Papers in This Issue

The July 2012 issue of the Journal features 17 technical papers and one technical note. First, four papers in this issue are related to dynamic effects on bridges. The paper “Seismic Fragility of Retrofitted Multispan Continuous Steel Bridges in New York” by Agrawal et al. discusses various retrofit measures, such as elastomeric bearings, lead rubber bearings, viscous dampers, and jacketing by carbon fibers to improve the seismic performance of multispan continuous steel highway bridges. The authors have investigated the effectiveness of these retrofit measures through comparisons of seismic fragility of as-built and retrofitted multispan continuous steel bridges. Results show that both elastomeric and lead rubber bearings reduce the fragility of bridge piers significantly through isolation effects. Wrapping of piers by FRP increases the effective ductility of piers through confinement and shifts the failure mode of an FRP-wrapped pier to rupture of FRP at much higher peak ground acceleration. The use of viscous dampers in combination with elastomeric bearings is effective in reducing fragilities of both pier ductility and bearing displacements. In the paper “Determination of 18 Flutter Derivatives of Bridge Decks by an Improved Stochastic Search Algorithm” by Xu et al., the authors investigate the determination of 18 flutter derivatives of bridge decks from three-degree-of-freedom free vibration data using an improved stochastic search algorithm (ISSA) combined with the unified least-square (ULS) method. The ISSA is capable of circumventing the local optimum dilemma in pursuing the optimal solution experienced in the traditional ULS method. The validity and accuracy of the ISSA are demonstrated by one numerical example and two long-span cable-stayed bridge deck sections. The merit of using different lengths of vertical, torsional, and lateral vibration data in flutter derivatives identification is also investigated. The identification error and modal participation flutter are easily examined through a decomposition of modal components from the original vibration data. The underlying complexities in aeroelastic parameter identification are studied, and the causes of low accuracy of some flutter derivatives are identified. Based on the comparative investigation on the aerodynamic characteristics of typical streamlined and bluff bridge decks, an improved understanding of the coupled bridge flutter is developed. In the paper “Blast Resistance of Steel Orthotropic Bridge Decks,” authors Son and Astaneh-Asl investigate the response of bridge decks to blast effects resulting from an explosion on bridge decks. A typical steel orthotropic deck used in long-span cable-supported bridges has been modeled using MSC Dytran nonlinear finite-element (FE) analysis software and has been subjected to simulated explosion. Simulations have been carried out by varying the size of the explosive device in terms of equivalent TNT, the axial compressive force present in the deck, and high-strain-rate mechanical properties of the material of the steel used in the orthotropic deck. By conducting dynamic analyses, the behavior of the orthotropic decks under blast loads has been established and their failure modes have been identified. In this system, to limit the effect of blast to a localized area of the bridge and to prevent catastrophic progressive collapse of the span, special fuses have been developed to be placed between two deck segments. In the paper “Code Formulas for Ship-Impact Design of Bridges,” authors Wang et al. have developed four FE models of ships with dead weight tonnage (DWT) varying from 3,000 to 50,000 t for numerical collision simulation. The equivalent static loadings are defined, and their simplified formulas are obtained by a fitting procedure based on the data from the numerical simulations of ship–rigid wall collisions. Simplified formulas from AASHTO’s code and others are compared with these simplified formulas. The geometric effects of bridge foundations on the ship’s impact forces are investigated with an example pile cap, and a geometric modification factor is proposed for an ideal rectangular block to modify the ship’s impact forces estimated by the code’s formulas.

