Journal of Bridge Engineering

In this paper, current and proposed methodologies for assessing the robustness and redundancy of bridge structures are discussed and an overview of the latest advances in research is presented along with proposals for implementation in bridge design and evaluation codes. In particular, European and North American standard codes and guidelines, including the Eurocode, AASHTO, and the Canadian Standards Association (CSA), as well as some national European codes and related guidelines, are compared. Research projects, including those of the European Cooperation of Science and Technology (COST) and the U.S. National Cooperative Highway Research Program (NCHRP), represent key links between research activities and the need for implementing concepts of structural redundancy and robustness into the codes. The activities undertaken by COST and NCHRP constitute attempts to synthetize, simplify, and implement theoretical concepts that have been discussed over many years. This review shows that the U.S. approach concentrates on developing tools and criteria for the numerical evaluation of the capability of a bridge structure to continue to carry load after the failure of a member, whereas the European approach, still in a more theoretical phase, lacks specific guidelines for bridges and uses available building recommendations. This paper also presents a critical assessment of the state of the art and argues that criteria implementable in future generations of bridge codes should be based on quantifiable measures of risk, which is a subject still in its very early stages of development.

The ability to accurately predict traffic loading is essential for cost-effective bridge maintenance and repair programs. The traffic load model currently used in the United States for the design of long-span bridges was developed over three decades ago. In the meantime, vehicle characteristics and traffic patterns have changed. The Eurocode for traffic loading is more recent, but was calibrated only for bridges up to 200 m long. In this work, weigh-in-motion traffic records from 11 different sites across Alabama are used to establish congested traffic loading. Traffic microsimulation is used to generate congestion based on real traffic data. Influence lines for two typical long-span bridges, one cable-stayed bridge, and one suspension bridge are determined using finite-element models. These are used in the microsimulation model to estimate the bridge-load effects caused by congested traffic. These results are extrapolated to find the characteristic lifetime maximum values that are used to evaluate the Eurocode load model to assess its suitability for long-span bridges. In a similar way, the current American load model for long-span bridges, commonly known as the ASCE model, is evaluated to see if it accurately reflects the congested traffic loading that is currently found on American highways. Recent research has suggested the use of the AASHTO HL-93 load model to estimate the effects of traffic loading on long-span bridges, and this model is also evaluated in this work.

In order to coordinate the design of RC structures all over Europe, the new Eurocode has been implemented in all member states. In Germany, for example, the unified codes with their national annexes are exclusively valid as of July 1, 2012. Some changes to Eurocode No. 2 have large effects on the design of new buildings and the safety verification of old ones. This applies particularly for the shear design of RC members without stirrups. The structural safety of RC bridge decks, which are mostly built without shear reinforcement, under static and cyclic loads, is the focus of this paper. The shear capacity of the new design regulations will be compared with test results to demonstrate the inaccuracy of the Eurocode No. 2 approach and the need for further investigations. Then results of a big test series with slabs under wheel loads, straight and haunched beams under monotonic loading, and RC beams under cyclic loads will be discussed. The main purpose of the presented research is to improve the shear design of RC bridge deck slabs, which are mostly built without stirrups. This paper does not deal with prestressed slabs, and further standards and design approaches are not part of the paper.

Prestressing strands are commonly used in pretensioned prestressed concrete bridge construction. Transfer length is an important parameter for structural design. This paper presents a comparative study on strand transfer length provisions from Eurocode 2 and North American practice, and identifies similarities and differences between both models. A database of measured transfer lengths according to several authors has been compiled and compared with predictions according to code provisions. The intervals of predictions are smaller than those corresponding to the experimental results, and they are smaller when code provisions are more simplified. The interval from Eurocode 2 is greater than that from American Concrete Institute (ACI) code 318, which, in turn, is greater than the interval from AASHTO. The number of underestimated cases is lower for Eurocode 2 because of the higher predicted values, but situations in which a short transfer length is unfavorable are neglected by all models because they are not good predictions of shorter measured transfer lengths. When a transfer length estimation criterion is based on an allowable free end slip, more cases are excluded from the ACI code 318 provisions.

The effects of time-dependent phenomena on concrete prestressed girder bridges are investigated. The study concerns the case of bridges built directly in their final configuration and that of bridges built by a sequence of stages in which geometry, restraints, and loads vary until the final configuration is achieved. An analytical approach based on the principles of aging linear viscoelasticity and the age-adjusted effective modulus method is followed. The paper has two aims: the first is to provide an efficient and simplified tool for the evaluation of the structural response in the early stages of design; the second is to compare the results of the analyses on actual cases of bridges when different shrinkage and creep prediction models are used. Numerical applications show that the influence of stress redistribution on bending moments, attributable to the change in static schemes, is reduced by the load-balancing effect of the dead load and prestressing and that bending moment diagrams for bridges in service do not change significantly with the different prediction models. Opposite results are found on deflections, which differ significantly in reference to the model used. Differences between predictions models are underlined, suggesting that designers should manage them with prudence to predict the long-term performance of concrete. Finally, Eurocode predictions always underestimate final deflections and stress redistribution because of the static scheme variation.

