Civil Engineering

 

Date of this Version

Fall 12-3-2012

Comments

A DISSERTATION Presented to the Faculty of The College of Engineering at the University of Nebraska-Lincoln In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Engineering (Construction), Under the Supervision of Professor Terri R. Norton. Lincoln, Nebraska: December, 2012

Copyright (c) 2012 Mehdi Mohseni

Abstract

Dynamic loads can cause severe damage to bridges, and lead to malfunction of transportation networks. A comprehensive understanding of the nature of the dynamic loads and the structural response of bridges can prevent undesired failures while keeping the cost-safety balance. Dissimilar to the static behavior, the dynamic response of bridges depends on several structural parameters such as material properties, damping and mode shapes. Furthermore, dynamic load characteristics can significantly change the structural response. In most cases, complexity and involvement of numerous parameters require the designer to investigate the bridge response via a massive numerical study.

This dissertation targets three main dynamic loads applicable for railway and highway bridges, and explores particular issues related to each classification: seismic loads; vehicular dynamic loads; and high-speed passenger train loads. In the first part of the dissertation, highway bridge responses to the seismic loads are investigated using fragility analysis as a reliable probabilistic approach. The analysis results declare noticeably higher fragility of multispan curved bridges, compared to straight bridges with the same structural system.

Structural reliability of steel tension and compression members in highway bridges, and the effects of the vehicular dynamic load characteristics are studied in the second part of the dissertation. Latest available experimental data have been used to re-evaluate current US design criteria for axially loaded steel members. The obtained results indicate conservative design of steel tension members for yielding of gross cross section, (βmin=3.7 compared to the target reliability βT=3.0) and fracture of the net section (βmin=5.2 compared to the target reliability βT=4.5). In addition, all monitored steel sections designed for axial compression show adequate safety in all cases.

Lastly, the resonance of railway bridge superstructures under passing high-speed passenger trains is examined and their dynamic response are presented using dynamic load factor diagrams, applicable in strength limit state design of railway bridges. Applying proposed response curves can guide designers to estimate the structural response of railway bridges in the initial design phase, and avoid any possible resonance by changing the superstructure system, or modifying design parameters and the consecutive vibration frequency.

Adviser: Terri R. Norton

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