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Re: Increases in Foundation Bearing Pressure For Wind and Seismic Loading

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Tom,
 
The 1/3 increase in allowable stress has been applied to soil bearing values from the time I began learning the Building Code, and soils engineers have consistently included it in their recommendations and structural engineers have based their designs on it in every project that I can think of.  It seems to be a concept that has stood the test of time.  Do you know of unsatisfactory foundation performances related to use of the 1/3 increase?
 
To me, the 1/3 increase seems believable and reasonable by the following rationale: In general the strength values for soils are based on saturated soil -- dry soil has greater strength.  In saturated soil, support of short-term loads is shared by water pore-pressure and the inter-grain bearing strength.  Time is required for the pore-pressure to dissipate under an increase of load -- not much pore-pressure dissipation occurs during the time of a wind or seismic pulse.  The 1/3 increase is taking advantage of either the support provided by the temporarily available water pore-pressure before it dissipates, or of the greater strength of dry soil, if it is dry.  That's a non-geotechnician's simple understanding, and I'm open to enlightenment by the real experts.
 
Nels Roselund
Structural Engineer