Base Shear Distribution

For seismic analysis using equivalent force method per IBC, the base shear is distributed vertically using equations involving the weight at each floor x the height / sum of the weight x height for all floors. For a single story slab on grade building, this results in 100% of the base shear being applied at the roof level. However, I have seen published design examples where the lateral force applied at the roof level is the weight of the roof + 1/2 the weight of the walls. Which is correct?


If it is a single story flexible diaphragm on shear walls, I take (1/2 the perpendicular walls + roof weight) in designing the roof for seismic.

If it is a single story rigid diaphragm, I think you should take (1/2 the perpendicular walls + 1/2 the parallel walls + the roof weight) when designing the roof diaphragm for seismic.

I have also seen examples that do this differently than I have stated above. I do not design the diaphragm to take the total base shear that includes the weight of the full height of all walls.

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