Base Shear distribution

[sonnyson]

For seismic analysis using equivilent 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?

 

[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.

 

[tngolfer]

Similar to haynewp, I (and my office) only use half the wall height (plus parapet if present) for our seismic weight acting at the roof.

 

[MarcbSE]

Agree with haynewp and tngolfer. The reason you do this is that you don't need to consider the weight of the parallel walls because they resist their own shear and the load will never reach the roof diaphragm.

You do need to consider 1/2 the weight of the perpendicular walls because that seismic shear will have to be transferred into the parallel shear walls. The rest goes directly into the foundation.

 

[strguy11]

Similar to MarcbSE, although for a metal stud wall with brick we include the weight of all walls and only ingore the parallel walls if they are concrete or masonry, with the thought being they are rigid enough to resist their own shear, while the metal stud walls wont be.

 

[MarcbSE]

strguy11-
I assume you are referring only to metal stud walls that are not shear walls? Obviously if the exterior walls are sheathed (or strap braced) shear walls they will resist their own shear.

If the exterior walls are not shear walls and all bracing is through braced frames or the like, then I would also include the weight of the parallel walls in the diaphragm shear as strguy11 said. That is assuming the braced frames are not located immediately adjacent to the exterior walls.

 

[strguy11]

Correct.