Showing papers in "Journal of Bridge Engineering in 2000"

Evaluation of Seismic Damage to Memphis Bridges and Highway Systems

Abstract: This paper presents a procedure for the evaluation of the expected seismic damage to bridges and highway systems in Memphis and Shelby County, Tenn. Data pertinent to 452 bridges and major arterial routes were collected and implemented as a geographic information system database. The bridges were classified into several bridge types using a bridge classification system modified from the NBIS/Federal Highway Administration coding guidelines. The bridge damage states considered are no/minor damage, repairable damage, and significant damage. The fragility curves corresponding to these damage states were established for various bridge types. Given an earthquake with a moment magnitude of 7.0 occurring at Marked Tree, Ark., the intensity of ground shaking and liquefaction-induced permanent ground deformation in Memphis and Shelby County were estimated, and then the expected damage to bridges and highway systems was determined. The results can be used to prioritize bridges for retrofitting, to prepare a pre-earthquake preparedness plan, to develop a postearthquake emergency response plan, and to assess the regional economic impact from the damage to highway transportation systems.

Design of mechanical viscous dampers for stay cables

Abstract: External dampers have been utilized in a number of cable-stayed bridges to suppress transverse cable vibrations. However, simple and accurate damper design recommendations that concurrently consider all important cable parameters are lacking. Previous efforts have been based on the idealization of cables as taut strings. In this paper, the governing differential equation for vibration of cables containing a viscous damper is converted to a complex eigenvalue problem containing nondimensional cable parameters. Next, a parametric study is conducted involving repeated solutions of the eigenvalue problem for a wide range of nondimensional parameters. The effects of dampers on first mode vibration frequencies and first mode cable damping ratios are presented in nondimensional format. It is shown that, for the range of parameters involved in most stay cables, the influence of cable sag is insignificant, whereas the cable bending stiffness can have a significant influence on the resulting cable damping ratios. Simplified nondimensional relationships are proposed for calculating damper-induced changes in the first mode cable damping ratios. Results of lab tests on a scaled model cable are compared with estimated values using the formulation presented. Example problems are also presented for comparison and design purposes.

AASHTO-LRFD Live Load Distribution Specifications

Abstract: The live load distribution factors contained in the AASHTO-LRFD Bridge Design Specification present a major change to the AASHTO-LFD specifications that have been in effect for more than 50 years. This change has generated some interest in the bridge engineering community and has raised some questions. The AASHTO-LFD formulas are based on the girder spacing only and are usually presented as S/D, where S is the spacing and D is a constant based on the bridge type. This method is applicable to straight and right (i.e., nonskewed) bridges only. The new formulas are more complex and consider more parameters, such as bridge length and slab thickness. It may not be obvious to the engineers what added accuracy and flexibility (e.g., skewed bridges) is gained by the increased complexity. This paper will present the background on the development of the formulas and compare their accuracy with the S/D method. A discussion on the extension of the single girder design (using formulas) to the skewed bridges is also pr...

Field Performance of FRP Bridge Repairs

Abstract: Many reinforced concrete bridges throughout the United States on county and state highway systems are deteriorated and/or distressed to such a degree that structural strengthening of the bridge or reducing the allowable truck loading on the bridge by loading posting is necessary to extend the service life of the bridge. The structural performance of many of these bridges can be improved through external bonding of fiber-reinforced plastic (FRP) laminates or plates. This paper describes the rehabilitation of an existing concrete bridge in Alabama through external bonding of FRP plates to the bridge girders. Field load tests were conducted before and after application of the FRP plates, and the response of the bridge to test vehicle loadings was recorded. Results of the field tests are reported, and the effects of the FRP plates on the bridge response are identified. The repaired bridge structure exhibited a decrease in steel reinforcing bar stresses and vertical midspan deflections. These decreases ranged from 4 to 12% for various static and dynamic loading cases.

