Existing block wall as a shear wall

Does the wall work as an unreinforced ungrouted masonry wall? I'd look at my shear and flexural stresses as a starting point for making my decision.

EZBuilding said:

Does the wall work as an unreinforced ungrouted masonry wall?

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That is my question exactly. Would you assume that the existing wall acts as hollow and unreinforced? Below is a plan for reference with the wall in question highlighted in red

Torchman said:

I am wondering how you feel about using an existing block wall considering the unknowns, specifically, on one side of the addition, I am looking to use the existing wall as my shear wall. My approach is that it would require a global check of both structures?

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Per the Existing Building Code, if you increase the lateral demand to an existing element by more than 10% you "own it". By observation, this threshold will be met and if you attach to this wall you need to perform a check that considers all of the loads on the wall.

Torchman said:

Even with the global check, some unknowns that concern me: are there any bond beams? are there any ladder or vertical reinforcement? how much of it is filled? If you would use it, how would you go about investigation of bond beam and ladder reinforcement? Is scanning is the best way?

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I would have my initial calculations drive the need for further assessment. Does it work as an unreinforced hollow masonry wall? If so, further investigation may not be needed. If it does not work, one could consider exploratory methods or reinforcement efforts to improve the capacity.

Torchman said:

At which point, is it best just to design a lateral resisting system just for the addition on that side? It looks redundant, but that is just looks. Let me know what you think.

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A 3-sided diaphragm is a possibility, but your aspect ratios seem pretty tight. This all depends on your lateral demands, is this house in middle of the US 90 mph and minimum seismic or is it in LA or Miami? Until you have a sense for how hard the existing walls are working, it becomes hard to define the best path forward.

Agreed with the comments above. Would like to add that if you use it, it might affect your seismic R value, increasing the load on other new elements of the building.

At which point, is it best just to design a lateral resisting system just for the addition on that side?

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Not really. The lateral system works together. Expanding a building increases demand on the whole building. That's a very general comment and there are lots of nuances. But in general, you can't just decide to design a piece of shear wall for an addition only, even with flexible diaphragm assumption.