Simulating Cyclic Local Buckling-lnduced Softening in Steel Beam-Columns Using a Nonlocal Material Model in Displacement-Based Fiber Elements

摘要

Steel beam-columns subjected to cyclic loading (such as during earthquakes) may exhibit local buckling, which results in effective cross-sectional softening and localization of deformation. These phenomena are critical from the standpoint of performance and collapse assessment. Fiber-based elements are attractive for simulating beam-column response because they capture P-M interactions and the spread of plasticity and can be generalized to different cross sections from material-level calibrations. However, conventional fiber models typically employ softening constitutive material laws to represent local buckling. Without a regularizing length scale, this results in a nonelliptic boundary-value problem, leading to severe mesh dependence. A two-dimensional nonlocal fiber-based beam-column model is presented to address this issue for steel wide-flange sections subject to combinations of axial and cyclic lateral loads. The methodology includes the following elements: (1) a constitutive material model that is able to represent inelastic cyclic local buckling, (2) a nonlocal strain formulation that incorporates a physically based length scale, and (3) suggested practices for input selection and parameter calibration. Forty-two continuum finite-element models (encompassing a range of parameters including cross section, axial load ratio, moment gradient, and loading history) are constructed to inform as well as validate the presented methodology. The methodology simulates various aspects (load-deformation response, localized deformation, and column axial shortening) with accuracy and without mesh dependence. This is in contrast to conventional fiber models that exhibit severe mesh dependence. Limitations are discussed. (C) 2019 American Society of Civil Engineers.