level lateral load and seismic base shear

[VTHokie91]

I need some advice and clarification. I am working on a large concrete box, rectangular in plan, with a rigid diaphragm roof and precast walls. There is a small office mezzanine, bearing in a corner on exterior walls for 24' in each direction. There are interior columns supporting a ridge beam.

I have calculated seismic base shear using the dead weight of the full wall heights, the entire roof, the entire mezzanine, and equipment dead loads on the roof and mezzanine. I reported a single number, i.e. I did not split it into orthogonal components.

When calculating lateral forces using IBC eqn 16-42 (2006 ed), it appears that I should only use the tributary area of the orthogonal walls to the roof diaphragm to calculate level (roof in this case) loads. Because the building is rectangular, two different loads will be generated. I'm fine with that, but my concern is that the sum of the two numbers will not equal the base shear because I'm only using half of the wall height in each case.

Am I trying to compare apples and oranges here, or am I missing something else? Should I have not considered weight supported by columns in my base shear calculation?

Thanks in advance.

 

[TDIengineer]

Some methods do not include the wall self weights for the walls that are acting in-plane as the diaphragm doesn't see these loads. Therefore, as you have indicated you very well could have different diaphragm loading in each direction.

When it comes to checking the foundation system, you need capture back the in-plane wall self weight when checking overturning. I usually just add back in the wall self-weight times the base shear coefficient and then use half the height for the CG.

When checking mass, you only need to capture half of the column weight, as the lower half dumps it's lateral load into the foundation and base plate for that column.. the upper support of the column delivers half of the column mass to that diaphragm.

 

[TDIengineer]

In-plane wall shear stress and in-plane jamb steel design will also need to have the wall self-weight times base shear added if you're not including the walls in plane for your base shear in that direction, similarly as I described for check of the foundation.