Showing papers in "Journal of Bridge Engineering in 2014"

Performance Evaluation of Concrete Columns Reinforced Longitudinally with FRP Bars and Confined with FRP Hoops and Spirals under Axial Load

Abstract: Nowadays, AASHTO LRFD Bridge Design Specifications and the Canadian Highway Bridge Design Code contain flexural and shear provisions for the design of concrete bridge members reinforced with fiber-reinforced polymer (FRP) bars. Because of a lack of research, these standards do not recommend using FRP bars to resist compressive stresses in compression members. This paper reports on 14 full-scale circular RC columns tested under concentric axial load. The columns were reinforced with longitudinal FRP bars and confined with circular FRP spirals or hoops. Sand-coated glass-FRP (GFRP) and carbon-FRP (CFRP) reinforcement was used. The test parameters included configuration of the confinement reinforcement (spirals versus hoops), hoop lap length, volumetric ratio, and FRP reinforcement type (glass versus carbon). The test results indicate that the GFRP and CFRP RC columns behaved similarly to columns reinforced with steel. Using GFRP and CFRP spirals or hoops according to the provisions of the Canadian S...

Seismic Fragility Analysis of Cable-Stayed Bridges Considering Different Sources of Uncertainties

Abstract: Seismic fragility analysis is a complex process involving different sources of uncertainties. In this paper, an alternative procedure for seismic fragility analysis based on the uniform design method (UD) is proposed. UD is used to generate a reasonable sample to build probabilistic seismic demand models (PSDMs), which are used to generate the fragility curves of a cable-stayed bridge. Sensitivity analyses of different uncertainties are carried out by comparison of the fragility curves for different components. It is concluded that (1) the UD method can consider different sources of uncertainties in fragility analysis with reasonable accuracy; (2) the bridge system is more fragile than any component of bridges; and (3) the damage probability of fragility considering uncertainty in the ground motion, geometry, and material is larger than that considering only the uncertainty in the ground motion.

Shake Table Studies of Energy-Dissipating Segmental Bridge Columns

Abstract: Five one-third scale segmental bridge columns with plastic hinges incorporating different advanced materials were designed and tested on one of the shake tables at the University of Nevada, Reno. The columns were subjected to the Sylmar Earthquake record with increasing amplitudes until failure. All the models were cantilever with longitudinal steel dowels connecting the base segment to the footing. Unbonded posttensioning was used to connect the segments and to minimize the residual displacements. Energy dissipation took place mostly through the yielding of the longitudinal bars in the base segment. Conventional RC was used in the plastic hinge of a reference column. In one of the models, a built in elastomeric pad integrated with the footing and a concrete segment constituted the plastic hinge. The other two columns incorporated engineered cementitious composite (ECC) and unidirectional carbon fiber reinforced polymer (CFRP) fabrics at the lower two segments. The effectiveness of repair with CFRP wraps was also studied by repairing and retesting the reference column. The test results showed that the proposed models with advanced materials are suitable for accelerated bridge construction in high seismic zones because of their fast construction, high energy dissipation, minimal damage in the plastic hinge zone, and minimal residual displacement.

Quasi-Static Cyclic Testing of a Large-Scale Hybrid Sliding-Rocking Segmental Column with Slip-Dominant Joints

Abstract: This paper presents the findings of an experimental study that investigated the response properties of segmental cantilever columns incorporating internal unbonded posttensioning (PT) and slip-dominant (SD) joints. The SD joints exhibited controlled sliding that provided energy dissipation with low damage. All joints of these columns, except for the bottom one, were designed to be SD. The bottom joint was designed to be rocking dominant (RD) and exhibited rocking that offered self-centering to the system. Design objectives and equations are presented. These equations were used for the design of a large-scale cantilever column that was subjected to reverse lateral cyclic loading at its top end, reaching a maximum drift ratio of 14.9%. At small drift ratios (≤3%), the response was dominated by sliding of the SD joints that provided energy dissipation (damping). For medium drift ratios (between 3 and 10%), rocking at the bottom joint increased and provided self-centering properties to the system. For large drift ratios (>10%), the self-centering properties decreased, but the damping properties remained practically constant. Rocking at the SD joints remained small at all times. Minor spalling was observed in the SD joints, while concrete crushing was observed at the bottom joint.

