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Mill Certs/Yield Strength

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Post-Northridge earthquake reports evaluating the implications of
overstrength with respect to welded steel moment frame joint cracking have
repeatedly pointed out the danger of relying on mill certification as a
accurate predictor of local material properties.  My understanding from this
literature is that tests show significant variation in yield strength and
pre-fracture elongation (ductility) depending on where the test sample is
taken and, at least for thick material, whether it is pulled along or across
the axis of rolling.  Changes in the way mill sampling is done may be afoot,
but I think ASTM and the mills would tell you that a mill cert, intended as
evidence that a particular run of steel met material specs, may not provide
a sufficiently accurate measure of the average yield strength across the
entire cross-section of a particular member to serve as the basis of design.
This is probably why the Canadian code explicitly says not to use it that
way.  At this point AISC might well tell you not to use mill certs for
design too.  I would talk to them and seriously consider getting a qualified
independent lab to make additional tests for me before I relied on mill cert
as proof of higher than specified yield in a critical application.

Drew A. Norman, S.E.
Drew A. Norman and Associates