Simulating Local Buckling-Induced Softening in Steel Members Using an Equivalent Nonlocal Material Model in Displacement-Based Fiber Elements

Fiber-based elements are commonly used to simulate steel beam-columns because of their ability to capture P-M interactions and spread of plasticity. However, when mechanisms such as local buckling result in effective softening at the fiber scale, conventional fiber models exhibit mesh dependence. To address this, a two-dimensional (2D) nonlocal fiber-based beam-column model is developed and implemented numerically. The model focuses on hot-rolled wide flange sections (W-sections) that exhibit local buckling-induced softening when subjected to combinations of axial compression and flexure. The formulation upscales a previously developed nonlocal formulation for single-fiber buckling to the full frame element. The formulation incorporates a physical length scale associated with local buckling along with an effective softening constitutive relationship at the fiber level. To support these aspects of the model, 43 continuum finite element (CFE) test problems are constructed. These test problems examine a range of parameters, including the axial load, cross section, and moment gradient. The implemented formulation is validated against CFE models as well as physical steel beam-column experiments that exhibit local buckling-nduced softening. The formulation successfully predicts postpeak response for these validation cases in a mesh-independent manner , while also capturing the effects of P-M interactions and moment gradient. To enable convenient generalization, guidelines for calibration and selection of the model parameters are provided. Limitations are discussed along with areas for future development.