Optimal Sensor Placement for Modal Identification of Bridge Systems Considering Number of Sensing Nodes

Abstract: A series of optimal sensor placement (OSP) techniques is discussed in this paper. A framework for deciding the optimum number and location of sensors is proposed, to establish an effective structural health monitoring (SHM) system. The vibration response from an optimized sensor network reduces the installation and operational cost, simplifies the computational processes for a SHM system, and ensures an accurate estimation of monitoring parameters. In particular, the proposed framework focuses on the determination of the number of sensors and their proper locations to estimate modal properties of bridge systems. The relative importance of sensing locations in terms of signal strength was obtained by applying several OSP techniques, which include effective influence (EI), EI-driving point residue (EI-DPR), and kinetic energy (KE) methods. Additionally, the modified variance (MV) method, based on the principal component analysis (PCA), was developed with the assumption of independence in modal ordinates at each sensing location. Modal assurance criterion (MAC) between the target and interpolated mode shapes from an optimal sensor set was utilized as an effective measure to determine the number of sensors. The proposed framework is verified by three examples: (1) a numerically simulated simply supported beam, (2) finite-element (FE) model of the Northampton Street Bridge (NSB), and (3) modal information identified using a set of wireless sensor data from the Golden Gate Bridge (GGB). These three examples demonstrate the application and efficiency of the proposed framework to optimize the number of sensors and verify the performance of the MV method compared to the EI, EI-DPR, and KE methods.

Energy Harvesting from Train-Induced Response in Bridges

Abstract: The integration of large infrastructure with energy-harvesting systems is a growing field with potentially new and important applications. The possibility of energy harvesting from ambient vibratio ...

Observer Kalman Filter Identification for Output-Only Systems Using Interactive Structural Modal Identification Toolsuite

Abstract: Several modal identification techniques have been developed in the past few decades, and their use is rapidly expanding due to new focus on the instrumentation of major structures. This paper focuses on the expansion of the eigenvalue realization algorithm (ERA)–observer Kalman filter identification (OKID) to identify modal parameters of output-only systems (OO) by splitting the state-space model into deterministic and stochastic subsystems (ERA-OKID-OO). The performance is then compared with other output-only identification methods in terms of the level of accuracy and efficiency. A newly developed software package [Structural Modal Identification Toolsuite (SMIT)] is used to provide a uniform and convenient way of utilizing several system identification (SID) methods, including variations of ERA, auto-regressive with exogenous terms (ARX) models, system realization using information matrix (SRIM), and numerical algorithms for subspace state space system identification (N4SID). The main purpose o...

Distributed Strain Behavior of a Reinforced Concrete Bridge: Case Study

Abstract: To effectively assess and manage aging infrastructure, sensing technologies that produce accurate and useful quantitative data are required. Distributed fiber optic strain measurement systems are one potential technology that could fulfill this requirement. This case study investigates the performance of a distributed fiber optic strain measurement technology with high accuracy and spatial resolution during a load test on a RC bridge. The measurements from the fiber optic system are compared with conventional strain gauges and linear transducers, and good agreement between the sensors was found. The fiber optic measurements are also used to examine the bridge support conditions as well as the load distribution between beams. The results show that assumptions made about the support conditions during design do not match the actual bridge behavior, although the load distribution between beams is as expected.

Identification of Vehicular Axle Weights with a Bridge Weigh-in-Motion System Considering Transverse Distribution of Wheel Loads

Abstract: A modified two-dimensional (2D) Moses algorithm for acquiring the field-calibrated influence line (IL) of an existing bridge is presented, based on strain data acquired continuously at a high scanning rate with calibration vehicles of known axle weights and axle spacings crossing an instrumented bridge. Considering the transverse distribution of the wheel loads on each girder attributable to the 2D behavior of the slab-girder bridge, the IL of each of the girders can be calculated, which does not require the girders to possess identical material and geometric properties. By using the calculated IL of each girder as a reference, a modified 2D Moses algorithm was derived to identify axle weights of moving vehicles, taking into consideration the transverse distribution of the wheel loads on each girder. Mathematical equations to calculate ILs and axle weights were derived, and the proposed algorithms were implemented by a computer program. The accuracy of the IL calculation and axle weight identification was verified through a field test of a bridge on U.S. Route 78 in Alabama. The identified axle weights showed agreement with the static measurements from weighing pads and with results from the bending-plate weigh-in-motion (BPWIM) system near the instrumented bridge.

