Damage Analysis of Hanshin Expressway Viaducts during 1995 Kobe Earthquake. I: Residual Inclination of Reinforced Concrete Piers

The damage suffered by elevated viaducts of the Hanshin Expressway Kobe Route during the 1995 Kobe earthquake is described with emphasis on reinforced concrete (RC) piers. Although many piers were severely damaged, it is also true that the damage to many piers appeared moderate or even mild. On the other hand, a number of piers suffered from large residual inclination in spite of the apparently light damage. By considering that the large residual inclination of piers included severe earthquake-induced damage, it is pointed out that almost all the RC single piers from P35 to P350 received consistently severe damage. The cause of large residual inclination, especially in apparently nondamaged piers, is studied. A dynamic analysis of a single RC pier is conducted to study the relationship between residual inclination and residual deformation of a pier. As a result, we find that the flexural residual deformation of a pier cannot explain the observed large residual inclination, but it is suggested that the pulling out of reinforcing bar from the footing can be a primary cause of the observed large residual inclination.     

Seismic Response of Multiple Span Steel Bridges in Central and Southeastern United States. II: Retrofitted

Part I of this two-part paper evaluated the seismic response of typical multispan simply supported (MSSS) and multispan continuous steel girder bridges in the central and southeastern United States. The results showed that the bridges were vulnerable to damage resulting from impact between decks, and large ductility demands on nonductile columns. Furthermore, fixed and expansion bearings were likely to fail during strong ground motion. In this paper, several retrofit measures to improve the seismic performance of typical multispan simply supported and multispan continuous steel girder bridges are evaluated, including the use of elastomeric bearings, lead-rubber bearings, and restrainer cables. It is determined that lead-rubber bearings are the most effective retrofit measure for reducing the seismic vulnerability of typical bridges. While isolation provided by elastomeric bearings limits the forces into the columns, the added flexibility results in pounding between decks in the MSSS steel girder bridges. Restrainer cables, which are becoming a common retrofit measure, are effective in reducing the hinge opening in MSSS bridges with steel bearings. However, when used with elastomeric bearings, the restrainer cables negate the isolation effect of the bearings.     

Damage Analysis of Hanshin Expressway Viaducts during 1995 Kobe Earthquake. II: Damage Mode of Single Reinforced Concrete Piers

The damage mode that single reinforced concrete (RC) piers of the Hanshin Expressway Kobe Route suffered during the 1995 Kobe earthquake is discussed. On the Kobe Route, many single RC piers suffered from flexural mode damage; however, some suffered from shear failure, and most shear failure occurred in piers with rectangular cross sections. The flexural and shear capacity of each pier are calculated based on the design documents, and the ratio of flexure to shear capacity, r, is calculated by taking into account the mass of the pier column. It is found that the damage mode (flexure or shear) in the severely damaged single RC piers from P1 to P350 can be explained by the value of r, either >1.0 (flexural mode) or <1.0 (shear mode).     

Seismic Response of Multiple Span Steel Bridges in Central and Southeastern United States. I: As Built

The seismic response of typical multispan simply supported (MSSS) and multispan continuous steel girder bridges in the central and southeastern United States is evaluated. Nonlinear time history analyses are conducted using synthetic ground motion for three cities for 475 and 2,475-year return period earthquakes (10 and 2% probability of exceedance in 50 years). The results indicate that the seismic response for the 475-year return period earthquake would lead to an essentially linear response in typical bridges. However, the seismic response for a 2,475-year return period earthquake resulted in significant demands on nonductile columns, fixed and expansion bearings, and abutments. In particular, pounding between decks in the MSSS bridge would result in significant damage to steel bearings and would lead to the toppling of rocker bearings, which may result in unseating of the bridge deck.     

Reduction of Local Scour in the Vicinity of Bridge Pier Groups Using Collars and Riprap

The present study examines the use of independent and continuous pier collars in combination with riprap for reducing local scour around bridge pier groups. The efficiency of collars was studied through experiments. The data from the experiments were compared with data from earlier studies on single piers with collars and bridge pier groups without collars. The data showed that in the case of two piers in line, combination of continuous collars and riprap results in the most significant scour reduction of about 50 and 60% for the front and rear piers, respectively. In other cases for two piers in line, independent collars showed better efficiency than a continuous collar around both piers. It was also shown that efficiency of collars is more on a rectangular pier aligned with the flow than two piers in line. Experiments however, indicated that collars are not so effective in reduction of scouring around two transverse piers.     

