Papers in This Issue

The May 2013 issue of the Journal features nine technical papers, one technical note, and one discussion covering different areas of bridge engineering. The first two papers deal with ratings and load factors for bridges. In the paper “Development of State-Specific Load and Resistance Factor Rating Method,” authors Ghosn et al. investigate the application of the AASHTO Load and Resistance Factor Rating (LRFR) Manual for Bridge Evaluation to adjust the live load factors based on site-specific or state-specific information. They describe in detail the process followed during the reliability-based calibration of an LRFR methodology for New York State (NYS) bridges based on weigh-in-motion data collected from several representative sites. This methodology is applied to the rating of existing bridges, the posting of understrength bridges, and the checking of permit trucks. The new NYS-LRFR live load factors provide uniform and consistent levels of bridge safety and reliability over all pertinent bridge classes and configurations. The reliability levels of the proposed NYS-LRFR reflect the experience gained from the evaluation of existing bridges under current loading conditions. In the paper “Reliability-Based Calibration of Load and Resistance Factors for Design of RC Bridges under Multiple Extreme Events: Scour and Earthquake,” authors Alipour et al. develop a multihazard reliability-based framework to evaluate the structural response of RC bridges under the combined effects of pier scour and earthquake events. This framework is utilized to calibrate the scour load modification factors for design of bridges located in high seismic areas through a series of case-study bridges. For each bridge case, the joint probability of failure because of scour and earthquake hazards is determined for a range of expected combinations of these two extreme events. The occurrence probability of each scour-earthquake scenario is probabilistically identified by taking into account all the major sources of load uncertainty through scour risk and seismic hazard curves. Furthermore, the uncertainties inherent in the structural response of bridges are included in the framework to improve the accuracy of estimated failure probabilities. The calculated probabilities are then compared with the maximum acceptable probability of failure (or its equivalent target reliability index) given by current design codes to obtain scour load modification factors. The developed framework provides a reliable approach for the calibration of the code specifications in extreme event situations and can be extended to other combinations of natural hazards.