Seismic Response of Bridges with Rocking Foundations Compared to Fixed-Base Bridges at a Near-Fault Site

Abstract: This paper numerically investigates the three-dimensional seismic response of six reinforced concrete bridges hypothetically located in Oakland, California, 3 km from the Hayward fault. Three of the bridges are 17 m tall and three are 8 m tall. Three types of column-foundation designs are studied: (1) columns that form flexural plastic hinges, which are conventionally designed according to CALTRANS seismic design criteria; (2) columns on rocking pile foundations that are designed to remain elastic; and (3) columns designed to remain elastic that are supported on rocking shallow foundations. The bridges with rocking foundations use lead rubber bearings at the abutments to enhance strength, stiffness, and hysteretic energy dissipation. Using two components of horizontal excitation, three-dimensional nonlinear response history analyses are performed for two seismic hazard levels with return periods of 975 and 2,475 years, respectively. At both levels of shaking, the conventionally designed bridges experience substantial inelastic deformations and damage in the columns, whereas the bridges with rocking foundations result in negligible structural damage and a nominally elastic response and small residual deformations.

Pre-Earthquake Multi-Objective Probabilistic Retrofit Optimization of Bridge Networks Based on Sustainability

Abstract: Planning retrofit actions on bridge networks under tight budget constraints is a challenging process. Because of the uncertainties associated with this process, a probabilistic approach is necessary. In this paper, a probabilistic methodology to establish optimum pre-earthquake retrofit plans for bridge networks based on sustainability is developed. A multicriteria optimization problem is formulated to find the optimum timing of retrofit actions for bridges within a network. The sustainability of a bridge network and the total cost of retrofit actions are considered as conflicting criteria. The sustainability is quantified in terms of the expected economic losses. The uncertainties associated with seismic hazard and structural vulnerability are considered. The methodology is illustrated on an existing bridge network. Genetic algorithms are used to solve the multicriteria optimization problem. The effects of deterioration on bridge seismic performance are considered. The effects of the time horizon on the Pareto optimal solutions are also investigated.

Bridge Remaining Strength Prediction Integrated with Bayesian Network and In Situ Load Testing

Abstract: This paper proposes a new framework for predicting remaining bridge strength that integrates a Bayesian network and in situ load testing. It discusses the uncertainty of important factors on corrosion damage and develops a stiffness degradation model for corroded beams based on experimental investigations. Following this, the authors develop a Bayesian network that includes corrosion damage, stiffness degradation, load-deflection response, and other factors to predict structural strength degradation. A numerical example using an existing RC bridge demonstrates the general procedures. The comparison between the theoretical and the experimental deflections from load testing shows that the proposed methodology can efficiently improve prediction accuracy and reduce prediction uncertainty.

Seismic Design of Rocking Shallow Foundations: Displacement-Based Methodology

Abstract: This paper proposes a direct displacement-based design (DDBD) methodology for seismic design of rocking shallow foundations for ordinary bridges under earthquake loads. A multilinear model is developed to represent the backbone curve of the nonlinear moment-rotation behavior. In addition, a new empirical relationship is proposed that correlates the initial rotational stiffness to the moment capacity of a rocking foundation; this correlation is proposed as an alternative to calculation of stiffness based upon elasticity theory. In the proposed design procedure, a bridge system consisting of a deck mass, a rocking foundation, and a damped elastic column is integrated into a single element from which the equivalent linear damping and period can be determined. The DDBD methodology uses the equivalent system damping and period along with a design displacement response spectrum to determine the seismic displacement demand. The approach is checked by comparing displacements predicted by this method to th...