Impact Factors for Curved Continuous Composite Multiple-Box Girder Bridges

The results from a parametric study on the impact factors for 180 curved continuous composite multiple-box girder bridges are presented. Expressions for the impact factors for tangential flexural stresses, deflection, shear forces and reactions are deduced for AASHTO truck loading. The finite-element method was utilized to model the bridges as three-dimensional structures. The vehicle axle used in the analysis was simulated as a pair of concentrated forces moving along the concrete deck in a circumferential path with a constant speed. The effects of bridge configurations, loading positions, and vehicle speed on the impact factors were examined. Bridge configurations included span length, span-to-radius of curvature ratio, number of lanes, and number of boxes. The effect of the mass of the vehicle on the dynamic response of the bridges is also investigated. The data generated from the parametric study and the deduced expressions for the impact factors would enable bridge engineers to design curved continuous composite multiple-box girder bridges more reliably and economically.     

Dynamic Analysis of Curved Continuous Multiple-Box Girder Bridges

The use of horizontally curved composite multiple-box girder bridges in modern highway systems is quite suitable in resisting torsional and warping effects induced by highway curvatures. Bridge users react adversely to vibrations of a bridge and especially where torsional modes dominate. In this paper, continuous curved composite multiple-box girder bridges are analyzed, using the finite-element method, to evaluate their natural frequencies and mode shapes. Experimental tests are conducted on two continuous twin-box girder bridge models of different curvatures to verify and substantiate the finite-element model. Empirical expressions are deduced from these results to evaluate the fundamental frequency for such bridges. The parameters considered herein are the span length, number of lanes, number of boxes, span-to-radius of curvature ratio, span-to-depth ratio, end-diaphragm thickness, number of cross bracings, and number of spans.     

Seismic Design of Lifeline Bridge using Hybrid Seismic Isolation

This paper presents the merits of a hybrid seismic isolation system used for the seismic design of a major bridge. The bridge is analyzed for two different arrangements of seismic isolation systems. The first arrangement consists of friction pendulum bearings at all substructure locations; the other incorporates a hybrid system where laminated elastomeric bearings are used at the abutments and friction pendulum bearings at the piers. Analysis results have demonstrated that the hybrid seismic isolation system provided a structure with a fundamental period long enough to attract smaller seismic forces, while controlling the magnitude of isolation bearings displacements. It also provided a more uniform distribution of seismic forces among substructure elements. As a result, higher seismic forces on the piers were reduced, allowing for a more economical design of substructures. The hybrid seismic isolation system helped to control the wind-induced vibrations and reduced the sizes of the isolation bearings.     

Seismic Performance Assessment of Simply Supported and Continuous Multispan Concrete Girder Highway Bridges

Seismic evaluations of typical concrete girder bridges are conducted for both a multispan simply supported and a multispan continuous girder bridge common to the Central and Southeastern United States. These evaluations are performed for an approximate hazard level of 2% in 50?years by performing nonlinear time history analyses on three-dimensional analytical models. The results show significant vulnerabilities in the reinforced concrete columns, the abutments, and also in unseating of the girders. In general, the longitudinal loading of the bridges results in larger demands than the transverse loading. However, the simply supported bridge sustains bearing deformations in the transverse direction which are on the same order as their longitudinal response. These results suggest that both longitudinal and transverse loading are significant and should be considered when performing seismic hazard analyses of these bridges.     

