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Allowable Soil Pressures for Earthquake Loads

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I have stumbled across a transitional problem with the adoption of the 1997
UBC related to allowable soil pressures.

The 1997 UBC has (1809.2) deleted any specific mention of a 1/3 stress
increase in allowable soil pressures for earthquake loads leaving any
increase up to the geotechnical engineer.  This is consistent with previous
codes where it was noted that the allowable stress increase for earthquake
loads could be greater than 1/3 where justified.

As you probably have noted there are several sets of load combinations that
you can use depending on whether you are dealing with ultimate capacities
or working stress.  If you use Equation 12-11 in section 1612.3.1 you will
see a 0.75 factor which is equivalent to taking a 1/3 increase by reducing
the loads.

  D + 0.75( L +(Lr or S) + (W orE/1.4))     Equation (12-11)

On the other hand if you use equations in section 16.12.3.2 there is no
0.75 factor and it states that      ".... a one-third increase shall be
permitted..." when checking wind or earthquake loads. 

As you can see the way the allowable soil pressures are used has changed
and is dependent on the load combinations you have used.   If you used the
recommended allowables from a geotechnical report issued prior to the
adoption of the 1997 UBC and used equation 12-11 you would effectively take
the typical 1/3 increase twice.  What should happen is the geotechnical
engineer should make it clear what set of load combinations he assumed
would be used when he developed his recommendations.

My belief is that some geotechnical engineers are not aware of this new
provision.  You may want to check with the geotechnical engineer to see
what was his intention.


Mark Gilligan SE