Uniform and Pitting Corrosion Modeling for High-Strength Bridge Wires

Abstract: Cable inspections revealed that severe corrosion of steel wires is one of the main failure mechanisms of cables. Accelerated corrosion experiments were conducted to evaluate the variation in the uniform and pitting corrosion depths of high-strength steel wires over time. The measured uniform corrosion depth followed a lognormal distribution with time-dependent corrosion variables at both the zinc coating corrosion stage and the steel corrosion stage. The block’s maximum pitting factors from a different exposure time were proven to be drawn from the same underlying continuous population and followed Gumbel distribution. The regression models of the scale and location parameter with increased surface area were obtained based on the experimental block’s maximum pitting factors. The extreme value distribution of the pit depth of corroding steel wires could be predicted by the developed statistical uniform and pitting factor models.

Blast Load Effects on Highway Bridges. I: Modeling and Blast Load Effects

Abstract: Numerous terrorist events during the last decade, including the 2001 attack on the World Trade Center, have heightened concern about the safety of bridges during intentional/unintentional blast load effects. Analysis of highway bridges under blast loads requires accurate generation and application of blast loads and good understanding of the behavior of components of a bridge during high strain rate loading encountered during blast loads. In this paper, a new approach for the application of blast loads on bridge components has been presented. This approach can apply realistic loads and can simulate both reflection and diffraction of blast loads. Using this approach, verification of simulation of blast loads in LS-DYNA has been carried out by using available blast tests on two types of beams. A high fidelity model of a typical three-span highway bridge has been developed for investigation of blast load effects on a three-span reinforced concrete bridge. It is observed that the range of demands impo...

Seismic response analyses of the Yokohama bay cable-stayed bridge in the 2011 great East Japan earthquake

Abstract: Yokohama Bay Bridge, with a total span length of 860 m, is the second longest span cable-stayed bridge in East Japan, and one of the most densely instrumented bridges in Japan. On March 11, 2011, the Great East Japan (Tohoku) Earthquake hit northeastern Japan with magnitude of MW 9.0, notably the largest earthquake in Japan’s modern history. Intensity 5+(PGA 1.4–2.5 m/s2) of the maximum scale 7 of Japan Meteorogical Agency’s seismic intensity was recorded on the bridge site. This paper describes seismic response analyses of the bridge, system identification, performance evaluation of link-bearing connections (a seismic isolation system), and postearthquake field observation. Response analyses show that transverse vibration dominated the response of girder and tower, with a maximum girder displacement of 62 cm. The large transverse vibrations induced pounding between tower and girder on the tower–girder connections as shown by the periodic impulses on acceleration records. Meanwhile, longitudinal accelerations indicate that the link-bearing connections functioned properly during the earthquake. System identification reveals nonlinearity of the response as evidenced by variations in natural frequencies and mode shapes during large excitation. Despite these conditions, the bridge did not suffer any structural damages, since the ground motions experienced during the earthquake were less than the design and seismic retrofit ground motions.

Curvature Monitoring of Beams Using Digital Image Correlation

Abstract: A method for measuring longitudinal strains with the height at a section, and thus the curvature, using a technique based on digital image correlation (DIC), is presented. The background to this technique is introduced as well as previous work in this area. The accuracy of DIC under ideal conditions is established using artificially generated images that represent beams with various curvatures. The practical accuracy of DIC is established by comparing the strains measured using DIC to those predicted by elastic theory and measured using strain gauges for a steel beam. The correlation between these results is found to be excellent. DIC is then used to measure curvatures in RC beams and these results are compared with analytically predicted results with good agreement. The choice of an appropriate gauge length for RC is discussed and is shown to be one of the significant advantages of using DIC as opposed to strain gauges in both laboratory testing and field monitoring of bridge structures.