Effect of Skewness on the Distribution of Live Load Reaction at Piers of Skewed Continuous Bridges

This paper presents a study of the skewness effect on live load reactions at the piers of continuous bridges. Two prestressed concrete I-beam bridges and one steel I-girder bridge were selected for the study. To evaluate the skew effect, the skew angle of the bridges was varied from 0 to 60°. Live load reaction at support and shear at the beam ends of the selected bridges were determined using finite-element analysis. The comparison of the distribution factors of live load reactions and shear revealed that the distribution factor of reaction at piers was higher than that of shear at beam ends near the same support. The increase in the reaction distribution factor was more significant than that in the shear distribution factor in the interior beam line when the skew angle was greater than 30°. The LRFD shear equations and the Lever rule method could conservatively predict live load reaction distribution for piers in exterior beam lines but underestimate live load reaction distribution in interior beam lines. It is recommended that more research be performed for the distribution factor of live load reaction to quantify the responses.     

Feasibility of a 1,400 m Span Steel Cable-Stayed Bridge

This paper describes the feasibility of 1,400 m steel cable-stayed bridges from both structural and economic viewpoints. Because the weight of a steel girder strongly affects the total cost of the bridge, the writers present a procedure to obtain a minimum weight for a girder that ensures safety against static and dynamic instabilities. For static instability, elastoplastic, finite-displacement analysis under in-plane load and elastic, finite-displacement analysis under displacement-dependent wind load are conducted; for dynamic instability, multimodal flutter analysis is carried out. It is shown that static critical wind velocity of lateral torsional buckling governs the dimension of the girder. Finally, the writers briefly compare a cable-stayed bridge with suspension bridge alternatives.     

Collapse during Construction of a Precast Girder Bridge

This paper describes the collapse during construction of a precast concrete girder bridge crossing Highway 15, north of Montréal. The bearing conditions for the girders at the time of collapse are discussed. The tie rods that were used to restrain the movement of the girder top flanges were tested to determine their effective stiffness and strength. Rotational and sliding tests were also performed on the pot bearing units. These experimental results were used in an analytical study to confirm the sequence of collapse of the girders. Bracing requirements during construction for precast girder bridges supported on pot bearings are discussed.     

Development and Implementation of a Continuous Strain Monitoring System on a Multi-Girder Composite Steel Bridge

This paper describes the implementation and evaluation of a long-term strain monitoring system on a three-span, multisteel girder composite bridge located on the interstate system. The bridge is part of a network of bridges that are currently being monitored in Connecticut. The three steel girders are simply supported, whereas the concrete slab is continuous over the interior supports. The bridge has been analyzed using the standard AASHTO Specifications and the analytical predictions have been compared with the field monitoring results. The study has included determination of the location of the neutral axes and the evaluation of the load distributions to the different girders when large trucks cross the bridge. A finite-element analysis of the bridge has been carried out to further study the distribution of live load stresses in the steel girders and to study how continuity of the slabs at the interior joints would influence the overall behavior. The results of the continuous data collection are being used to evaluate the influence of truck traffic on the bridge and to establish a baseline for long-term monitoring.     

Large Shear Studs for Composite Action in Steel Bridge Girders

Shear studs used in composite steel bridge construction are typically 19.1 mm (? in.) or 22.2 mm (? in.) in diameter. This paper presents the development and implementation of the 31.8 mm (1??in.) stud diameter. Because the 31.8 mm (1??in.) stud has about twice the strength and a higher fatigue capacity than the 22.2 mm (? in.) stud, fewer studs are required along the length of the steel girder. This would increase bridge construction speed and future deck replacement, and reduce the possibility of damage to the studs and girder top flange during deck removal. Studs also can be placed in one row only, over the web centerline, freeing up most of the top flange width and improving safety conditions for field workers. This paper provides information on the development, welding, quality control, and testing of the 31.8 mm (1??in.) stud. Information on the first bridge built in the state of Nebraska with the 31.8 mm (1??in.) studs is provided.     

Condition Assessment of Shear Connectors in Slab-Girder Bridges via Vibration Measurements

The paper presents a field study on condition assessment of the shear connectors in a full slab-girder bridge via vibration measurements. First, a model updating technique is employed to assess the condition of the whole structure, including boundary conditions, bearings, girders, slab, and shear connectors, from the accelerations on the slab measured in vibration testing. Then, a new damage index based on the difference of frequency response functions on the slab and the corresponding points on the girder is developed to evaluate the condition of shear connectors. The advantage of the new method lies in the fact that it does not need any reference data (undamaged data) for the structure. Compared with the results obtained using the model updating technique, the method is more reliable and accurate in assessing the condition of the shear connectors between the slab and girders. The effects of measurement noise on the damage identification results and the damage quantification are also studied through numerical simulation.     

