Section 10.6.3.4 of the AASHTO LRFD Bridge Design Specifications gives provisions for calculating sliding resistance of spread footings. For footings on clay, the specifications state that the sliding resistance shall equal the cohesion of the clay. This is my first use of this particular provision, and I was surprised by this. Under the previous specs (the Standard Specifications, i.e. "LFD" method) for retaining walls on spread footing, the requirement was to use the ADhesion of the clay (see figure 5.5.5A), which is typically some percentage of the cohesion (anywhere from ~.3 to 1.0). Is the new code requirement to use COhesion correct? Should this still be ADhesion? Or since it is to be used with factored loads and a factored resistance it is in fact the COhesion that should be used?
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