Next, six papers in this issue are related to bridge girders. The paper “Bottom Flange Confinement Reinforcement in Precast Prestressed Concrete Bridge Girders” by Patzlaff et al. investigates the effect of confinement reinforcement on the performance of prestressed concrete bridge girders. Of particular interest is the effect on transfer and development length of prestressing steel and on the shear capacity of prestressed girders. The experimental investigation presented in this paper includes testing the flexural and shear capacities of 610-mm-(24-in-) deep T-girders and 1,100 mm (43.3 in.) I-girders. The results of this investigation indicate that (1) neither the amount nor distribution of confinement reinforcement has a significant effect on the transfer length of prestress strands; (2) at the AASHTO calculated development length, the amount of confinement reinforcement does not have significant impact on either the nominal flexural capacity of bridge girders or bond capacity of the prestressing steel; however, the distribution of confinement reinforcement along the entire length of the girder results in improved ductility and reduced cracking under extreme loading conditions; (3) confinement reinforcement improves the anchorage of strands at girder ends and consequently the shear capacity of prestressed girders. The paper “Influence of Skew Angle on Continuous Composite Girder Bridge” by Nouri and Ahmadi investigates the effect of skew angle on continuous composite girder bridges using three-dimensional (3D) finite-element analysis (FEA). For this aim, 72 models of two-span bridges with different span ratios ($N=1$, 1.55, and 1.82), skew angles (0–60°), and different arrangements of intermediate transverse diaphragms are analyzed. All models are subjected to American Association for State Highway and Transportation Officials (AASHTO) HS20-44 loading. Results for skewed bridges are compared to the reference nonskewed bridge as well as AASHTO standard specifications and AASHTO-LRFD specifications. Results show that as the skew angle increases, the support moment in interior and exterior girders decreases rapidly. It decreases about 10% when skew angle is less than 20° and reaches 33% for a skew angle of 45°. The shear force increases in pier support at the exterior girders and decreases at the interior ones with increasing skew angle. For exterior girders, the ratio of shear force increases up to 1.3 for skew angle of 45°. The AASHTO standard specifications overestimate the maximum bending moment by 20% for skew angle of 30° and $N=1$, and 50% for 45°. The overestimation of shear force is about 10% for skew angle of 45°. The AASHTO-LRFD specifications overestimate the longitudinal bending moment and shear force. This overestimation increases with increase of skew angle and reaches 12% for skew angle of 20° and 45% for 45°. The results show that transverse diaphragms perpendicular to the longitudinal girders of the bridges are the best arrangement for load distribution. Comparing results of simplified relations for skewed decks with finite-element analysis shows that the results of proposed equations are conservative for continuous-skewed bridges. The results are for bridges with specific configurations investigated and may change if the presumed conditions vary, although the trend should be similar. The paper “Buckling Behavior of Steel Bridge I-Girders Braced by Permanent Metal Deck Forms” by Egilmez et al. documents the results of a research investigation focused on improving the bracing potential of bridge deck forms. Modifications to the connection details have been developed to improve the stiffness and strength of the forming system. The research includes buckling tests on a 15-m-(50-ft-) long twin-girder system with permanent metal deck forms (PMDF) for bracing. In addition to demonstrating the behavior of the bracing systems, the twin-girder tests have also been used to validate computer models of the bracing systems so that parametric FEA studies could be conducted. Buckling test results demonstrate that modified connection details make PMDF systems a viable bracing alternative in steel bridges that can significantly reduce the number of cross-frames and diaphragms required for stability bracing of steel bridge I-girders during construction. The paper “Modeling Structural Performance of Second-Generation Ultrahigh-Performance Concrete Pi-Girders” by Chen and Graybeal investigates the applicability of the concrete damaged plasticity (CDP) model in modeling the behaviors of a prestressed second generation UHPC pi-girder. The computational aspects include discussion of the various parameters that influence the accuracy of the model and investigation of the scenarios in the limit that are useful for further optimization of the girder. The CDP model has been found to be consistent and reliable in replicating the observed structural response of both the UHPC pi-girder and a modified structural configuration referred to as the UHPC pi-girder with joint. The FEA modeling techniques developed are expected to be valuable in the future development of additional UHPC structural components. The paper “Live-Load Analysis of Posttensioned Box-Girder Bridges” by Hodson et al. presents an evaluation of flexural live-load distribution factors for cast-in-place box-girder bridges. The response of a typical box-girder bridge was recorded during a static live-load test. This test involved driving two heavily loaded trucks across an instrumented bridge on selected load paths. The instruments used to record the response of the bridge were strain gauges, displacement transducers, and tilt sensors. This measured data was then used to calibrate a finite-element modeling (FEM) scheme using solid elements. From this FEM, the theoretical live-load distribution factors and load ratings for the test bridge were determined and compared to those predicted in the AASHTO-LRFD specifications. A parametric study of cast-in-place, box-girder bridges using the calibrated finite-element modeling scheme was then used to investigate how different parameters such as span length, girder spacing, parapets, skew, and deck thickness affect the flexural live-load distribution factors. Based on the results of the parametric study, a new equation that more accurately predicts the exterior girder distribution factor is proposed. The paper “Practical Approach for Estimating Distribution Factor for Load Rating: Demonstration on Reinforced Concrete T-Beam Bridges” by Catbas et al. presents a methodology with rapid experimental testing to determine critical parameters for practical analysis of highway bridges. This approach is demonstrated on a reinforced concrete T-beam bridge population. The authors illustrate that the moment distribution factors (DFs) of single span T-beam bridges can be determined by using skew angle, modal frequency and the flexibility coefficient, where frequency and flexibility coefficient can be identified by means of rapid impact test that can be conducted using an impactor, such as a falling weight deflectometer (FWD). To demonstrate the methodology, FEMs of 40 single span T-beam bridges are analyzed to obtain the modal frequency and flexibility coefficients. Next, the maximum moment values of these bridges are obtained using the FEMs and simple beam analyses under HL-93 truckload. A multiple regression analysis is conducted to generate an equation to determine the moment DF (i.e., the ratio of FEM response to beam analysis) as a function of modal frequency, flexibility, and skew angle. This equation is then verified by using additional FEMs, where moment approximation to FEM results with the new approach is 6%, whereas this approximation is in the order of 30% with the conventional beamline analysis given in the AASHTO code. Finally, this approach is demonstrated by using experimental data from four real-life bridges for the computation of moment values as well as the load ratings. The new approach can conservatively accommodate live-load increase for the four existing bridges.

