**QUESTION**

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?

**REPLIES**

**haynewp**

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.

**SOURCE**

https://www.eng-tips.com/viewthread.cfm?qid=215483

Above is a snippet.