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Minimum
bearing area under the house was determined using
the combined weight of the structure and its contents
and an assumed soil bearing strength. The total weight
of the structure, the floor live load, and the maximum
downward wind pressures (or roof snow pressures) were
combined to obtain an upper conservative estimate
of load to be supported by the soil.
An
allowable soil bearing strength of 1000 pounds per
square foot was used to determine minimum bearing
area. This was based on examining the 1975 soil survey
data published by National Resources Conservation
Service (NRCS) for the District of Columbia. The soil
family, which is denoted as Udorthent, has an approximate
sand content of 40%, silt content of 30%, and clay
content of 30% at the 15 inch depth. It is interesting
to note that the NRCS survey states "These areas
have poor potential for almost any use. Most areas
of this unit are subject to subsidence and, therefore,
have poor potential for use as building sites."
Similarly,
anchorage requirements were based on conservative
estimates of soil strength. Screw auger-type anchors
are specified to resist overturning moments of the
structure due to wind pressures. |