Development of truck weight regulations using bridge reliability model

Abstract: Historically, truck regulations have maintained controls on axle and gross weights with legal load formulas based on limiting allowable stresses in certain types of bridges. These stress limitations do not usually lead to consistent or defensible safety levels and also ignore the cost impact of the weight regulation on the national bridge network. This paper illustrates how new truck weight regulations can be developed to provide acceptable safety levels. Target safety levels are derived from existing AASHTO bridge evaluation and rating procedures applied to structures showing adequate performance levels. Reliability indices are used to relate the statistics of bridge load effects, based on either existing or proposed truck weight regulations, to the dynamic behavior and resistance variables of existing bridges. The sensitivity of the results to various assumptions and errors in the database is also analyzed. An accompanying paper reviews the consequences of adapting such a formula on the safety of existing bridges. The deterministic analysis as well as a reliability assessment are performed in the accompanying paper to review the consequences of adapting such regulations on the U.S. bridge network using the National Bridge Inventory files.

Vehicular Overloads: Load Model, Bridge Safety, and Permit Checking

Abstract: All states in the United States issue special permits for nondivisible and/or divisible truck overloads exceeding the weight limit of the highway jurisdiction. This causes stress levels higher than those induced by normal truck traffic. The rationality of such overstress levels has not been documented. This paper addresses several aspects of this issue. It presents (1) a method to develop live load models including overload trucks; (2) associated reliability models for assessing structural safety of highway bridges; and (3) proposed permit-load factors for overload checking in the load and resistance factor format. It shows that the proposed overload checking procedure leads to relatively uniform reliability of bridge structures. A sensitivity analysis is also presented here to assure that possible variations of the input data used to prescribe the proposed load factors will not adversely affect bridge safety. The proposed procedure is intended to be used by engineers responsible for checking overload permits. It may be included in evaluation specifications for highway bridges.

Survey and evaluation of damaged concrete bridges

Abstract: It is well known that the U.S. bridge inventory stands in need of repair. For a rational allocation of U.S. investment resources to bridge maintenance, life cycle cost and probabilistic methods must be used. This requires a quantitative estimate of the remaining strength over the intended lifespan for a given bridge. Although nondestructive evaluation methods are becoming established for bridge inspection purposes, specific recommendations for the application of these methods for individual bridges do not exist. This study focuses on reported damage and damage modeling for concrete bridges, with particular attention to Colorado bridges. A survey on degradation mechanisms is briefly presented. Bridge damage is reviewed for a variety of concrete bridges based on information in the literature and from field studies performed by the Colorado Department of Transportation. A catalog of damages and examples that illustrate the variety and severity of damage in these bridges are presented. For the bridges conside...

Dynamic Response of Elevated High-Speed Railway

Abstract: The study of the dynamic response of the elevated railway for the high-speed train in the Taiwan area at the preliminary design stage is presented. Two types of the elevated reinforced concrete railway, they being the simple-span and the three-equal-span box girders supported on piers; three types of the high-speed train, namely, the French T.G.V., the German I.C.E., and the Japanese S.K.S.; and the maximum operation speed 350 km/h are under investigation. The general dynamic stiffness matrix of a damped Timoshenko beam is employed for the structural analysis. The influence line of any dynamic response (also called the dynamic influence line) of the elevated railway subjected to the high-speed train, considered as a series of the moving loads, is calculated by the mode-superposition method. A preliminary design of the section of the railway is proposed for this study.

Effect of Bearing Pads on Precast Prestressed Concrete Bridges

Abstract: Precast AASHTO concrete bridge I-beams are often supported at the ends by elastomeric bearing pads. The bearing pad-bridge beam interface defines support boundary conditions that may affect the performance of the bridge. In this study, finite-element modeling was used to validate AASHTO bearing stiffness specifications. Stiffness characteristics of the Florida DOT bearing pads were theoretically determined under varying elastomer shear modulus values. Finite-element models of AASHTO Types III and V beams were subjected to simulated static truckloads. Vertical and horizontal spring elements simulating new bearing pads were incorporated at the ends of the beam models. A full section of a bridge on U.S. Route 27 was also modeled, and the results were compared with field tests. In general, the restraint effects of the bearing pads are beneficial to the performance of the beams and the bridge. The beneficial effect, however, is small for new bearing pads and more pronounced under a drastic increase in bearing stiffness due to aging and colder temperatures. Such a dramatic increase in bearing stiffness must be justified if the beneficial elements are to be utilized. Current Florida DOT bearing pads are serving the main purpose of their application, which is to provide minimum horizontal restraint force to the beams while allowing horizontal movement.

