Ultimate Strength of Highway Girder Bridges.

About 40 per cent of highway bridges in the United States are classified as structurally deficient. Tests show that many condemned bridges are adequate to carry service loads. Traditional evaluation methods do not provide an adequate estimation of the actual strength. There is an urgent need for efficient evaluation methods. This research deals with the development of a static nonlinear analysis to evaluate ultimate bridge strength based on methods of system reliability analysis. A bridge structure is considered to be a system of interacting members (girders, slab, diaphragms, connections, etc.). The mechanical and geometrical parameters (moduli of elasticity, ultimate stress, dimensions, etc.) are simulated using a Monte Carlo method. Correlations are estimated and included in the model. Live load is modelled as a truck with AASHTO wheel configuration. Wheel loads are gradually increased until the bridge fails as a result of excessive deformations. The transverse positions of truck on the bridge are modelled using actual survey data. Ultimate strength is evaluated for three types of bridges: composite steel girders, prestressed concrete girders and reinforced concrete T-beams. From several numerical examples the following conclusions are drawn: (1) The failure load of a bridge depends on the transverse position of a truck; (2) Reserve resistance exceeds the yield resistance by approximately 25 per cent. (3) Rosenblueth's point estimate can be used to determine the values of the mean and variance of the bridge resistance. (4) Considering the girder strengths as perfectly correlated will lead to underestimating of the value of the reliability index within 10 per cent compared with the case of independent girder strengths.