Next, seven papers in this issue are on different aspects of bridge engineering. The paper “Sensor Networks, Computer Imaging, and Unit Influence Lines for Structural Health Monitoring: Case Study for Bridge Load Rating” by Catbas et al. presents a novel methodology for structural health monitoring (SHM) of a bridge for bridge load rating using sensor and video image data from operating traffic. With the methodology demonstrated in this paper, video images are analyzed by means of computer vision techniques to detect and track vehicles crossing the bridge. Traditional sensor data is correlated with computer images to extract unit influence lines (UIL). With the use of laboratory studies, it is shown that UIL can be extracted for a critical section with different vehicles by means of synchronized video and sensor data. The results show that the synchronized computer vision and strain measurements can be obtained for bridge load rating under operational traffic. This is demonstrated through real-life data processed under a moving load on an instrumented bridge. A detailed FEM of the bridge is also developed and presented along with the experimental measurements to support the applicability of approach for load rating using unit influence lines extracted from operating traffic. This approach is further proved with different vehicles captured through video and measurements. The paper “Regularity Criteria for RC and PRC Multispan Continuous Bridges” by Grendene et al. presents a parametric study to develop criteria for structural regularity on reinforced concrete (RC) and precast RC (PRC) multispan continuous bridges, assessing whether an equivalent single-degree-of-freedom (ESDOF) system represents a multi-degree-of-freedom (MDOF) system correctly. The ESDOF system, determined by means of a nonlinear static analysis (NLSA), is compared, in terms of displacements, with the original MDOF through a nonlinear dynamic analysis. To this end, a sample of 32 bridges was considered to take into account the effect of stiffness ratio between different piers and between piers and deck. Together with the evaluation of the reliability of the regularity indexes found in literature, a comparative analysis has been carried out. The results show that the simplified system is representative of the bridge global behavior either when the deck is enough stiff to control pier displacements and deformations or when pier distribution is symmetric. The paper “Structural Behavior of Inferior-Deck Spatial Arch Bridges with Imposed Curvature” by Sarmiento-Comesías et al. focuses on the structural behavior and the effect of the geometrical configurations of inferior-deck arch bridges with imposed curvature. In this type of spatial arch bridges, the arch and the deck centroid lines are both contained in the same vertical cylinder. The aim of the study is to propose the most appropriate design for controlling the out-of-plane response. A simple analytical model representing the stiffness of the arch, the deck, and a hanger was effective in determining the main variables that control the behavior of the system. Afterward, the authors analyzed a series of linear 3D frame FE models of the complete bridge. The study demonstrates that nonplanar arches can be approximated by inclined planar arches. Parametric analyses have led to recommending a set of relevant design criteria for these bridges. The paper “Full-Scale Ultimate-Load Test of a Stress-Laminated-Timber Bridge Deck” by Ekholm et al. has investigated ultimate load tests on timber bridges. A full-scale test of a stress-laminated timber (SLT) deck, with a span of 4.9 m and thickness of 270 mm was performed to obtain deformations at various prestress levels as well as the ultimate load capacity. Prior to the ultimate load test, nondestructive tests (NDTs) were performed at three different prestress levels. Load was applied as an axle load positioned both centrically and eccentrically. Deflections were about 10% larger at a prestress level of 300 kPa compared to prestress levels of 600 and 900 kPa. For applied loads larger than 150–250 kN, the deflection of the deck was nonlinear at certain positions. This was most likely caused by large concentrated shear forces resulting in interlaminar slip between the laminates. The limit for linearity seems to be dependent on the prestress applied. A prestress of 600 kPa and an eccentrically positioned load was used for the ultimate load test. Failure occurred at a load level of 900 kN. Existing design codes and new procedures in development may be verified and calibrated against results in this paper. The paper “Bridge Timber Piles Load Rating under Eccentric Loading Conditions” by Andrawes and Caiza evaluates the load rating procedure that is