Viscoelastic Dampers at Expansion Joints for Seismic Protection of Bridges

Abstract: This paper presents results of a study on the use of viscoelastic dampers at expansion joints of highway bridges for preventing superstructure decks from falling off the seats and/or from colliding with each other in the event of a severe earthquake. The Kelvin and Maxwell models, consisting of an elastic spring and a linear viscous damper combined in parallel and in series, respectively, are considered for analysis. A 2-D finite-element analysis using bilinear hysteretic models for bridge substructure joints was performed on example bridges constructed with 1 or 2 expansion joints. It was demonstrated that the damper is effective in suppressing the relative displacements at the expansion joints without introducing a significant increase in ductility demands for the substructures. Results also showed that the spring component of the Kelvin and Maxwell models has little effect on the performance of the damper component. This study clearly indicated that the use of linear viscous dampers offers a practical solution to the seismic problem often arising in bridges with expansion joints.

Dynamic Response and Fatigue of Steel Tied-Arch Bridge

Abstract: The Toutle River Bridge is a steel tied-arch bridge, one that vibrates extensively and has sustained significant fatigue cracking. An experimental study into the cause of this behavior is described. Computer analyses of the bridge behavior are used to estimate the expected response and to establish appropriate locations for instrumentation. The instruments were installed and field tests were performed. Controlled tests were performed with trucks of known axle weight and spacing. Some controlled tests were performed with trucks traveling at known speed and in a specific driving lane with no other traffic on the bridge. Controlled tests were used to calibrate the instrumentation and establish the basic bridge behavior. The results showed that composite action had been lost in the heavily loaded stringers, and little amplification of dynamic response was noted. The measured periods of vibration generally compared well with computer predictions. Uncontrolled truck traffic was then measured for approximately o...

Simplified Model for Computer-Aided Analysis of Integral Bridges

Abstract: This paper presents a computer-aided approach for the design of integral-abutment bridges An analysis procedure and a simplified structure model are proposed for the design of integral-abutment bridges considering their actual behavior and load distribution among their various components A computer program, for the analysis of integral-abutment bridges, has been developed using the proposed analysis procedure and structure model The program is capable of analyzing an integral-abutment bridge for each construction stage and carrying the effects of applied loads on the structure members from a previous construction stage to the next The proposed analysis methods and structure models are compared with the conventional analysis method and structure model currently used by many structural engineers for the design of integral-abutment bridges The benefits of using the proposed analysis method and simplified structure model for the design of integral-abutment bridges are discussed It was concluded that it

Experimental verification of horizontally curved i-girder bridge behavior

Abstract: Past research has been conducted on the behavior of horizontally curved girders by testing scaled models and full-scale laboratory bridges and by analyzing numerical models. Current design specifications are based on this past research; however, little field data of in-service bridges exist to support the findings of the past research on which the current design criteria are based. The purpose of the present study was to gather field response data from three in-service, curved, steel I-girder bridges to determine behavior when subjected to a test truck and normal truck traffic. Transverse bending distribution factors and dynamic load allowance were calculated from the data collected. Numerical grillage models of the three bridges were developed to determine if a simple numerical model will accurately predict actual field measured transverse bending distribution, deflections, and cross-frame and diaphragm shear forces. The present study found that AASHTO specifications are conservative for both dynamic loa...