Bolted joints are broadly used for the connections of structural elements in steel bridges. Rules for design of bolted connections are currently under discussion in Europe for improving Part 1-8 of Eurocode 3, which deals with the sizing and structural design of joints. In this work, a wide comparison is made between the Eurocode and the codes of Italy, the United States, Canada, Australia, and Japan. General descriptions of the design criteria for typical connections in bridges related to materials, geometrical limitations, slip, shear, and bearing resistance are presented. An illustrative example to compare the various code provisions is given to quantitatively show their performance for a practical case.

In many design codes for roadway bridges, fatigue design involves passing a load model over an influence line for a critical location on the bridge and then determining the resulting nominal stress range. For fatigue design in the finite-life domain, this stress range is then multiplied by a damage equivalence factor to account for differences in the fatigue damage because of the load model and the expected real traffic. In general, the effects of simultaneous vehicle crossings are not considered in the calibration of the damage equivalence factor. In this paper, fatigue design procedures in the U.S., Canadian, European, and Swiss codes applicable for the design of steel or aluminum roadway bridges are first reviewed. A simulation-based study, conducted to investigate the effects of simultaneous vehicle crossings on the damage equivalence factors for North American roadway bridges, is then presented. Based on the results of this study, recommendations are made for amplifying the North American damage equivalence factors in cases where the effects of simultaneous vehicle crossings are expected to be significant.

This paper presents a study on the effectiveness of various casting techniques used to control slab tensile stresses during the construction of continuous steel-concrete composite bridge decks. The main features describing the early age behavior of concrete are outlined and available modeling options are considered. A practical procedure is presented for the evaluation of the slab stress state taking place at various construction stages based on the modular ratio approach suggested by Eurocode 4. The method, which can be implemented using commercial software, requires the evaluation of suitable modular ratios and creep coefficients to estimate the time-dependent effects of the concrete slab weight and the thermal, endogenous, and drying shrinkage components. This simplified procedure is then applied to a case study to evaluate, for various casting sequences, the contributions of the concrete weight and shrinkage components to the slab early cracking. Finally, the efficiency of various casting techniques in controlling the slab tensile stresses is presented and discussed.

The paper presents a methodology for evaluating the actual response modification factors ($q$ or $R$) of bridges and applies it to seven concrete bridges typical of the stock found in southern Europe. The usual procedure for analytically estimating the $q$-factor is through pushover curves derived for the bridge in (at least) its longitudinal and transverse directions. The shape of such curves depends on the seismic energy dissipation mechanism of the bridge; hence, bridges are assigned to two categories, those with inelastically responding piers and those whose deck is supported through bearings on strong, elastically responding piers. For bridges with yielding piers, the final value of the $q$-factor is found as the product of the overstrength-dependent component ($q_s$) and the ductility-dependent component ($q_{\mu }$), both estimated from the pertinent pushover curve. For bridges with bearings and nonyielding piers of the wall type, an equivalent $q$-factor is proposed, based on spectral accelerations at failure and at design level. In this paper, pushover curves are also derived for an arbitrary angle of incidence of seismic action using a procedure recently developed by the authors, to investigate the influence of the shape of the pushover curve on the estimation of $q$-factors. It is found that in all cases the available force reduction factors are higher than those used for design to either Eurocode 8 or AASHTO specifications.

Although many models of pedestrian dynamic loading have been proposed, possible bidirectional interactions between the walker and the excited structure are generally ignored, particularly for vertical vibrations. This shortcoming has arisen from scarcity of data on gait-adaptation strategies used in the presence of structural motion and, as a consequence, the absence of a credible fundamental pedestrian model capable of capturing the underlying relations between the two dynamic systems. To address this inadequacy of current approaches, a biomechanically inspired inverted-pendulum pedestrian model has been applied to the human-structure interaction problem. The behavior of the model is studied when subjected to vertical motion of the supporting structure, in particular, in relation to potential self-excited forces that can be generated. A mechanism has been identified by which the timing of pedestrian footsteps can be altered subtly, giving a net damping effect on the structure, without necessarily involving full synchronization. It has been found that depending on the ratio between the bridge vibration frequency and pedestrian pacing frequency, walkers can effectively act as positive or negative dampers to the structural motion, but it is expected that for a group of pedestrians with distributed parameters, their action is, on average, to add damping and mass.

