Base shear part of foundation design? 1

[cobadger]

I haven't had to do a seismic SDC D structural review in a while and would like some clarification. I am having another PE review the drawing, so I have some help there. That being said, I am reviewing a 1.5 story log house. It's rectangular with a 10' poured foundation, though the house has some large openings. I've calculated the base shear (V) and loft level shear as well as the Seismic load effects.

My question is how should I handle the base shear in terms of the foundation design. Do I merely take the base shear and divide by the perimeter of the house, for the house-to-foundation shear connection value or do I handle the main floor as a flexible diaphragm and distribute the shear loads to only the end walls, which are then designed accordingly?

Also, does the base shear induce any sort of lateral load (normal to the wall face) on the foundation walls or is the base shear supposed to be distributed by the main floor diaphragm to the end walls? I have a base shear of 1841 lbs./ft which would be tough for a standard 3/4" subfloor to handle.

Thanks for your help.

 

[msquared48]

Normally, even in a log home, the roof diaphragm takes most of the load gto the log walls and foundation directly without ever seeing the main floor diaphragm. I doubt that you really have the problem you think you have here with thew 1841# figure. And really, unless there is a basement, the floor diaphragm dead load should not even figure into the base shear.

Mike McCann
MMC Engineering

 

[asixth]

The shear will get to the foundations through the stiff lateral elements, if it is the walls that are taking the seismic forces from the roof diaphram to the foundations, then the foindations that support those walls will be required to take out the base shear.