Nonlinear flexural distribution behavior and ultimate system capacity of skewed steel girder bridges

Bridge designs are routinely developed using simplified 1D approximations of structural behavior and assume linear elastic behavior throughout the structure. More rigorous methods can reveal significant unquantified reserve capacity with 3D system behavior as ductile members experience yielding, but reserve capacity can be overestimated if cracking in a concrete deck is neglected. The primary objective of this study is to characterize the influence of concrete deck nonlinearity on 3D system behavior and capacity of skewed steel girder bridges. Structural behavior is examined throughout the range of potential applied loads, from initial linear elastic behavior to initial yielding and ultimate conditions. Additionally, this study provides a secondary benefit by comparing the applicability of two general load distribution characterization methods: response fractions (displacement, strain, or curvature ratios) versus extracted and integrated results from a rigorous analytical model. Analytical models, accounting for material inelasticity in the steel girders, are constructed for each of two experimentally tested bridges. Skew is varied parametrically from 0 degrees to 60 degrees. Concrete deck is modeled with a tension cracking stress limit consistent with typical design assumptions in United States practice. Results are also presented for the 60 degrees case with linear elastic deck to directly compare the influence of cracking versus noncracking deck. The analyses indicated that neglecting concrete cracking noticably affected capacity by overestimating slab plate flexure between abutments, resulting in increasingly unconservative reserve capacity evaluations with increasing skew. Bridges with small skews evaluated using only elastic analysis were found to be reasonably well characterized using response fractions feasibly obtained from diagnostic load tests. However, bridges with high skews and bridges evaluated using ultimate capacity from detailed finite element analysis, regardless of skew, will receive unnecessarily conservative flexural capacity ratings if response fractions are relied upon to represent analogous internal load effects.