Retaining Wall Design with Seismic Loading
Retaining Wall Design with Seismic Loading
(OP)
When designing retaining walls I have referred to the lateral pressures provided by the geotechnical reports. The reports have always included standard active pressure and an active pressure with seismic loading included (e.g. 55 pcf active and 85 pcf active with seismic loading). I have always designed with the seismic loading which is the higher of the two and still designed the walls to with a 1.5 safety factor against sliding and overturning. I just barely noticed a section of the building code (IBC 1807.2.3) which states an exception to the 1.5 safety factor as, "Where earthquake loads are included, the minimum safety factor for retaining walls sliding and overturning shall be 1.1".
From this, it appears that the retaining walls have been over designed at that if the larger active pressure with seismic loading included is used the safety factor can be reduced to 1.1. I have worked with several engineers and none have ever mentioned this before. Am I missing something or is it alright to use a lower safely factor when including seismic loading for the active pressure?
From this, it appears that the retaining walls have been over designed at that if the larger active pressure with seismic loading included is used the safety factor can be reduced to 1.1. I have worked with several engineers and none have ever mentioned this before. Am I missing something or is it alright to use a lower safely factor when including seismic loading for the active pressure?
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RE: Retaining Wall Design with Seismic Loading
RE: Retaining Wall Design with Seismic Loading
It seems perfectly logical that an extreme limit state check should be allowed a more relaxed factor of safety when compared to a strength limit state.
On a side note, I've never seen earthquake loads given as a trapezoidal load distribution. Mononobe-Okabe forces typically are expressed as a uniform load but I'm no geotech.
Also, do you consider the inertial weight of the wall itself?
RE: Retaining Wall Design with Seismic Loading
Just because the code says you can, doesn’t mean you have to. I interpret the section as it states to design to a minimum of 1.1 or greater. Conservatism is never a bad thing. Do you know what soil types are behind the wall? You could also have other potential issues if you’re designing for seismic purposes as well which is why I ask.
The bottom line is, the code is what we go by. If you want to add a little margin, it never hurts. It covers you in case there are any unknowns and provides a better design, even if it’s a little beefier than it has to be.
Erica