Shake Table Studies and Analysis of a Two-Span RC Bridge Model Subjected to a Fault Rupture

Abstract: The objective of this study was to investigate the fault rupture effects on the seismic response of a bridge system crossing an active fault. A large-scale two-span bridge model supported on three bents, one on each of the University of Nevada, Reno (UNR), shake tables, was tested under earthquakes with incoherent motions that simulated fault rupture. The results were compared with those from an identical bridge model subjected to coherent ground motions in a previous shake table study. It was found that fault rupture substantially affected the damage type and location in the bridge bents. The most severely damaged bent in the current bridge was a relatively flexible bent near the fault. However, under coherent motions, the shortest bent experienced the severest damage. Linear static and nonlinear dynamic analyses of the test model revealed that existing analytical techniques are adequate in estimating the response of the most critical bent subjected to fault rupture.

Analysis of structural health monitoring data from Hammersmith Flyover

Abstract: There has recently been considerable research published on the applicability of monitoring systems for improving civil infrastructure management decisions. Less research has been published on the challenges in interpreting the collected data to provide useful information for engineering decision makers. This paper describes some installed monitoring systems on the Hammersmith Flyover, a major bridge located in central London (United Kingdom). The original goals of the deployments were to evaluate the performance of systems for monitoring prestressing tendon wire breaks and to assess the performance of the bearings supporting the bridge piers because visual inspections had indicated evidence of deterioration in both. This paper aims to show that value can be derived from detailed analysis of measurements from a number of different sensors, including acoustic emission monitors, strain, temperature and displacement gauges. Two structural monitoring systems are described, a wired system installed by a...

Repair of Reinforced Concrete Bridge Columns Containing Buckled and Fractured Reinforcement by Plastic Hinge Relocation

Abstract: This paper describes a new repair technique that involves the use of plastic hinge relocation to restore strength and deformation capacity of RC bridge columns. Summarized is the overall repair concept and experimental results that include the reversed cyclic testing of three large-scale bridge columns that were previously damaged, repaired using the proposed methodology, and then subsequently retested. To date, two different repair alternatives were executed using unidirectional carbon fiber sheets in the hoop and longitudinal directions, the latter anchored into the RC footing with 30-mm-diameter carbon fiber anchors. A method for predicting the force-displacement responses of columns repaired in this manner was also developed and found to give reasonable results. Also included in this paper are design considerations, which are carried out in the steps needed to design a repair system to relocate the plastic hinge in a column containing buckled longitudinal reinforcement. The responses show that...

Blast Load Effects on Highway Bridges. II: Failure Modes and Multihazard Correlations

Abstract: Bridges with different seismic design levels and concrete compressive strengths have been analyzed for three levels of underdeck blast loads. It is observed that there are several other damage modes besides failure of bridge columns that may contribute to a complete collapse of the bridge. In general, it is demonstrated that an increased seismic resistance leads to improved performance during blast loads. Both concrete strength and seismic capacity are equally effective for bridges designed with higher seismic resistance. Extensive simulations have been performed to establish a correlation between the ratio of ductility and strength reduction factor, μ/R, and pier top displacement for concrete with different compressive strengths. It has been observed that the pier top displacement decreases drastically for μ/R>6. Moreover, it is observed from results of 27 simulations that bridge piers with μ/R>6 survive high levels of blast loads (without failure of piers).

Study on Wind-Induced Vibration Control of a Long-Span Cable-Stayed Bridge Using TMD-Type Counterweight

Abstract: It has been widely acknowledged that a tuned mass damper (TMD) can effectively control the wind-induced vibration of the main deck of long-span bridges. However, the unfavorable effect on static characteristics of the increased dead load cannot be avoided if the TMD is installed straight on the main deck. A TMD-type counterweight is designed in this paper, where the counterweight originally designed for reducing the live load–induced displacements at the central span are taken as the mass block in the TMD. The Sutong Cable-Stayed Bridge (SCB), with a main span of 1,088 m, is taken as an example. The buffeting responses of the bridge with the stationary counterweight and the proposed TMD-type counterweight are compared, and the control performance of the bridge with and without auxiliary piers is also investigated. Results indicate that the TMD has notable effects on reducing the vibration of the main deck without auxiliary piers, whereas the impact is not significant for the presence of the auxili...