A new prestressed concrete-steel composite (PCSC) girder system is developed to provide a viable alternative for steel and prestressed concrete I-girders in bridges. The PCSC girder is composed of a lightweight W-shaped steel section with shear studs on its top and bottom flanges to achieve composite action with the pretensioned concrete bottom flange and the cast-in-place concrete deck. The PCSC girder is lightweight, economical, durable, and easy to fabricate. The proposed fabrication procedure is similar to those of prestressed concrete girders and does not need specialized equipment, materials, and forms. A service design procedure is proposed using the age-adjusted elasticity modulus method to evaluate the time-dependent stresses and strains in the PCSC girder caused by creep and shrinkage effects of concrete and relaxation of strands. The strength design method is proposed for the design of PCSC girders at prestress release. A design procedure is proposed to assist engineers to accomplish economic design and production of PCSC girders, and design examples are presented to illustrate the design procedure.

In this paper, finite-element analysis (FEA) of the prestressed concrete-steel composite (PCSC) girder is performed to investigate strain and stress distributions in the girder sections and determine the influence of stud distribution on stresses in the concrete bottom flange. Approaches of FEA are discussed for the material and element models of steel, concrete, and strands, and element models of the bond between the concrete and strand and the shear studs, loading and boundary conditions, and convergence issues. A PCSC girder specimen is fabricated and instrumented in the structural laboratory to validate the proposed fabrication and design procedures. FEA and service design using the age-adjusted elasticity modulus method (AEMM) are both validated using the strain profiles at different sections and values of concrete surface strains and camber/deflection. Test results indicate that the cracking moment, ultimate moment, and ultimate shear of the PCSC girder can be well predicted using the AEMM and the AASHTO LRFD bridge design specifications.

Concrete box-girder bridges are one of the most commonly used bridge types owing to ease of maintenance and economy in construction. However, current design codes do not realistically consider the effects of haunches and web restraints on the strength enhancement of the top deck slab in the box-girder bridges. The purpose of this paper is to propose a rational model that can consider the effects of arching and haunches on the prediction of ultimate load capacity of deck slabs in concrete box-girder bridges under vertical loading. A rational frame model for the transverse direction of box girders that considers the effects of rigid joints and nonprismatic elements was formulated in this study. The flexural strength enhancement of the upper slab from the restraints of webs and arching action also was derived. To verify the model, a comprehensive experimental program was set up, and several full-scale concrete box girders were tested. The displacements and strains of steel bars and concrete were measured for test box girders during the vertical loading process. It was found that the cracking patterns and failure modes of box girders vary greatly depending on the existence and size of haunches. The present test results also indicate that an increase in haunch size increases the ultimate load capacity of the deck slab of a box girder greatly. The load capacity of a haunched deck slab is more than two times greater than that of the deck slab without haunches depending on the size of haunches. This effect is taken into account in this analysis through the consideration of arching action for the upper slab with various haunch sizes. Comparison of the proposed theory with test data shows good agreement on the ultimate loads and displacements. This study indicates that the theory without considering arching action greatly underestimates the ultimate load capacities of deck slabs in box-girder bridges. Therefore, the effects of haunches and arching action must be considered realistically for more advanced and economical design of box-girder bridges.

In this paper, the importance of displacement history and its effects on performance limit states, the relationship between strain and displacement, and the spread of plasticity in RC structures is explored. An experimental study is underway to assess the performance of 30 circular, well-confined, bridge columns with varying lateral displacement history, transverse reinforcement detailing, axial load, aspect ratio, and longitudinal steel content. Eight of these columns, with similar geometry and detailing, were subjected to various unidirectional displacement histories including standardized laboratory reversed cyclic loading and re-creations of the displacement responses obtained from a nonlinear time-history analysis of multiple earthquakes with distinct characteristics. Longitudinal reinforcing bars were instrumented to obtain strain hysteresis, vertical strain profiles, cross section curvatures, curvature distributions, and fixed-end rotations attributable to strain penetration. Results have shown that the limit state of reinforcement bar buckling was influenced by load history, but the relationship between strain and displacement along the envelope curve was not. The main impact of load history on bar buckling is its influence on accumulated strains within the longitudinal reinforcement and transverse steel.

Because concrete-filled steel-tube (CFST) arch bridges usually reside in areas with strong temperature effects, which influence concrete creep behavior considerably, this paper aims to present an approach to evaluate the temperature influence on the creep behavior of CFST arch bridges, which has not been reported until now. Based on the microprestress-solidification theory for concrete creep, the age-adjusted effective modulus method and a proposed temperature estimation method were integrated into a finite-element model to calculate the creep behavior of a half-through long-span CFST arch bridge. The comparison between the predictions and measurements validated the efficiency of this approach and also demonstrated the necessity of considering temperature history in the creep-effect analysis of CFST arch bridges. The numerical method proposed in this paper provides a practical way to evaluate the temperature-involved creep effects on CFST structures.