Journal of Bridge Engineering

Using equations proposed by the AASHTO LRFD specifications, live-load distribution factors in bridge superstructures were calculated based on the linear-elastic behavior of the system. In this study, the applicability of these equations in predicting girder distribution behavior in the presence of high material nonlinearity and oversized loading scenarios was investigated. Two representative composite steel girder bridge superstructures, which are in service in the state of Michigan and had been previously subjected to a live-load testing program, were selected for this study. Sixteen cases were analyzed to study the effect of boundary conditions, loading position, and load configuration on the girder distribution behavior of the selected bridges as they approach their ultimate capacities. Comparing the results obtained from nonlinear finite-element analysis with those proposed by the AASHTO LRFD specifications demonstrated that the code-specified values for the distribution factors are overly conservative. Much of the variation in distribution behavior occurs when plasticity propagates in steel girders, whereas the initiation of material nonlinearity in the concrete deck (cracking) has a negligible effect on the evolution of distribution behavior. The results of this investigation also highlight the importance of accurate modeling and analysis in the load rating practices of existing in-service bridge superstructures.

Observations of a U.K. suspension bridge show that thermal expansion and contraction cycles do not follow simple linear relationships with a single temperature value and that time lag and temperature distribution can be significant factors. This investigation explores these effects by simulating the transient thermal and quasistatic response of the Tamar Bridge with separate finite-element models of the bridge and suspension cables. Thermal loads are determined by calculated solar radiation intensities and temperature data from the bridge monitoring system. Because cloud cover plays an important role in the levels of solar radiation, cloud coverage was estimated indirectly using monitored temperature differences between the top and bottom of the suspended structure. The results demonstrate that peak temperatures of the suspended structure and cables occur at different times. The lag is caused by differing material properties and the surfaces’ ability to absorb and lose heat. Transient phenomena manifest in the structural responses such as the tower sway.

This paper proposes a novel Level I damage-detection technique for short- to medium-span road bridges using weigh-in-motion (WIM) technology. The technique is based on the input provided by two WIM systems: (1) a pavement-based WIM station located in the same route as the bridge (which gives vehicle weight estimates without the influence of the bridge), and (2) a bridge-based WIM (B-WIM) system that estimates vehicle weights based on the deformation of the bridge. The authors show that the ratio of estimations of vehicle weights by both systems is a reliable and robust indicator of structural integrity, even for WIM systems with relatively poor accuracy. They further show that this indicator is more sensitive to damage than the traditional method based on variation in natural frequencies.

Single-support bar modular joints are widely used in large-span bridges because of their competitiveness and their ability to accommodate larger movements than conventional multiple-support bar joints can accommodate. Despite their general acceptance and use, detailed fatigue testing and design guidelines well suited for these joints are still not well documented. This has been the main impetus for carrying out this experimental and analytical investigation on the fatigue performance of a single-support bar modular bridge expansion joint with welded stirrups. Three identical subassemblies were tested in fatigue to establish the experimental fatigue $S\text{-}N$ curve for the main critical details. In addition, a few variants of stirrup specimens were separately tested under static and fatigue loadings to investigate failure modes and the effects of important geometric parameters such as bottom-plate thickness on the stress range and fatigue resistance of the stirrups. The tested joint subassemblies and stirrup specimens developed characteristic fatigue cracks in and around the stirrups and their welded connections. These cracks are classified into three distinct categories, and expressions for computing their associated stress ranges and fatigue resistance are presented. This study reveals that secondary bending moments within the stirrup make a major contribution to the stress range at critical sections of the stirrup. This contribution should be included for adequate prediction of the fatigue life of single-support bar modular joints and can be reasonably determined with basic models using one-dimensional (1D) beam elements for the center beam and for the stirrup. The importance of secondary bending moments within the stirrup is strongly related to the relative stirrup bottom plate-to-vertical leg bending stiffness and to the relative stirrup vertical leg-to-center beam bending stiffness.

Dynamic impact of moving vehicles on bridges is an important and long-standing issue in the design and evaluation of bridges and has received much attention from researchers and engineers. The use of the dynamic impact factor (IM) to account for the impact effect of vehicles has been widely accepted in bridge engineering. Accurate evaluation of the IM will lead to safe and economical designs of new bridges and provide valuable information for condition assessment and management of existing bridges. Nevertheless, agreement on the evaluation of IMs is yet to be reached. Numerous studies have shown that the evaluation of the IM is a difficult task because it is influenced by a large number of parameters and uncertainties. As a result, different forms and values of IMs are specified by different bridge design codes and this disagreement has been debated in many studies in the past few decades. Furthermore, some field tests observed that the IMs in design codes are overestimated while many other field tests have suggested that code provisions may lead to underestimation of IMs, indicating the need to develop a more accurate assessment method for IMs. It is the objective of this paper to review and summarize the important methodologies and findings of the study of the dynamic IM of highway bridges conducted over the past two decades. While reviewing the advances achieved in the past two decades, much effort was made to identify the remaining controversies and gaps left in this field. Therefore, it is hoped that this review can also provide necessary background information for researchers and engineers to further examine these problems and help identify future research directions in this field.

Generally, in existing bridge management systems, the deterioration is modeled based on visual inspections where the corresponding condition states are assigned to individual elements. In this case, limited attention is given to the correlation between bridge elements from a structural perspective. In this process, the impact of the history of deterioration on the reliability of a structure is disregarded, as it may lead to inappropriate conclusions. The improved estimate of service life of a bridge deck may help decision makers enhance intervention planning and optimize life-cycle costs. The objective of this research is to evaluate the system reliability of conventional bridges that were designed based on existing codes. According to the methodology developed in this study, the predicted element-level structural conditions for different time intervals are applied in the nonlinear finite-element model of a bridge superstructure, and the system reliability indexes are estimated for different time intervals. This method has been applied in simply supported traditional RC bridge superstructures designed according to Canadian bridge design standards. Based on the reliability estimates, these conventional bridges designed based on the current codes are found to be in good condition during the initial stages of their service life, but their condition degrades faster once corrosion in steel reinforcements is initiated and spalling of concrete becomes evident. The system reliability deterioration model can be integrated into existing bridge management systems by replacing the existing condition index by the reliability index, or by adding it to the assessment process as an additional parameter.

This paper presents the results from a dynamic analysis of a cable-stayed concrete-filled steel tube arch bridge under vehicle loading. The study is carried out based on a three-dimensional vehicle-bridge coupled model. A finite-element model for the bridge is first developed and validated based on field measurement data. A truck specified by industry standards is adopted for vehicle loading in the analysis and is simulated as a multidegree-of-freedom vehicle model. Three important indexes, including the dynamic impact factor, perceptible level of vibration, and ride comfort of the bridge, are investigated. A parametric study is conducted to investigate the effects of a few important parameters, including the vehicle loading condition, vehicle speed, and road surface condition, on the three indexes. Results from the analysis show the following: (1) the dynamic impact factors, which vary between different bridge components/locations, are significantly affected by the three parameters; the impact factors of key structural components of the bridge studied are generally below the value of 0.33 specified in previous design specifications; (2) the perceptible level of bridge vibration is greatly affected by the road surface condition and vehicle loading condition; pedestrians can feel it to be slightly hard to walk on the bridge when two trucks move side by side under poor road surface conditions; and (3) the ride comfort of the bridge decreases as the road surface condition becomes worse, and drivers can feel a little uncomfortable under poor road surface conditions. Because all three indexes studied are found to be greatly affected by the road surface condition, establishing and maintaining a regular program of maintenance is very important to assure both the safety and serviceability of the bridge studied.