published_or_final_version / Civil Engineering / Master / Master of Philosophy

The effects of thermal loads on steel bridges are not well understood. Although thermal effects are discussed in the AASHTO specifications, the appropriateness of the recommended thermal gradients is questionable. Thermal effects on the bridges can impact the design of the steel superstructure, the support bearings, and even the bridge piers. Previous field monitoring of steel trapezoidal box girder bridges has shown that thermal stresses on the order of ±5 ksi were not uncommon under regular daily thermal cycles. Stresses induced during annual thermal cycles may be potentially larger than those during daily thermal cycles. Recent data has shown that the bearings that are to allow the girders to expand and contract freely due to thermal movements are not frictionless. Because of the bearing friction, the supporting piers must flex to accommodate the bridge movements. In curved girder applications, questions have been raised by designers and contractors regarding the proper orientation of guided bearings. This research study includes field measurements, laboratory tests and finite element parametric analyses. The bearings of nine bridges in the Houston area have been instrumented and monitored for more than a year to measure bearing movements due to changes in temperature. Instrumentation of the steel girders on one of the Houston bridges was made utilizing thermocouples and vibrating wire strain gages to measure temperature distribution and thermal stresses. In addition, strain gages and thermal couples were applied to the steel girders and concrete bridge deck on a simple twin box girder bridge located at the Ferguson Structural Engineering Laboratory in Austin, Texas. The data from the field monitoring and laboratory tests were used to validate a finite element model. Based on this model, a detailed parametric study was conducted to investigate the effects of bridge configuration. It is found that under the given weather conditions, the most critical thermal loads are achieved under the following bridge configurations: N-S bridge orientation, shorter lengths of the concrete deck overhang, deeper steel girder webs, thinner concrete decks, and larger spacing between two box girders. To evaluate the effect of environmental conditions and obtain extreme thermal loads for design purposes, the most critical configuration of bridge sections was modeled for thermal analysis with Texas weather data from 1961 to 2005 as the input environmental conditions. Four cities were considered to bound Texas weather conditions. Based on the thermal analyses, a 45-year sample data of thermal parameters were used to describe the temperature field over a section. Extreme value analyses of the sample data were performed to obtain the relationship between thermal loads and return periods. The thermal loads with 100-year return period were compared to the ones suggested by AASHTO. The thermal loads with 100-year return period were used to investigate structural response. The effect of bearing orientation and the point of fixity were studied. A rigid body model was proposed to estimate thermal movements at the ends, which matched those obtained from field monitoring and finite element analysis. The maximum possible thermal stresses were also evaluated. Design suggestions are put forward based on the analysis. / text

Fire resistance design and analysis is an under-studied and under-codified area of bridge engineering. With the lessening of conservatism in bridge design, the aging or our bridge infrastructure, and the increase in the ground transport of highly-flammable and -combustible materials, it is essential that the bridge engineering community better understand and incorporate methods for modeling the effects of fire on bridges. Typical fire resistance analysis looks at the response of individual structural components. Analysis for the component of a bridge is nowhere more important than for that of the main cables of suspension bridges. As such, we will survey and introduce the necessary analysis techniques to provide the bridge engineering community with the knowledge and tools to understand fire modeling and both rapidly and accurately assess their effects on suspension bridge main cables. The work of this dissertation is twofold. In the first portion, we address proper fire modeling techniques for bridge conditions and apply them in a sequential thermal-mechanical analysis of a three-dimensional model main cable with thermally-dependent material and mechanical properties. Although fire modeling has been addressed in a variety of scenarios, including extensive studies for building design and analysis as well as tunnel design and analysis, the types of fires, fire geometries, and air conditions associated with bridge fires vary drastically. Our work identifies the time to failure for our particular main cable example and subsequently compares both the temperature and strain distributions for temperature-dependent and temperature-independent models. Although the three-dimensional analysis is complete, we hope to emulate and expand on the work done in the building fire engineering community and bring to the literature methods to produce significant two-dimensional temperature distributions for when a main cable component is either partially or fully-exposed to fire. As such, the main fire modeling analyses mentioned in the previous paragraph lay the groundwork for our pursuit of closed-form analytical solutions necessary to rapidly and accurately assess the time-dependent temperature distribution within a cable cross-section exposed to fire. These solutions are formed with different approaches depending on the fire scenario in question. They include a separation of variables (eigenfunction) approach, sinusoidal transforms, Laplace transforms, Green's function solutions, and a semi-analytical hybrid method. We validate each of the approaches numerically using three different fire models.