30-05-2012, 03:04 PM
Combined Footings
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A combined footing is usually used to support two columns of unequal loads. In such a
case, the resultant of the applied loads would not coincide with the centroid of the
footing, and the consequent the soil pressure would not be uniform.
Another case where a combined footing is an efficient foundation solution is when
there are two interior columns which are so close to each other that the two isolated
footings stress zones in the soil areas would overlap.
The area of the combined footing may be proportioned for a uniform settlement by
making its centroid coincide with the resultant of the column loads supported by the
footing.
There are many instances when the load to be carried by a column and the soil
bearing capacity are such that the standard spread footing design will require an
extension of the column foundation beyond the property line. In such a case, two or
more columns can be supported on a single rectangular foundation. If the net
allowable soil pressure is known, the size of the foundation B x L can be determined.
Cantilever or Strap Footings.
A strap footing is used to connect an eccentrically loaded column footing to an
interior column.
The strap is used to transmit the moment caused from an eccentricity to the interior
column footing so that a uniform soil pressure is generated beneath both footings.
The strap footing may be used instead of a rectangular or trapezoidal combined
footing if the distance between columns is large and / or the allowable soil pressure is
relatively large so that the additional footing area is not needed.