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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 posted 04-17-2023 06:14 PM 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.