Seismic soil bearing pressure and overturning for grade beams

[apaulos15]

I was wondering if any of you guys could help me out on a problem I have been wrestling with. I'm doing a seismic retrofit for an apartment building in the Bay Area, and my design includes adding new stud walls to create new brace lines for shear. I'm running a new grade beam under these new walls, which I intend to span the whole length of the building so I could dowel it into the existing foundation on either end. Due to the layout of the building, my shearwalls won't always be centered at the middle of the grade beam. For example, I might have a 5'-0" long shearwall at the end of the grade beam, while the grade beam spans 25'-0". My question is, how would you approach determining the soil bearing pressure due to the overturning moments? The wall would be designed entirely to resist shear and would have very little gravity loads. All the textbooks I have been referencing for foundations subjected to overturning seem to be based on the fact that the moment is applied at the center of the footing, whereas my overturning moment would be off center. Any input or advice is appreciated!

 

[theonlynamenottaken]

The point of application of a concentrated moment does not affect the soil bearing pressure magnitude and distribution if you are assuming that your footing acting rigidly. If you have any vertical force being applied concurrently with the moment at that point then its eccentricity would affect your bearing pressure distribution and magnitude. However, if your foundation does not act rigidly then you'll need to consider the flexural stiffness of your foundation and its interaction with the soil below. With flexible behavior the point of application of a concentrated moment does matter.

My personal threshold for rigid-action with a foundation is ~4:1. For instance, an isolated footing that is 4'-0" square would act rigid if it was ~12" thick or thicker (2x cantilever span of 2'-0" = 4'-0", 4'-0" / 4 = 1'-0"). So, unless your grade beam is going to be (25' / 4) ~6' thick or more then it will behave flexibly. To analyze a flexible foundation you should treat it as a beam-on-elastic-foundation. This approach considers the flexural properties of the grade beam, and considers the elasticity of the soil idealizing it as a series of compression-only springs.

Note that if you are combining any non-spring supports (i.e. the pinned, doweled-to-existing foundation ends of your grade beam) with a series of subgrade springs then you'll probably need to run this through structural analysis software.