Comparison of Measured and Computed Stresses in a Steel Curved Girder Bridge

Abstract: Steel curved I-girder bridge systems may be more susceptible to instability during construction than bridges constructed of straight I-girders. The primary goal of this research is to study the behavior of the steel superstructure of a curved steel I-girder bridge system during all phases of construction and to ascertain whether the actual stresses in the bridge are represented well by linear elastic analysis software developed for this project and typical of that used for design. Sixty vibrating wire strain gauges were applied to a two-span, four-girder bridge, and elevation measurements were taken by a surveyor's level. The resulting stresses and deflections were compared to computed results for the full construction sequence of the bridge as well as for live loading from up to nine 50-kip trucks. The analyses correlated well with the field measurements, especially for the primary flexural stresses. Stresses due to lateral bending and restraint of warping induced in the girders and the stresses in the c...

Slippage of neoprene bridge bearings

Abstract: Elastomeric bearing pads are more appropriate than mechanical bearings to support bridge loads for many reasons. Two of the most significant reasons are cost and maintenance. Failure modes for bearing pads include crushing, delamination, and slippage. The most notable of these failure modes is slippage or “walking” out of place. The writers' objective in this paper is to establish a methodology to determine why some neoprene bearings are slipping under bridge girders. The methods used to investigate this phenomenon are discussed. A general survey of state DOTs was performed. The results are presented to show how some states have remedied this problem. During the investigation period, numerous bridge engineers and researchers gave their opinions as to why bearings slip. These theories are presented. This investigation has discovered that bearing slippage occurs on a daily basis.

Long-Term Behavior of Continuous Precast Concrete Girder Bridge Model

Abstract: Continuous concrete box girder bridges composed of precast reinforced and prestressed concrete beams with a U cross section and a cast-in-place top slab are frequently used for medium spans due to their competitiveness. The service behavior of such bridges is very much influenced by their segmental construction, due to time-dependent materials behavior that makes it difficult to accurately predict the stresses, strains, and deflections at long term. A 1:2 scale model of a two-span continuous bridge was tested in order to study its behavior during the construction process and under permanent loads. Time-dependent concrete properties, as well as support reactions, deflections, and strains in concrete and steel, were measured for 500 days. Important time-dependent redistributions of stresses and internal forces throughout the bridge were also measured. The test results were compared with analytical predictions obtained by means of a numerical model developed for the nonlinear and time-dependent analysis of segmentally erected, reinforced and prestressed concrete structures. Generally good agreement was obtained, showing the adequacy of the model to reproduce the structural effects of complex interactive time-dependent phenomena.

Deterioration assessment and rehabilitation design of existing steel bridge

Abstract: Diagnostic truck-load tests and microstructural analysis were applied for structural deterioration assessment of a steel truss bridge and its rehabilitation design. The feasibility and potential benefits of using advanced field experimental techniques within a structural-identification framework have been demonstrated. Experimental information, coupled with visual inspection, engineering experience, and intuition, increased the level of confidence in the results of the condition-assessment process, permitting a more rational and cost-effective rehabilitation design. Based on the condition assessment, a two-step rehabilitation is recommended for the bridge: Restoration for a safe service life of 5–7 years, and a subsequent effort for preservation beyond.

Effect of Changing Truck Weight Regulations on U.S. Bridge Network

Abstract: Historically, truck weight regulations have maintained controls on axle and gross weights with legal load formulas based on limiting allowable stresses in certain types of bridges. These stress limitations do not usually lead to consistent or defensible reliability levels and also ignore the impact of the weight regulation on the existing highway bridge network. This paper is the second part of a two-paper series. The companion paper by the first writer illustrated how new truck weight regulations can be developed to provide an acceptable reliability level. The target reliability level was derived from bridge structures designed to satisfy AASHTO standard design specifications that showed safe and adequate performance levels under current truck loading conditions. In this part of the two-paper series, a deterministric load capacity evaluation as well as a reliability assessment are performed to review the consequences of adapting such regulations on the existing U.S. bridge network. A sensitivity analysis shows how changes in the safety criteria used to develop the truck weight regulations would affect the existing bridge network. Detailed load capacity evaluations and reliability analyses are also performed on a representative sample of bridges to provide specific examples of expected changes in rating and safety levels if the proposed truck weight regulation is to be adopted.

