Hani Nassif

Multiple Presence Statistics for Bridge Live Load Based on Weigh-in-Motion Data

A study was done to determine truck load spectra for bridges in New Jersey with emphasis on multiple truck presence statistics. Truck weight data were collected by the New Jersey Department of Transportation from 25 weigh-in-motion sites throughout the state between 1993 and 2003 (with some gaps). The sites encompass a variety of site-specific conditions, including truck volume, road and area type, and number of lanes. For each truck, the recorded parameters include the time of passage, speed, travel lane, number of axles, and axle loads and spacings. Of particular interest are the frequency and correlation among trucks simultaneously occurring on a bridge either as following, side by side, or staggered. This multiple presence directly affects simulation, future extrapolation of live load effects, and code calibration. In fact, the case of two trucks occurring side by side has been considered the governing case for the design of bridges during the development and calibration of the AASHTO LRFD Bridge Design Specifications. However, until recently, multiple presence statistics have been based on few observations of truck traffic at a small number of sites. Statistics are presented for various truck loading cases, and the effects of truck volume, area type, road type, and bridge span length on such statistics are examined. It is observed that truck volume and bridge span length have a significant effect on the frequency of multiple truck presence, whereas area and road type have only slight effect. It is also observed that the rate of increase in the percent occurrence of following loading events is lower for bridge span lengths of up to 100 ft (30 m) as compared with longer spans, whereas for staggered loading patterns the opposite is true. The frequency of side-by-side trucks was found to remain relatively constant with respect to span length.

Optimization of Design Details in Orthotropic Steel Decks Subjected to Static and Fatigue Loads

In recent decades, orthotropic steel decks (OSDs) have been routinely incorporated into long-span bridges. The most widely used method to reduce stress concentration, improve fatigue performance, and control crack propagation is to cut out the diaphragms (or subfloor beams) into which the OSDs frame. However, the capital cost of cutout fabrication in the United States is high and may not be economically feasible. Study is required of cost-effective modified design details without cutouts as well as comparisons with their corresponding flexural and fatigue performance against current design details that use cutouts. Alternative design details (e.g., deck ribs welded directly to the transverse diaphragms using full-penetration welds) with thicker deck plates, but without cutouts, were investigated for potential improvements in fatigue resistance and capital cost. A parametric study was conducted with calibrated finite element models of a portion of the Bronx–Whitestone Bridge in New York City to study the effects of cutouts, deck plate thickness, and other important parameters on fatigue performance. Various traffic load combinations and truck types were considered with the use of an elaborate weigh-in-motion database. Results detail the equivalent stress ranges at critical locations in the OSDs that were calculated to quantitatively estimate fatigue lives for two OSD models: one with cutouts and one without. On the basis of these comparisons, recommendations related to overall structural performance were made to ensure a safe and rational design for various OSD options in long-span bridges.

Effect of Substructure Stiffness on Performance of Steel Integral Abutment Bridges Under Thermal Loads

This paper presents a study that investigated the effect of substructure stiffness on the performance of short- and medium-length steel integral abutment bridges (IABs) built on clay under thermal load effects. Various parameters, such as pile size and orientation, pile type, and foundation soil stiffness, were considered in the study. Detailed, three-dimensional (3-D), finite element (FE) models were developed to capture the behavior of IABs. Field measurements from a IAB were used to validate the 3-D FE model developed with LUSAS software. With the use of validated models, a parametric study was carried out to study the effect of these parameters on the performance of IABs under thermal loading with AASHTO load and resistance factor design temperature ranges. The study showed that the substructure stiffness had a significant effect on the stress level induced by thermal loads in various components of the substructure and superstructure. The results also showed significant variations in displacement and stress between interior and exterior locations in relatively wide IABs. The study showed that prestressed concrete piles could form a viable alternative to steel H-piles for short-span bridges. The stress level from thermal loading in the various components of the bridge could be reduced significantly if the top part of the pile were placed in an enclosure filled with crushed stone or loose sand.

