Controlling Lateral Force: Seismic vs. Wind 2

[sundale]

What is the general consensus in regard to a what is deemed the "controlling" base shear in a lateral analysis?

Namely, say you have E = 20 kip seismic base shear (ultimate) and a W = 10 kip wind base shear (service). IBC load combinations would say 0.7E to convert it to a service load: E = 14 kip (service). BUT, to calculate the seismic base shear, you also divided the force coefficient by the (inelastic) response modifier R.

Is directly comparing a "service" E = 14 kip (with implied inelastic assumption) to a service W = 10 kip (with an elastic assumption) really correct?

I (perhaps too) simply compare 0.7E vs 1.0 W for determining which one "governs" the base shear force requirement. For deflections, I will check check the seismic P-delta stuff and then also a servicability drift using 0.7 W.

 

[JAE]

It doesn't matter which one governs the base shear. Technically each and every element of the structure must be checked against both. In many cases, the wind may govern some elements and the seismic may govern others in the same building.

For instance, you might have a 90 mph wind area and a Seismic Design Category B. Say you have a long, narrow structure where the main wind force applied to the short wall faces doesn't produce much lateral effect while for seismic, due to the long building, there is plenty of mass and thus higher lateral forces developed than the wind.

But for the wind perpendicular to the long face, the wind would probably be higher.

But in designing an concentrically braced frame, the brace must be designed straight-up for wind and then also checked for the seismic requirements - say from the AISC seismic code which requires the special load combinations (with the omega factor) used on the connections.

A collector beam at the eave could be sized for flexure due to lateral wind forces on it, but its longitudinal design as a collector would possibly require the seismic loads to govern as these elements require the omega factor applied using the special load combinations.

To answer your main question - I use the comparison of 0.7E vs 1.0 W....but I've never heard of a "base shear force requirement".

It is a mistake to simply look at base shear wind vs. seismic, pick the higher one and ignore the other in the design of the structural elements.

 

[MillR]

I agree with JAE and want to add: even down to a single holddown, you ought to still be comparing seismic and wind because the factors for dead load resistance are different and without direct comparison you can't know which controls. You can't decide ahead of time which controls for the building or even just look at which controls for each direction.