Modeling the Reduction in Load Capacity of Highway Bridges with Age

Abstract: Bridge management systems focus on optimizing the life-cycle cost of preservation and improvement activities in a network of structures. Pontis, the predominant bridge management system currently implemented in the United States, considers load-carrying capacity as static during an incremental benefit-cost approach. Including consideration of load-carrying capacity, deterioration may enhance this model. To accomplish this goal, a method to relate load-carrying capacity to physical bridge deterioration using a combination of regression analysis and Markov chains is presented. The method is applied to historic bridge data from the United States and Hungary.

Behavior of transverse confining systems for steel-free deck slabs

Abstract: Extensive research conducted over the past eight years in Canada has led to a concrete deck slab of girder bridges that can be entirely free of any tensile reinforcement. This slab, known as the steel-free deck slab, derives its strength from its internal arching action, which is harnessed longitudinally by making the slab composite with the girders, and transversely by restraining the relative transverse movement of the top flanges of adjacent girders. Two steel-free deck slabs have already been built, in which the transverse confinement is provided by welding steel straps to the girders. This paper presents test results on two other kinds of transverse confining systems, which are applicable to both steel and concrete girders. It is shown that the steel-free deck slab, in addition to being more durable than slabs with steel reinforcement, can also prove to be more economical.

Effect of Friction on Shear Connection in Composite Bridge Beams

Abstract: In the design of new composite steel and concrete bridge beams, the shear connectors are assumed to transmit all of the longitudinal shear forces at the interface between the concrete slab and the steel beam. However, in practice, the forces on the shear connectors are modified by friction resistances at the interface. The effect of friction on the fatigue endurance of shear connectors is first illustrated through a specially developed finite-element analysis procedure. Then a simple mathematical assessment model is proposed that allows for the beneficial effect of friction on the fatigue endurance of shear connectors in composite steel and concrete bridge beams. This procedure can extend the design life of the shear connectors in existing composite bridge beams, as it can be used to estimate their remaining endurance and their remaining strength and, if necessary, to determine the effect of remedial work on increasing the endurance of the shear connectors.

Bridge Deck Aeroelastic Admittance Revisited

Abstract: The subject matter of the present paper falls within the context of the analysis of the response to wind of long-span bridges. Although the paper stands alone, it should properly be seen as focusin...

Thrust and guide devices for launched bridges

Abstract: Incremental launching is a competitive construction method for medium-span (40–65 m) prestressed concrete bridges. It does not constrain the length and width of the superstructure, and bridges longer than 1 km and wider than 20 m have been successfully launched. This method is hardly constrained by the bridge layout, as varying plan curvatures can be solved by shifting launch supports and varying vertical curvatures by shimming the bottom edges of the superstructure. The launch of a prestressed concrete bridge involves enormous forces and requires the guide and control of big volumes. The devices used for this purpose are described along with their design criteria and optimum fields of utilization, and several suggestions derived from many years of launching practice are given.

Relaxation in High-Strength Bolted Connections Using Galvanized Steel

Abstract: In recent bridge construction in Connecticut involving heavy sections, galvanized coating thicknesses were in excess of 12 mils. The current American Association of State Highway and Transportation Officials specification is based on research conducted with coating thicknesses not greater than 6 mils. Since thicker coatings on steel can reduce shear capacity in high-strength bolted connections, there was concern that the thick galvanized coating could compromise the performance. In this research, tests were conducted to study the relaxation and the shear capacity of high-strength bolted connections with galvanized coating thicknesses up to 20 mils. Both normal and oversize holes were studied. The research shows that the shear capacity of connections with thick galvanized coatings is reduced due to the loss in the clamping force. The test results have been used to develop design guidelines.

