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Re: seismic at retaining wall

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> > I got no response from this so I'm resending.  I'm
> > sure others have been faced with this...
> >
> > How would you analyze a retaining wall that extends
> > to form a 10 foot high masonry fence for seismic forces?
> > With a parking lot at the retained side, would the surcharge
> > load need to be included?  What weight would you
> > use for the seismic mass? (Fence portion or the wall
> > in its entirety?)  It seems there are several ways
> > to look at it and all yield somewhat different results.
> >
> > Mark Pemberton, P.E..

If you read a geotech report, they will typically give you a higher active
pressure for seismic loading.  In the past, I have assumed that
the load factor for a seismic active pressure loading would be 1.4.
For a standard soil pressure loading, the load factor is 1.7.  My
reasoning is that I'm willing to take more risk during a seismic
event, than for typical everyday loading.  In  most of the reports
I have seen, the difference between the seismic active pressure, and
the standard active pressure is not significant enough to make up the
difference in load factors.

Now for your question.  I assume that you would have two controlling
cases.  The first would be standard soil pressure loading with the surcharge
included at a load factor of 1.7.  The second case is the seismic loading.
It seems unreasonable to include the live load surcharge along with the
seismic loading.  The UBC only "makes" you include a portion of the live
load in the seismic dead weight if it is over a 100 psf.  I'm assuming a
lot live load of 50 psf.  The weight for the seismic mass would include the
masonry fence, and the concrete wall of the retaining wall.  You don't
need to include the footing.  In checking your soil bearing, I believe that
it is reasonable to take a 1/3 increase.

My feeling is that you will probably get nearly identical results for the
two cases.

Hope this helps.

Dan Goodrich, P.E.