Seismic load, horizontal and vertical

[jdengos]

I know in seismic we always calculate lateral base shear, which is actually horizontal seismic load.
code also require the vertical seismic load at the same time, which is 0.3*Sds*DL, which is much bigger load for both uplift and downward. But we never consider in our design before, am I missing anything?

 

[structSU10]

Its built into the seismic analysis load combinations. see ASCE 7-10 12.4.2.3

 

[JAE]

You absolutely should include the vertical component. E = Ev + Eh

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[cal91]

In LRFD you'll have

(0.9-0.2*sds)*D + E

or

(1.2+0.2*sds)*D+0.5L+E

These must always be accounted for. Vertical acceleration creates more uplift and/or more down force. In some earthquakes, some buildings (especially those close to the epicenter) will experience a vertical acceleration greater than the horizontal acceleration.

 

[WoodDesignCat]

Doing residential design the California Bay area I have checked vertical seismic forces (and vertical wind forces) on a number of jobs. In almost every case I have done the load combinations included dead load, live load, and vertical seismic/wind loads have been smaller or about the same as the load combinations that only have dead load and live load. Unless someone has had other experience, for light frame structures it isn't a huge issue. I imagine for concrete buildings it will be a larger issue.

 

[cal91]

Gravity only members will of course usually be governed by 1.2D + 1.6L instead of (1.2+0.2sds)D + L .

For members that are already governed by earthquake load combinations, it is critical to include the affects of vertical acceleration, even in light frame construction.

Members where this is critical would be end posts at shear walls in light frame construction, or beams columns and braces in brace/moment frames.

If wind governs then the vertical acceleration combos will probably not be critical.