PhD Final Defense – Ricardo Dorado

Influence of Skew and Integral Abutments on Stress Distribution in Steel I-Girder Bridges

Advisors: Professor Larry A. Fahnestock and Professor James M. LaFave

Abstract:

Skewed steel I-girder integral abutment bridges (IABs) are increasingly used due to their structural efficiency and elimination of deck joints. Response of such bridge systems to live load (traffic) and thermal effects, however, remains insufficiently understood. This study therefore investigates the combined effects of skew and integral abutment restraint on live load and thermal behavior of skewed steel I-girder bridges representative of current Illinois bridge practice, through long-term field monitoring of two skewed steel I-girder bridges with different abutment types (stub versus integral) and via numerical parametric studies. Three-dimensional finite element models developed in ABAQUS show good agreement with field-measured thermal response and demonstrate the importance of realistic representation of support restraint and soil-structure interaction effects. Live loading parametric studies establish that skew angle, support restraint, diaphragm configuration, and bottom flange unrestrained length strongly influence girder bottom flange lateral bending stresses in both stub and integral abutment bridges, with significant stresses observed even below current AASHTO and IDOT bridge skew thresholds for lateral bending stress consideration. Long-term field monitoring further reveals that substantial thermally-induced stresses develop in skewed steel I-girder bridges, with the monitored IAB generally developing greater demands and more complex behavior than the stub abutment bridge. Parametric studies with temperature variation confirm that superstructure axial stresses are relatively insensitive to skew angle and bridge length, partly because progressive softening of foundation restraint allows additional thermal movement without a proportional increase in axial stress. Strong-axis girder bending generally decreases with increasing skew and is concentrated near the integral abutments at low skew angles, whereas weak-axis girder bending increases with skew and is concentrated primarily near the piers and abutments. Consequently, the governing thermal response transitions from strong-axis bending near the integral abutments at low skew to weak-axis bending near the piers at high skew. This increased importance of pier regions at high skew is also reflected in the substantial longitudinal and transverse forces developed in fixed pier bearings under extreme thermal loads. Overall, the findings demonstrate that restrained thermal effects can produce significant, location-dependent girder stresses, and they provide a basis to account for the coupled effects of skew and integral abutment restraint, to evaluate the spatial distribution of girder demands, and to support design and evaluation of longer and more highly skewed IABs.