Bridge Structures

This paper presents a simplified view of cable vibrations in cable-stayed bridges. The different types of cable vibrations are described with an emphasis on parametric excitation which is more of a structural than aerodynamic problem. Countermeasures to eliminate or reduce cable vibrations are discussed with examples of cable vibrations.

In recent years several theories have been put forward in relation to lateral forces imparted on bridges from crowd movements. It is now widely accepted that the interaction between the crowd and the structure, particularly when the crowd pacing frequency is close to the lateral natural frequency, is the major factor determining the lateral response. However, very little work has been done with individual pedestrians in order to determine the relationship between the lateral force induced by a single pedestrian and the structural response. Equally, most literature concerning lateral forces induced from walking is based on results from fixed force plate tests and hence no assessment of any interaction is made. This paper examines the lateral response of a GRP (glass-fibre reinforced polymer) cable-stayed footbridge to individual pedestrian crossings at a range of pacing frequencies. Two lateral load modelling approaches are considered. The first approach, derived from back analysis of the measured bridge response, was found to be ineffective in predicting the measured response accurately. A second modelling approach, incorporating a spring-damper to represent a moving pedestrian, which thereby accounts for the interaction between the structural response and the mass, stiffness and damping characteristics of the body of the traversing pedestrian, is demonstrated to be more effective in simulating the bridge response.

Several tied arch and truss bridges have experienced horizontal floorbeam cracking in the region of the upper web-to-flange weld or in the web gap between the top flange and the connection angles where simple web shear connections have been used. A recent study on a 189 m (620 ft) span tied arch bridge located in Pittsburgh, Pennsylvania over the Monongahela River has evaluated the cause of this type of cracking and resulted in an effective retrofit strategy. Remote monitoring of two retrofitted floorbeam connections was conducted for a period of almost 40 days as random vehicles crossed the bridge. The measurements have confirmed that the primary cause of the observed cracking is the relative longitudinal displacements between the top flange of the floorbeam and the face of the tie girder. This relative movement was focused within the horizontal web gap between the connection angles and the bottom of the top flange. This resulted in the development of horizontal fatigue cracks along the web-to-flange weld in the floorbeam. Several cracks were observed to branch and begin to turn downward into the web. In order to alleviate this problem, a portion of the floorbeam and connection angles were removed at the top of the connection in order to provide sufficient flexibility to allow the required movement and reduce stresses. Instrumentation consisting of strain gages and LVDTs (displacement sensors) was installed at the upstream and downstream side of an interior floorbeam and used to verify the performance and effectiveness of the retrofit. The exact geometry of the retrofit was determined through a parametric finite element study, which was subsequently confirmed with field-measured data. The retrofit provides sufficient flexibility at the connection without producing high stress ranges at critical locations.

This paper succinctly outlines the evolution of curved girder design specifications from the 1980 AASHTO Guide Specifications for Horizontally Curved Girders through to the LRFD provisions for curved girder design accepted for inclusion in the AASHTO LRFD Bridge Design Specifications at the June 2004 meeting of the Subcommittee on Bridges and Structures. The multi-path approach to develop the background for these specifications is described, and some of the more significant changes to the design process are outlined.

Closed stiffener orthotropic decks can be advantageous for ballasted or non-ballasted railway bridges. This has been achieved successfully on a number of tied arch railway bridges for the high-speed railway network in Belgium. In these bridges, the orthotropic plate contributes to both the deck plate action and the main load carrying system as the horizontal lower chord or arch tie. Although the basic structural action is comparable to that of road bridges, specific differences exist. These differences, which originate from a substantial discrepancy of the load transfer, are addressed in this paper. Railway loads considerably exceed applicable loads for road bridges, which result in significantly larger rib and floor beam dimensions. This has a significant impact on the transverse load spreading in the plate and on the moment redistribution due to floor beam flexibility. Load spreading describes the phenomenon in which a concentrated load is spread throughout a continuous medium. For typical railway bridge deck dimensions, the load spreading is nearly negligible. This mechanism is demonstrated in the paper. On the other hand, the axle loads are spread over a large area, reducing the contact pressure at the deck plate. Consequently, the potential for fatigue at the stiffener to deck plate joint is also reduced. In addition, the ballast layer bridges the plate areas between stiffener webs reducing deck plate stresses even further. This has serious consequences on acceptable deck plate dimensions, which differ substantially from the recommended values for road bridges. Therefore a different set of basic input data is considered for orthotropic decks of railway bridges, leading to a more efficient use of steel. All conclusions and recommendations presented in this paper are based on numerical Finite Element simulations and strain gauge measurements on a number of bridges.

