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 Yield Point for Steel SMF Plastic Hinge Modeling for NLTHA

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Matthew Bosch-Willett's profile image
Matthew Bosch-Willett posted 04-17-2023 05:24 PM

ASCE 41-17 permits the use of the generalized force-deformation relation for deformation-controlled steel elements depicted in Figure 9-2 using the modeling parameters and acceptance criteria specified in Table 9.7-1 for nonlinear procedures.  The yield point, Qy, for flexural action in beams is determined as the expected plastic moment capacity of the section (i.e., Ry*Fy*Zx) in lieu of the nominal plastic moment strength (i.e., Fy*Zx).  What is the reason for this approach?  I would expect that this approach artificially delays the onset of hinge formation (since Qy is defined at a stress higher than the minimum expected yield stress of the material), thus resulting in less conservative deformations.

Ronald Hamburger's profile image
Ronald Hamburger Best Answer

There are many uncertainties associated with nonlinear modeling of structures including the variability of material strength, record-to-record variability in ground motion, level of damping, actual ductility available in each element, etc.  In order to obtain an unbiased prediction of performance, both ASCE 7 and ASCE 41 specify they use of "median" properties when conducting modeling and analysis.  The "expected" strength is intended to be a best estimate of median probable strength.