Max Seismic Shear in parallel single-story walls of a rectangular building with a flexible diaphragm

[jochav52802]

Good Evening,

I'm studying for the Structural portion of the California PE Exam, and one of the problems asks me to determine the question posed in the title of this post.

In their solution, they take the diaphragm force and evenly distribute it to each of the walls that are parallel to the seismic force, however, they then proceed to add the diaphragrm's seismic force to only half of the parallel wall's seismic selfweight to come up with the maximum force in the parallel walls.

My questions is whether this is correct, as intuition would tell me that I should add the parallel wall's full seismic force to the diaphragm force in order to determine the maximum shear wall force. I understand that it's standard practice to assume that half of the perpendicular wall seismic load is distributed up to the diaphragm, but I nor my colleagues are following why a similar approach would be used for the parallel walls since they're the lateral-load-resisting-system that carry the main building loads to the foundations.

I'd appreciate any support in understanding why my thinking may be off.

Many thanks in advance!

Best regards,

jochav5280

 

[Just Some Nerd]

The other half has its weight added to the diaphragm below typically, or at least I think that's what you are asking about. You would also add the bottom half of the weight of any walls/columns above

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Why yes, I do in fact have no idea what I'm talking about

 

[AaronMcD]

Personally, I always use half the wall. First, it's easier to use half of all walls rather than consider perpendicular and parallel walls separately. Second, parallel walls that are not part of the SFRS should be treated the same as perpendicular walls. I suppose the lower half of the shear wall itself contributes, but (1) it is a very small portion of total seismic weight, (2) half the wall is more accurate for overturning forces, and (3) upper levels usually have higher force, and the wall mass is mid-height. So the lower half of the wall doesn't contribute much, if at all, after taking half the wall and pushing the mass up to the diaphragm.

Technically I think using half of all walls could underestimate slightly. But that's 2% of the total weight, and we have safety factor of 2 for ASD design, and R factors with 10% jumps, and equivalent lateral force being nowhere near 2% accurate, and I almost never design anything to within 2% anyway. The contractor misses a nail and that's 2% right there.

If I have one shear wall with very heavy cladding, I do sometimes take the full height for seismic weight.

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