Early-Age Cracking in High Performance Concrete Decks of a Curved Steel Girder Bridge

Highway flyover bridges in New Jersey experienced extensive early-age cracking on the high performance concrete deck recently. There were transverse cracks observed predominantly in the positive moment regions and spider cracks concentrated in the negative moment region over the box beam. A typical two-span curved continuous bridge with high performance concrete (HPC) deck, steel girders and integral box-beam was selected to identify factors that affect early-age HPC deck cracking so as to mitigate the cracking issues on existing concrete continuous bridges. Extensive field tests were performed and various structural responses were collected for model calibration and structural analysis. Cracking issues due to effects of vertical temperature differentials, staging, shrinkage, settlement, tilt of piers, dead load, live load, etc., were quantitatively investigated and summarized through parametric study. Of prime interest has been the identification of the contributions of various effects on deck cracks. The present study provides better understanding of the behavior of curved composite bridges, which can be efficiently used to reduce the risk of cracking at early ages.

Vibration Versus Deflection Control for Bridges with High-Performance Steel Girders

The use of high-performance steel (HPS) in highway bridges has proved successful in structural performance and in the cost-efficiency of the constructed bridges. However, the use of optional deflection criteria as specified in a subsection of the AASHTO LRFD Bridge Design Specifications may impede the use of HPS in highway bridges. Besides the deflection criteria, the current AASHTO specifications provide a depth-to-span limitation table for steel superstructure designs. The values in that table are based primarily on the use of Grade 36 steel and were initially a carryover from railroad bridge construction. Therefore, both the deflection criteria and the depth-to-span limitation need to be evaluated for bridges constructed with HPS. This paper presents an investigation of the vibration control (e.g., acceleration and velocity) of HPS bridges using a three-dimensional (3-D) dynamic computer model. The 3-D dynamic model was validated with the use of field test data on various bridges, including a three-span continuous steel girder bridge. A suite of typical bridges designed with various slab thicknesses and span-to-depth ratios was selected for this study. In particular, the effects of the steel girder depth and concrete slab thickness on bridge vibration were identified. The analysis results indicated that bridge vibrations were better controlled with the choice of optimal concrete slab thicknesses (i.e., by adding to the mass and moment of inertia of a composite girder) rather than with specifications of the span-to-depth ratio limits, deflection limits, or first natural frequency.

Effect of Overweight Trucks on Bridge Deck Deterioration Based on Weigh-in-Motion Data

Highway agencies are responsible for the optimal expenditure of taxpayer dollars allocated to highway infrastructure. Truck size and weight are regulated by federal legislation, and every state highway agency has its own legal load limits. In addition, state agencies issue permits for trucks with gross vehicle weights that are above legal load limits. However, the effect of overweight trucks on the service life of bridge structures, especially concrete decks, is not explicitly quantified. Detailed research on deterioration models for bridge decks was conducted. Condition ratings of bridge decks in New Jersey from the National Bridge Inventory were used to derive the deterioration of decks over time, and the expected service lives of decks on different highways were obtained. Weigh-in-motion data from stations in New Jersey were used to extract two data sets: “all trucks” and “legal trucks.” The “all trucks” data set was used to develop a deck deterioration model as a function of equivalent wheel load that could be used to estimate expected service life. Finally, bridge life-cycle cost analysis was conducted under two scenarios, one with and the other without overweight trucks, to quantify the economic impact of such trucks on bridge decks. The results indicate that overweight trucks caused more damage on New Jersey state highways than on Interstate highways because of a larger proportion of overweight trucks, heavy wheel loads from overweight trucks, and fewer axles per truck.