Design for Shear in Curved Composite Multiple Steel Box Girder Bridges

Modern highway bridges are often subject to tight geometric restrictions and, in many cases, must be built in curved alignment. These bridges may have a cross section in the form of a multiple steel box girder composite with a concrete deck slab. This type of cross section is one of the most suitable for resisting the torsional, distortional, and warping effects induced by the bridge’s curvature. Current design practice in North America does not specifically deal with shear distribution in horizontally curved composite multiple steel box girder bridges. In this paper an extensive parametric study, using an experimentally calibrated finite-element model, is presented, in which simply supported straight and curved prototype bridges are analyzed to determine their shear distribution characteristics under dead load and under AASHTO live loadings. The parameters considered in this study are span length, number of steel boxes, number of traffic lanes, bridge aspect ratio, degree of curvature, and number and stiffness of cross bracings and of top-chord systems. Results from tests on five box girder bridge models verify the finite-element model. Based on the results from the parametric study simple empirical formulas for maximum shears (reactions) are developed that are suitable for the design office. A comparison is made with AASHTO and CHBDC formulas for straight bridges. An illustrative example of the design is presented.     

Distribution of Wheel Loads on Continuous Steel Spread-Box Girder Bridges

Composite concrete-steel spread (multispine) box girder bridges remain one of the most common types constructed. Current design practices in North America recommend few analytical methods for the design of such bridges in simply supported construction. However, the effects of continuous construction have not been dealt with fully. In designing a continuous bridge, it is important to determine the maximum negative and positive stresses, maximum reactions, and shears in the bridge subjected to various loadings. This paper presents an extensive parametric study using a finite-element model in which 60 continuous bridge prototypes of various geometries, each subjected to various loading conditions, are analyzed for the distribution of flexural stresses, deflection, shears, and reactions. The parameters considered in the study are span length, number of spread boxes, and number of lanes. Distribution factors for maximum flexural stresses, deflection, shears, and reactions, suitable for design, are deduced for AASHTO truck loading. Results from tests on five box girder bridge models verify the finite-element model. A design example is presented to illustrate the use of the deduced formulas for the distribution factors.     

Experimental Study on Moment–Plastic Rotation Capacity of Hybrid Beams

The moment–inelastic rotation behavior of hybrid steel girder bridges is experimentally investigated. Six welded girders having compact flanges and webs are statically loaded under three-point bending condition in order to simulate the interior pier section of a two-span continuous girder. Six girders, three with hybrid sections and three with homogeneous sections, are designed with three types of web slenderness ratios, resulting in three pairs of hybrid and homogeneous girders. The inelastic rotation capacities obtained from the experimental tests are then compared between hybrid and homogeneous girders. In addition, the results are compared with the prediction moment–inelastic rotation curve proposed in 1998 by White and Barth. It is concluded that, under the condition in this study, hybrid girders have more deformation capacity than homogeneous girders, and that the prediction curve is more conservative for a specimen with higher web slenderness ratio.     

R-Factor Parameterized Bridge Damage Fragility Curves

Damage fragilities describe the probability that a bridge will incur certain (discrete) damage states conditioned on the intensity of the earthquake it may experience. Reinforced concrete box girder highway overpass bridges are prevalent among the total inventory of bridges in California. For this class of bridges, a method for computing damage fragilities for three damage states (concrete cover spalling, longitudinal bar buckling, and column failure) based on the bridge force reduction factors (R-factors) is derived in this paper. Bridge damage fragilities are described by equations relating the median intensity and uncertainty of ground motion to discrete damage states of column concrete cover spalling, column bar buckling, and column failure using the bridge R-factor as the principal parameter describing the bridge structure. Such damage fragility equations are furnished for earthquake intensities measured using pseudospectral acceleration (Sa) and cumulative absolute displacement (CAD) in both the bridge longitudinal and transverse directions.