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"Caisson Footings"

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ICBO's alternate provision of 1994 UBC 1807.10/1991 UBC 2908(j) to
establish presumptive side friction values was the result of static load
tests similar to the procedures in Methods 2 and 3 of 1994 UBC 1807.4 that
were performed  during 1958-1960.  Crumbled newspaper was stuffed into the
bottom of the holes before casting the piers to eliminate influence on
results due to end bearing.  Load testing of rectangular spread footings
was performed at the same time in the same ground conditions and the
settlements of both spread footings and piers were measured after set.  The
results were correlated to empirically arrive at the one-sixth ratio that
has been in the UBC since 1964.  The comparisons were based on yielding,
not capacity. 

For uplift, presumptive side friction values (which were based on long-term
loading with FS=2) are very conservative even after increasing the
allowable by one-third, but side friction load may be reduced by applying
0.85*DL.  
The limitation is usually the capacity of the pier under eccentric load.
For projects where it is worthwhile, tests for axial uplift are described
in ASTM 3689.  

Another problem with the use of the nonconstrained equation of 1994 UBC
1806.7.2.1 is (4) the method is limited to a depth of 12 feet.  For lateral
loading of deeper and larger drilled shafts there are several enduring
references, for ground support and strength design, which give clear
diagrams and equations: (A) geotechnical; "Design of Concrete Foundation
Piers", 1968, by F. Randall, PCA CR017.01D, and (B) structural; "Capacity
of Reinforced and PS Concrete Pile Sections", 1979, by W. Gamble, ASTM STP
670.


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*   Lawrence B. Karp          Siesta Valley Ridge     510 254-1111   *
*   Geotechnical Engineer     Orinda, CA  94563       Fax 254-2825   *
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