currently used in rating timber piles supporting multiple-span simply supported bridges. For simplicity, these piles are often rated under concentric loads, and the effect of bending in the piles is neglected. Recent studies have shown, however, that under highly eccentric live loads, the effect of bending moments in the piles is of great importance and could have a dramatic effect on the load rating of the piles. This paper proposes an alternative structural load rating method for timber piles based on the National Design Specifications, which takes the effect of combined compression-flexure behavior of piles into consideration. This method is used to conduct a parametric study to investigate the effect of several geometric and structural parameters on the load rating of bridge timber piles using 3D FEMs of concrete-deck bridges supported on group of timber piles. The results show that the proposed load rating method produced significantly less ratings for piles with moderate to high levels of deterioration, as compared to that obtained by the conventional approach. Among the studied parameters, the pile length is found to have the most significant impact on the pile load rating. The paper “Quality Assurance of Measured Response Intended for Fatigue Life Prediction” by Leander and Karoumi presents an approach for a quality assurance of the measured response based on established statistical methods. The stress range spectra, the product of the monitoring program intended for fatigue assessment, are analyzed. The aim of the analysis is to find deviant spectra and identify corrupt gauges. An additional aspect is the length of the monitoring period that is required for obtaining reliable results. A case study of a monitored Swedish steel railway bridge is incorporated in the paper to illustrate the approach. Some statistical distributions for the monitored stress ranges are also presented and incorporated in a fatigue assessment. The paper “Experimental Study on Repair Methods of Corroded Bridge Cables” by Nakamura and Suzumura investigates applications of six repair methods to corroded cable specimens. These specimens were exposed to severe corrosive environments to compare the effectiveness of these approaches. Two different types of test cables were used in this study: parallel wire strands and spiral strands. The first test group used parallel wire strand cables consisting of 19 nongalvanized steel wires, generally used as main cables of suspension bridges. Repair methods applied to these cable specimens are coating with zinc or epoxy resin paint or zinc powder paste, filling with epoxy resin or oil, and dehumidification method. Specimens were accelerated to corrode in laboratory for 15 months. By investigating mass loss caused by corrosion and appearance during this period, the effectiveness of six repair methods were compared. The dehumidification method was found to be the most effective for surface wires, followed by the epoxy resin paint and filling, the zinc powder paste, and the zinc and epoxy resin paint on the surface. The oil filling was not very effective compared with other repair methods. The corrosion of the inside wires was much less than that of the surface wires. The second test group used spiral strand cables consisting of seven galvanized steel wires. This test aimed at hangers of suspension bridges and stays of cable-stayed bridges. The same repair methods and corrosion acceleration methods were used. By investigating mass loss caused by corrosion and appearance of both inside and surface wires during the 16-month period, most of the proposed repair methods were found to be very effective. This study proves that even if cables are corroded, appropriate repair can be effective in preventing further corrosion.

The technical note “Response Modification Approach for Safe Extension of Bridge Life” by Gastineau et al. proposes an approach to extend the fatigue life of vulnerable steel bridges through a response modification apparatus, consisting of a mechanical amplifier and a response modification device that provides supplemental stiffness and damping to the bridge. Because of the relatively small deflections encountered under typical service loads, the use of a mechanical amplifier allows for a smaller apparatus and enables a more efficient device to provide adequate response modification forces to the bridge. The use of a scissor jack as the mechanical amplifier is proposed for use in bridge applications, and its utility in concert with a passive stiffness device is demonstrated by application to a simple beam structure. Reductions in moment of approximately 37% and safe life extensions of 300% are achieved on a simple beam model with the proposed response modification apparatus.