, replace it with a steel moment frame or something similar to take the seismic forces at that wall, and assume the walls on the other side are okay for their own lateral loading.

 

[Celt83]

ASCE 41-17 addresses the in-plane strength of unreinforced masonry and they also have some recommended lower bound strength values.

 

[Luceid]

Celt is right - ASCE 41 would probably be a good place to start