The remaining seven technical papers are in diverse areas of bridge engineering. In the paper “Longitudinal Joints with Accelerated Construction Features in Decked Bulb-Tee Girder Bridges: Strut-and-Tie Model and Design Guidelines,” He et al. focus on the development of a strut-and-tie model (STM), which is used in the design of previously developed longitudinal joints with accelerated construction features. Four joint specimens reinforced with tight bend diameter U-bars are used to validate the STM. The validated STM is capable of calculating the ultimate moments of the joints. The joint capacity increases with an increase in concrete strength and overlap length, and the capacity reaches the maximum value when the U-bar spacing is twice the overlap length. Design recommendations on U-bar spacing, overlap length, diameter of lacer bars, and concrete strength of the closure pour material are developed for full-strength joints. In the paper “Imaging Tools for Evaluation of Gusset Plate Connections in Steel Truss Bridges,” Higgins and Turan propose a new methodology that enables rapid and accurate collection of field measurements for connection plate geometry. The method uses close-range photogrammetry techniques to rectify images taken with consumer grade cameras using flat-field and fisheye lenses. The technique is demonstrated on full-scale gusset plates in the laboratory and in the field. Dimensional measurements from the processed images provide results that are as good or better than conventional field measurements and with tolerances below what most engineers require for calculation of gusset plate connection capacity. This technique provides a new tool for bridge engineers to quickly collect gusset plate geometry that can be used in connection evaluations and rating, and can further enhance bridge management tasks. In the paper “Bending Strength of a Horizontally Curved Composite I-Girder Bridge,” Yoo et al. investigate the interaction of vertical bending and lateral flange bending of girders, which is responsible for effectively reducing vertical moment carrying capacity of the section. They have derived the interaction equations for predicting the nominal bending strength of horizontally curved I-girders of compact-flange sections subjected to vertical moment and torsion. The limitations and applicability of the derived equations for practical designs are analyzed and demonstrated. In the paper “Synchronization among Pedestrians in Footbridges due to Crowd Density,” Pimentel et al. investigate excessive lateral vibrations in footbridges because of synchronization of pedestrian movements caused by an increase in crowd density. A test program to investigate whether synchronization occurs because of an increase in crowd density is carried out for a range of pedestrian densities, complementing data previously published on this subject. Head movement of pedestrians walking both in groups and in the flow was recorded by a video camera. An examination of the video indicates no synchronization because of densification. However, the lateral sway of the pedestrians’ body increases with an increase of the density. By employing an existing model of an inverted pendulum to estimate lateral forces applied by a pedestrian and using the collected data as input to the model, a steady increase of lateral force because of the rise in density was observed. In the paper “Stress-Laminated Timber Decks Subjected to Eccentric Loads in the Ultimate Limit State,” Ekholm et al. carry out comparisons between several linear design methods with ultimate-load test of a full-scale stress-laminated-timber bridge deck subjected to an eccentric load. Some of the linear hand calculation methods show significant discrepancies in results, depending on the load position. There are also variations in the results from finite-element models, depending on the material properties assigned to the deck. All the design methods fail to predict the deflection of the tested deck when loaded to failure. A larger deflection is observed in the full-scale test than that predicted by the design methods. As a result, the linear design method could underestimate the bending stresses in the deck. Hand calculation methods are also unable to calculate the transverse forces and moments necessary for design according to Eurocode 5. In the paper “Basic Performance of the Composite Deck System Composed of Orthotropic Steel Deck and Ultrathin RPC Layer,” Shao et al. propose an innovative composite bridge deck system consisting of an orthotropic steel deck stiffened with a 45-mm reactive-powder-concrete (RPC) layer. Based on the analysis of HuMen Bridge (a suspension bridge with conventional orthotropic steel bridge deck system with a span length of 888 m) and two types of full-scale model tests, a comparison investigation is conducted between a conventional orthotropic deck system where the asphalt wearing course has been ignored and the proposed orthotropic deck system that includes an integral concrete wearing course. The proposed composite bridge deck system proves to be considerably effective in reducing the stress range caused by service vehicle loads when installed in long-span steel suspension bridges. The thin RPC layer can be reliably integrated with the deck plate through stud shear connectors. The fact that tensile stress of the RPC layer is up to 42.7 MPa before cracking occurs demonstrates that cracking will not appear in the RPC layer under the service vehicle loads. The analysis, which is investigated on the premise that the dead load weight of the bridge deck of HuMen Bridge is approximately the same for the two different deck systems, demonstrates that the stress of the orthotropic steel deck is significantly reduced with the application of the proposed composite deck system. The analysis with the three-dimensional finite-element model indicates that the transverse tensile stress of the deck plate is reduced by 71%, whereas that of the connection between the deck plate and the longitudinal troughs is reduced by 72%. This concludes that the risk of causing fatigue cracks of the steel bridge deck system can be considerably reduced during the entire life cycle of the bridge. In the paper “Cyclic Behavior of FRP Concrete Bridge Pier Frames,” Li et al. present results of testing of four 1/6-scale two-column bents: a control RC, a glass fiber-reinforced plastic (FRP)-concrete (GFF), a carbon FRP-concrete (CFF), and a hybrid glass/carbon FRP-concrete frame (HFF). Each frame is tested under reverse cyclic lateral loading with a constant axial load. Specimen HFF with hybrid FRP tubes demonstrates the highest moment capacity and initial stiffness, with an increase of 200 and 70%, respectively, over the control specimen. Specimen GFF shows no sign of cracking up to a drift ratio of 15% with considerable residual strength, whereas specimen CFF has the least ductility. Glass FRP tubes extend the plastic hinge length of the pier columns to twice that of the control RC frame.

In the technical note “Load Rating of Pile-Supported Bridges Susceptible to Scour,” Sayed et al. present an investigation on the importance of assessing the substructure load rating. Case histories for bridges susceptible to scour are analyzed. The analysis and evaluation of the actual bridges presented generally demonstrate that the bridge load rating is limited by the substructure load-carrying capacity and the tolerable deformation. The deformation limits/requirements of the soil-foundation system control the substructure load rating and, hence, the bridge load rating. The examples clearly illustrate that a reduced load rating is experienced when scour occurs. Thus, bridges impacted by scour need to be judged based on both super- and substructure components. The findings suggest the need to augment the current bridge load-rating procedures to include the substructure load rating in a quantitative manner.

This issue of the Journal also contains a discussion of “Fatigue Evaluation of Rib-to-Deck Welded Joints of Orthotropic Steel Bridge Deck,” which was published in the Journal of Bridge Engineering, July/August 2011, Vol. 16, No. 4, pp. 492–499.