You need to apply your seismic forces in 3 directions!
A Structural Engineers Practical Guide To Seismic Design: Vertical Seismic Load Effect, Ev
Seismic forces act on your structure in both the horizontal AND vertical directions.
ASCE 7-16 Ch 12.4 guides you through the seismic load effects.
Ev is dependent on your site Sds and Dead Load.
It also is reversible!
Ev adds to your downward load combinations but subtracts from your uplift combinations. It makes your loading situation worse no matter what!
While it may be a small part of your load, it is important to include, especially in high seismic locations.
many folks don’t think of seismic as having a vertical load component. In higher seismic regions, this can indeed make a big difference in your load combinations. For example, if Sds=1.0, the 1.2 factor on the Dead Load is bumped up to 1.4 (1.2+0.2). That’s an increase of 17%.
That can have a real impact and cannot be ignored.
I’ve always thought of it as a separate load, but it really IS just a DL increase that you can put a % increase to.
In Europe (Eurocode 8) the vertical component has to be considered only for high seismic zones. In general, it can be neglected. Some of the interesting differences in the US vs. European approach.
We take Ev as 0.2*Sds*D as per ASCE 7.Some time we also include orthogonal effect for both horizontal direction if required for irregularities and specific SDCs. But That’s the standard equation as long as you don’t fall into the exceptions portion of 12.4.2.2