Impact of Damper Stiffness and Damper Support Stiffness on the Efficiency of a Linear Viscous Damper in Controlling Stay Cable Vibrations

Abstract: Accurate prediction of optimum damper size and its corresponding maximum attainable modal damping ratio is essential for the design of a linear viscous damper to control cable vibrations on cable-stayed bridges. The stiffness within the damper and the damper support would affect both the required damper size and the resulting equivalent modal damping ratio of the damped cable and thus influence the damper efficiency. An experimental study on a cable-damper system is conducted to investigate the individual and the combined effects of damper stiffness and damper support stiffness on the performance of a linear viscous damper. A finite-element model of the corresponding cable-damper system is developed to verify the experimental results and further study these two parameters within the typical ranges of cable and damper properties used on real bridges. Results show that higher damper stiffness and/or lower damper support stiffness would have an adverse impact on damper performance. Increasing the stiffness of a damper and/or its support would result in a larger optimum damper size. However, the maximum attainable damping ratio would decrease with larger damper stiffness but increase if the support is more rigid. To facilitate practical design, a set of asymptotic relationships has been proposed, of which the optimum damper size and the maximum achievable damping ratio are expressed concisely as functions of nondimensional damper properties in terms of its location, stiffness, and support stiffness. Design examples are given to illustrate the various applications of the proposed refined damper design tool.

Improving Fatigue Evaluations of Structures Using In-Service Behavior Measurement Data

Abstract: Conservative models and code practices are usually employed for fatigue-damage predictions of existing structures. Direct in-service behavior measurements are able to provide more accurate estimations of remaining-fatigue-life predictions. However, these estimations are often accurate only for measured locations and measured load conditions. Behavior models are necessary for exploiting information given by measurements and predicting the fatigue damage at all critical locations and for other load cases. Model-prediction accuracy can be improved using system identification techniques where the properties of structures are inferred using behavior measurements. Building upon recent developments in system identification where both model and measurement uncertainties are considered, this paper presents a new data-interpretation framework for reducing uncertainties related to prediction of fatigue life. An initial experimental investigation confirms that, compared with traditional engineering approaches, the methodology provides a safe and more realistic estimation of the fatigue reserve capacity. A second application on a full-scale bridge also confirms that using load-test data reduces the uncertainty related to remaining-fatigue-life predictions.

Detection of Delamination in Concrete Bridge Decks by Joint Amplitude and Phase Analysis of Ultrasonic Array Measurements

Abstract: The accuracy and precision of low-frequency (center frequency of approximately 55 kHz) ultrasonic testing for detection and characterization of delamination in concrete bridge decks were evaluated. A multiprobe ultrasonic testing system (with horizontally polarized shear-wave transducers) was used to detect built-in delamination defects of various size, depth, and severity (i.e., thickness) in a test specimen—a 6.1 m×2.4 m×216 mm (20 ft×8 ft×8.5 in.) reinforced concrete slab-built to simulate a concrete bridge deck. The collected data sets were reconstructed applying synthetic aperture focusing technique (SAFT). The reconstructed measurement results were then used to assess the condition of the concrete slab at individual points [point-by-point data collection and two-dimensional (2D) reconstruction] as well as along lines, where data were collected at smaller steps and reconstructed in a three-dimensional (3D) format. The local-phase information was also calculated, superimposed on the reconstruc...

Time-Dependent Analysis of Long-Span, Concrete-Filled Steel Tubular Arch Bridges

Abstract: Concrete-filled steel tubular (CFST) arch bridges have gained popularity over the last decades for use in long-span applications. At service conditions, these bridges are influenced significantly by the time-dependent behavior of the concrete. This paper presents a finite-element model that was developed using commercial finite-element software and is capable of describing the time-dependent behavior. The proposed approach can account for the construction process, time effects, and geometric nonlinearity. The time-dependent behavior of the core concrete in the arch ribs was modeled using European guidelines and the integral-type creep law, implemented with the finite-element model with a user-defined subroutine. The accuracy of the proposed method was validated against real site measurements recorded for a representative arch bridge. As part of this work, the necessity of considering the variation of the time of first loading and the geometric nonlinearity has been discussed. Finally, a simplified method was developed based on the results of the refined finite-element model and is recommended for possible use in day-to-day routine design.

