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The evaluation of existing bridges has received little attention in the structural reliability literature, yet probabilistic methods allow the evaluation of the adequacy of a structure better than deterministic analysis. Bridge test data can be used to improve the accuracy of load and resistance models. In this paper, available safety analysis procedures are reviewed. Bridge evaluation can be also carried out on the basis of tests, and various bridge test types are discussed. A discrepancy is observed between the analytical and test results. Measured member forces are consistently lower than calculated values. This indicates the importance of load sharing and redundancies. The hidden safety reservecan be revealed by use of system reliability methods. The reliability model can be significantly improved by use of test data to modify the load and resistance parameters. The approach is demonstrated on an example of a steel truss bridge.

Live load is an important load component for highway bridges. The paper summarizes the available data base and formulates the approach to calculating maximum moments and shears for various time periods. The liveload data base consists of trucksurvey results. Moments and shear forces are calculated for surveyed trucks and various spans. The maximum load effects for time periods from one day to 75 years are derived by extrapolations and simulations. Calculations are performed for singlelane and twolane bridges. For single lanes, the maximum moment and shear are caused by a single truck for spans up to about 100 ft 30 m. For longer spans, two trucks following behind each other govern. For two lanes, the maximum effect is obtained for two trucks sidebyside, with fully correlated weights. For the maximum 75year moment or shear, each sidebyside truck is represented by the maximum 1.5month truck. The effect of a 1.5month truck is about 15% smaller than that of a 75year truck.

Uncertainties in structures result from natural hazards, manmade hazards, variations within common practice, and accepted departures from common practice. However, the major source of structural uncertainty and failures is departure from accepted practice, i.e., human error. The purpose of an error control strategy is to lower the frequency of errors which have large consequences. Sensitivity functions which relate structural errors to structural safety are bases for a control strategy. Sensitivity analysis is performed for several practical cases to demonstrate calculation of sensitivity functions and its importance in establishing an error control strategy.

Reliability indices are calculated for single timber beams and timber bridges with deck planks and stringers. Load and resistance models are developed on the basis of the truck survey and material test data available. Three common groups of species and two material grades are considered. The reliability indices are calculated using the Rackwitz and Fiessler procedure for the single beams, and Monte Carlo simulations for the whole bridges. The reliability indices for single beams are about 15–40% lower than those for whole bridges. The difference can be attributed to a distribution of load between the stringers because of the mechanical properties of wood, in particular the modulus of elasticity, modulus of rupture and their correlation.

Current inspection requirements for several countries are reviewed. Observations and modeling of general corrosion of steelgirder bridges are described. A highway steelgirder bridge is assumed to be subjected to dead load and heavytruck traffic load. Three failure modes of the steel girders are considered, i.e., bending, shear, and failure of the stiffened web at the supports. The safety margins for each failure mode and an event margin for repair are described, and the reliability index is defined. The three failure modes are modeled as a series system, and expressions for the failure probability after inspections have taken place are shown. The optimization problem is formulated, with an objective function and constraints, to select an optimal inspection strategy. For example, a typical highway steelgirder bridge is investigated. The reliability indices are calculated for each girder as a function of time. Based on this an inspection strategy is proposed.

A corrosion damage model is developed for steel girder bridges. The bridge loadcarrying capacity is determined with regard to bending, shear, and bearing. The capacity loss depends on the location and rate of corrosion. The evaluation is demonstrated on two typical steel bridges in an aggressive environment. Results indicate that the governing criteria may change over time. This change is dependent on the relative thicknesses of the girder web and flange. The web in shear and bearing is shown to be typically the critical element, particularly for shorter span bridges with no bearing stiffeners.

Coldformed channels are joined by means of spot welds to form various configurations. They are used in typical electrical cable and conduit supports, also in power plants. The present design procedures follow AISI specifications, which gives the allowable load per spot weld as a function of the thickness of connected plates. Tests were carried out to establish the distribution of spot weld capacity. It has been found that the strength strongly depends on technology of welding. It is recommended to review the AISI provisions and specify special allowable loads for spot welds connecting coldformed channels. The allowable loads could vary depending on type of connection backtoback, sidetoside, sidetoback and technology of welding degree of correlation between adjacent welds.