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Concrete Mu

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I would appreciate the list's comments on the "effective coefficient of
friction" given in the PCI Design Handbook vs. the coefficient of friction
given in ACI318-99 or 97UBC with regards to the design of a corbel on a
precast concrete column.

Section 6.7 "Shear Friction" of the PCI Handbook says, "An 'effective shear
friction coefficicent', mu-sub-e, may be used when the [shear friction]
concept is applied to precast concrete connections."  This PCI section
references by footnote a March-April 1978 PCI Journal Article by A.F. Shaikh
titled, "Proposed Revisions to Shear-Friction Provisions."

When calculating this effective coefficient of friction for the monolothic
condition of a corbel cast on a column I am limited by the maximum value of
3.4, given the material strengths and corbel geometry.

I also note the PCI Handbook uses this mu-sub-e in their corbel, ledge, and
beam bearing example problems throughout the chapter.

But.... There is no mention of a mu-sub-e in the 97UBC or ACI318-99.  All
formulae and provisions reference plain old mu with a value of 1.4 for
monolithic application (normal wt. concrete).

While I'm already familiar with the concept of "when it comes to shear don't
play with fire so use the method that produces the most conservative
results", I would apprieciate any relevant experience or insight list
members have with the mu factors.

Thanks in advance,

Brent Koch, P.E.