Effect of Construction Loads and Vibrations on New Concrete Bridge Decks

Abstract: This paper presents an investigation on the effects of construction loads and vibrations on typical newly poured reinforced concrete bridge decks. The main objective of this paper was to perform dynamic finite-element analyses of concrete bridge decks under construction loads and vibrations. Two types of bridges were modeled by effectively simulating the loads encountered during construction. Separate analyses were performed to simulate one truck as well as two side-by-side trucks. Additional analyses were performed to simulate the loads during construction and pouring sequence. The loads imposed by concrete mixer trucks, including the full concrete load and the load imposed by a trailer unit carrying the pump, were taken into consideration during the modeling of the bridges. The results obtained consisted of vibration modes and frequencies for each structure in addition to top and bottom stresses in each principal direction. A separate analysis was performed that took into account the sequence of pour to...

Bridge Rating with Consideration for Fatigue Damage from Overloads

Abstract: This paper presents a proposed rating model that incorporates the fatigue damaging effects of overloads. This is achieved by introducing a fatigue index in the rating equation. The index, which app...

Behavior of Curved I-Girder Webs Subjected to Combined Bending and Shear

Abstract: Two previous papers by the writers described the buckling and finite-displacement behavior of curved I-girder web panels subjected to pure bending, presented a theoretically pure analytical model, and presented equations that describe the reduction in strength due to curvature This paper describes the buckling and finite-displacement behavior of curved web panels under combined bending and shear Unlike straight girder web panels, the addition of shear in curved panels is shown to increase the transverse “bulging” displacement of the web prior to buckling The accompanying decrease in moment carrying capacity is analyzed in a manner similar to that used for the combined bending and shear nominal strength interaction for straight girder design Preliminary recommendations are made toward forming design criteria for curved webs

Evaluation of strength formulations for horizontally curved flexural members

Abstract: In 1992, the U.S. Federal Highway Administration, along with 13 states, began a project to study the behavior of horizontally curved steel bridges. The project is referred to as the Curved Steel Bridge Research Project and involves studying the behavior of curved members through theoretical, analytical, and experimental research. In this paper, detailed finite-element models representing a curved 3-girder test frame, are used to evaluate the effects of curvature on the bending strength of curved I-girders, linear-elastic static, and buckling. Combined material and geometric nonlinear analysis are conducted using models that represent the test frame and component test specimens that will be inserted into it. Results are compared to various predictor equations developed from analytical work by the authors and to work by other researchers. It is demonstrated that the predictor equations of the authors are accurate in representing the behavior of the system. Limitations and needed improvements are described as well.

Overload and Ultimate Load Behavior of Posttensioned Slab Bridge

Abstract: A three-tenths-scale model of a posttensioned slab bridge is constructed in a laboratory and tested in the overload and ultimate load ranges Dimensions of the slab are 169 × 533 × 023 m In addition to uniformly distributed longitudinal posttensioning, a band of tendons is located in a narrow region directly above the supporting columns A large array of strain gauges, linear variable differential transformers, and load cells serve to gather data for each loading configuration Results are presented for a series of overloads that are placed on the slab in a variety of critical locations Ultimate positive bending moment is imposed on one span and an ultimate negative moment is applied to the slab in the region of the supporting columns Complementary nonlinear finite-element analyses of the overload and ultimate loadings are also presented Results show that the posttensioned slab bridge supported a load-causing moment equivalent to 48 times the factored moment of an AASHTO MS 18 loading

Improving Standard Bridges with Attention to Cast-in-Place Substructure

Abstract: This paper focuses on improving the efficiency and aesthetics of standard highway bridges with attention to cast-in-place substructure design. Standard short- and moderate-span bridges are predominantly functional and nondescript. Recent developments in superstructure design have led to improved efficiency of construction and material use, with a resulting slender, attractive appearance. However, typical substructure design remains a component where creativity can be played out for considerable improvements to the overall bridge appearance. A review of current cast-in-place substructure systems is presented including individual, wall, hammerhead, and multicolumn bents. Design recommendations for improving overall bridge appearance and the efficiency of the substructure are presented. Attention is given to integrating aesthetics with material and construction efficiency and with economic considerations. Recommendations include addressing structural expression, visibility through the bridge, shaping of the ...