Statistical Bridge Signatures

Abstract: Instrumentation of bridge structures provides a stream of data representing operational structural response under loading The authors define the term bridge signature as the expected response of a particular bridge under loading, as measured by different instruments In this research, the authors propose a new method to develop and evaluate a bridge signature The signature can be monitored over time and statistically evaluated to detect potential structural deterioration and damage An instrumentation system was implemented on the Powder Mill Bridge in Barre, Massachusetts, as a research prototype for the development of a structural health monitoring (SHM) system Heavy truck events due to daily traffic were collected using an automatic measurement system, which triggers above a given threshold of recorded strains Using the measured strain data due to daily traffic, a bridge signature was created using nonparametric statistical techniques Maximum experimental strain values from heavy truck eve

Flexural Members with High-Strength Reinforcement: Behavior and Code Implications

Abstract: High-strength steel reinforcement provides various benefits to the concrete construction industry, including a more efficient use of high-performance concrete, reduction of reinforcing bar congestion, and materials savings. Prior to the 2013 interim revisions of the AASHTO LRFD Bridge Design Specifications, the value of reinforcing steel yield strength used in design was limited to being no greater than 517 MPa (exceptions were permitted with owner approval for cases with a yield strength of less than 414 MPa). In 2007, National Cooperative Highway Research Program Project 12-77 was initiated to evaluate the AASHTO specifications with respect to the use of high-strength reinforcing steel and other grades of reinforcing steel having no discernible yield plateau. Among the objectives of this project was the investigation of ductility and crack control of flexural members using high-strength reinforcement. This research led to a number of recommendations that were subsequently incorporated into the 2013 interim revisions of the specifications. The flexural behavior and design of members reinforced with high-strength steel are presented. This paper also provides the background information for the AASHTO specification revisions related to strength reduction factors for flexure. The research demonstrates that the strain limits for high-strength reinforcement must be changed to achieve the curvature ductility comparable to that implicit with the current use of Grade 414 reinforcing steel. Moreover, the service load stresses in steel should be limited to 60% of the yield strength.

Experimental behavior of steel fixed bearings and implications for seismic bridge response

Abstract: Steel fixed bearings are commonplace structural elements for transmitting loads from superstructures to substructures, and they have typically occupied a role of elastic force transfer elements within the overall scheme of an earthquake resisting system (ERS). Recent revisions to design and guide specifications have acknowledged the possibility of bearings acting as fuses, but there is little research available to characterize bearing behavior for such design roles or the associated bridge response to be expected when bearings have fused. One design approach, adopted by the Illinois DOT (IDOT), applies capacity design principles and permits the bearings and superstructure to slide on the substructure. The intent of this design approach is to capture some of the beneficial aspects of conventional isolated systems, such as period elongation, reduction of force demands, and protection of substructures from large inelastic displacement demands, without incurring the additional design provisions and fabrication costs to satisfy the requirements for seismic isolation systems. To achieve this quasi-isolated bridge response, steel fixed bearings are used as fusing elements, where the steel pintles or anchor rods rupture, and the fixed bearing plates become free to slide on the supporting pier cap. A properly proportioned bearing will fuse prior to superstructure/substructure elements experiencing inelastic demands. The University of Illinois has been collaborating with IDOT to investigate the behavior of quasi-isolated bridge systems and to calibrate and refine IDOT’s ERS design and construction methodology. The research is composed of experimental testing to characterize fundamental bearing behavior, coupled with nonlinear global bridge modeling to evaluate limit state progression and estimate maximum displacement demands of the superstructure relative to the substructure. The cyclic response of full-scale steel low-profile fixed bearings demonstrates predictable sliding behavior, but based on current design procedures, these bearings are often overdesigned for use as fuses in quasi-isolated bridges. Consequently, a bridge, which in other respects may exhibit satisfactory quasi-isolated response, might also incur significant damage to the substructure unit where fixed bearings are provided. A parametric study of global bridge response demonstrates that the anchorage of fixed bearings to substructures could be reduced to limit the damage to the supporting substructure unit while incurring only a nominal increase in superstructure displacement demands.