Optimized Fiber Element Modeling Strategy for Concrete-Encased Steel Composite Columns: Focusing on Material Nonlinearity and Confinement Effects

Reliable numerical simulation of concrete-encased steel (CES) composite columns remains challenging, and practical fiber-element modeling can be sensitive to confinement representation and to discretization and integration choices. Although CES columns offer superior structural performance, accurate simulation is difficult due to the complex interaction between steel and concrete under cyclic loading. Current seismic design codes, such as ASCE/SEI 41-17, often simplify modeling parameters by underestimating composite action, which can lead to uneconomical and overly conservative assessments that do not fully reflect the confining effect of the concrete encasement and the buckling restraint of the steel core. This study proposes a practical guideline for constructing an accurate analytical model for CES columns using nonlinear fiber-element analysis, with a specific focus on material constitutive laws. To validate the proposed strategy, nonlinear analyses were conducted and compared against a comprehensive database of 79 experimental specimens compiled from previous studies. The predicted-to-test peak strength ratio shows a mean of 1.02 (standard deviation of 0.058). Sensitivity studies indicate that responses stabilize beyond ~23 fibers (<1.5% error), reducing computation time by ~40% on average (from 52 to 23 fibers) compared with dense discretization while maintaining reliable hysteretic response prediction.