Analytic Hierarchy Process to Improve Simple Bridge Security Checklist

Enhancement of bridge security is key to improved homeland security and entails several steps, including on-site assessment, analysis of security components, and implementation of some mitigation measures. A review of the nations current bridge security posture showed a need to develop methods to identify critical bridges as security hazards and to provide engineering standards and guidelines for security design to reduce the vulnerability of bridges to attack. In particular, a need was seen for a simple bridge security checklist to provide on-site assessment of bridge vulnerability and security risk. After the events of September 11, 2001, the New Jersey Department of Transportation asked Rutgers University to develop a checklist to be used by bridge inspectors to provide management with security data for the departments entire bridge inventory. Rutgers developed a concise checklist, which consisted of yes or no questions in three categories: occurrence, vulnerability, and importance. The overall risk of a structure was measured in terms of an equation risk. To improve this tool, a survey was administered to industry subject matter experts across the United States to determine the relative importance of each question. The data from the survey were analyzed with the use of the analytic hierarchy process, and new weights were assigned to each question. This paper provides the results of the survey and discusses the methodology, analysis of the survey results, and implementation of the updated tool.

Use of Fiber-Reinforced Self-Consolidating Concrete to Enhance Serviceability Performance of Damaged Beams

The use of fiber-reinforced self-consolidating concrete (FR-SCC) in repairing damaged concrete beams has been evaluated. An experimental program was conducted to design and test key fresh and hardened properties of SCC and FR-SCC mixtures. The designed FR-SCC mixtures included two types of supplementary cementitious materials (silica fume (SF) and slag (SL)) and two types of fibers (steel fiber (STF) and polypropylene fiber (PPF)) were used. To ensure good workability to repair congested areas, the optimized volume fractions of the STF were 0.25% and 0.50% compared with 0.10%, 0.15%, and 0.20% for the PPF. In addition, the flexural behavior of 10 beam specimens was investigated. The main reinforcement for the control beams consisted of #5 reinforcing bars, while the main reinforcement for the repaired beams was either #4 or #3 reinforcing bars that were introduced to simulate 35% and 65% reduction of the bar areas, respectively, due to corrosion. The results demonstrate that the optimized FR-SCC mixtures are effective repair materials and can develop adequate bond strength to existing concrete. The flexural test results showed that the repair mixtures were able to increase the cracking load for the repaired beams compared with the control beams. Such an increase is expected to contribute to extending the life of the damaged member or structure at the service load level. This paper also presents a comparison of the predicted values for the first-crack load strength using the ACI 544 code equation with the experimental data. Results showed that the code equation provides safe prediction.

Optimization of Deck Construction Staging for Multiple-Span Continuous Steel Girder Bridge

AbstractCracking in concrete deck has been the major concern to bridge designers as well as owners. Excessive cracking not only would affect the performance and serviceability of the bridge but als...

Dynamic Modeling and Field Testing of Railroad Bridges

In this paper, results for a study to investigate the impact of increasing the weight of freight railcar from 1,170 kN (263 kips) to 1,272 kN (286 kips) on typical bridges that are part of the New Jersey rail network are presented. Based on the field inspection reports, a number of critical bridges on New Jersey’s rail lines were selected and load-rated based on the current American Railway Engineering and Maintenance-of-Way Association (AREMA) specifications. Two-Dimensional (2D) dynamic models and field instrumentation and testing were adopted for the more accurate assessment of these bridges and to develop a refined methodology for evaluating and load-rating railroad bridges. The field study included instrumentation and testing under moving freight and passenger railcars. The steel bridge is simulated as a Bernoulli-Euler beam and the moving train is modeled using rigid-body dynamics. The method of modal superposition is adopted to compute the dynamic effects of the train-bridge interaction system. The dynamic model was validated with results from the field tests. The impact factor for fatigue of the bridge under moving freight and passenger train were compared with AREMA Mean Impact Factor. Results show that the impact factor for bridge fatigue is increased by the free vibration component. Moreover, for passenger car, when the running speed is above 170 km/h, the impact factor for fatigue is slightly larger than the AREMA Mean Impact Factor.