Reliability-based deterioration model for deflection limit state in steel girder bridges.

Composite steel girder bridges constitute a major part of the bridge population in the United States. However, they appear to deteriorate faster than other types, such that 45% are considered functionally obsolete or structurally deficient. Natural aging and deterioration of materials, environmental exposure, and improper design are certainly responsible for this fact. In the US, highway bridges are designed according to the AASHTO LRFD. Structural components have to satisfy ultimate and serviceability limit states. This dissertation is focused on the serviceability limit state of deflection. The deflection limitation is now optional, but was introduced in 1930's solely for vibration control. The objective of this study is to review the importance of the deflection limitation, and its effect on the reliability of the composite steel girders. The bridges behavior in the linear and non-linear domain is modeled using an advanced analytical model. Material properties, resistance and load components (including dead load and live load) are considered as random variables. The reliability analysis procedure is developed for calculation of reliability index, related to the probability of failure. The load and resistance parameters vary in time. The maximum expected live load increases with time. In contrast, strength and stiffness can decrease due to degradation of materials. Corrosion of the structural steel, coupled to the changes in concrete properties (reduction of modulus of elasticity due to creep and other effects) can result in larger deflections. The cumulative distribution functions of deflections are determined by Monte Carlo simulations coupled with Rackwitz-Fiessler procedure, and plotted on normal probability paper for various time periods. The computations are performed for various load cases, various environmental conditions, and for a wide range of span length and girder spacing. Two types of design are considered: moment-controlled and deflection-controlled. The structural performance is measured in terms of the reliability index, calculated for deflection limit state and serviceability limit state of the first yield of the structural steel girder. The results indicate that deflection is an important limit state as it often governs the design. Creep and corrosion can often result in weaker section of the girders and increased levels of deflection.PhDApplied SciencesCivil engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/125801/2/3224668.pd