The proposed Blennerhassett Island Bridge crosses the Ohio River and historical Blennerhassett Island connecting Wood County, West Virginia to Washington County, Ohio. The hybrid steel girders for the Ohio approach spans have span length of 52.1 m, 54.7 m and 42.6 m. One of the limitations presented in the AASHTO (1998) LRFD Bridge Design Specifications, with 2003 interims, is that when considering shear design for hybrid sections, there is no allowance for tension field action. The shear design of girders using these specifications required in excess of 120 transverse stiffeners per girder. The use of the tension field action for designing hybrid girders, which is allowed in the AASHTO (2004) LRFD Bridge Design Specifications, reduced the number of transverse stiffeners required to 36 per girder. This reduction in stiffeners reduced the cost by approximately 10% for each girder. The stresses in the web and the transverse stiffeners for the two designs are presented. Because there is a significant difference in the transverse stiffener requirements under each design, we were prompted to take a closer look at the tension field action anticipated. The tension field action involves the effects of out-of-plane forces on the transverse stiffeners due to the shear post-buckling response of the web panels. Therefore, a three-dimensional finite element model was used considering material and geometric non-linearity. The analysis demonstrated that the anticipated tension field did form as assumed in the design.

Modern bridge structures which are built by assembling slender arches, three-dimensional thin straight or curved members, and cable structures are often too deformable to be treated with the first-order elastic theory. The analysis of such structures requires the employment of large displacement theory. Additionally, a post-elastic nonlinear analysis is required to control and evaluate damage without collapse in the seismic design philosophies for bridge structures. Hence, it is essential to develop an effective procedure that considers both geometric and material nonlinearities for the three-dimensional analysis of bridges structural members. A general procedure for the analysis of linear and nonlinear behaviours of curved structures using the transfer matrix method (TMM) is presented. The procedure is a TMM extension to geometric and material nonlinear formulations, with large displacements, small deformations and localized plastic hinges. Some applications are presented for the nonlinear analysis of slender arch-bridge structures.

The objective of this paper is to present results of a study employed to identify the probable causes and location of cracking in bridge approach and transition slabs and the factors influencing the crack development. A finite element (FE) model has been developed to study the cracking phenomenon in bridge approach and transition slabs under vehicular live load and soil settlement. A field survey was conducted to determine the extent and probable causes of crack development in these slabs at various bridge sites in the State of New Jersey, USA. The data collected from field observations were compared with those predicted by the FE model to determine its reliability and consistency. The FE model was employed in conducting a parametric study to evaluate the effect of various designs as well as soil parameters on the cracking behavior of the slabs. The results from the parametric study showed that increasing the slab thickness would significantly increase the cracking load carrying capacity of the approach slab. However, an increase in soil settlement has an adverse effect on its cracking load carrying capacity. Based on the results from this study, two design alternatives, which exhibited better cracking resistance than the existing design detail, are also presented.

The application of externally post-tensioned segmental members is a very attractive solution compared with classical construction methods. The use of this solution results in smaller precast elements tied together by post-tensioned tendons, upbringing such advantages as fast and versatile construction, high quality control and lower overall cost. In this paper, a formulation based on the Finite Element Method is discussed. This formulation is used to numerically simulate the structural behavior of members composed of externally post-tensioned segments. These analyses not only allow serviceability limit states verifications when sections are fully compressed, but also allow verifications of ultimate limit states when joint openings and load transfers at the joints are considered. Additionally, to evaluate the accuracy of the numerical results of the computational model a comparison to experimental data from the literature is performed.

This paper describes a study evaluating increase in design strength, performance and capacity when including bridge parapets in a bridge analytical model. Traditionally, concrete parapets are treated solely as superimposed dead load in an overall bridge structural design. Parapets are detailed from a safety standpoint to resist impact forces from vehicle loading. However, in the global design models, the inherent strength of the parapets is not considered as a contributing factor to the overall strength of the bridge structure as a system. As part of an evaluation of global performance of bridge structures, the authors have prepared a set of three-dimensional finite element models of a simple span, composite concrete – steel stringer bridge. These models evaluate the overall strength and deflections under standard traffic loadings, with and without the stiffness contribution of the concrete parapets. These models predict a substantial increase in strength when including the parapets as a structural element rather than merely a superimposed dead load. Furthermore, standard details and construction procedures commonly in use to provide adequate strength for current code requirements seem to also provide sufficient strength to enable composite action between parapets and the bridge deck. The preliminary results from this analytical study indicate that current design practices are perhaps overly conservative in that they do not account for the system strength of the parapet. In some cases, ignoring the strength and stiffness contribution of parapets may be under-conservative, for example in pre-stressed concrete butted box beam bridges, longitudinal shear key joints may be adversely impacted by difference in stiffness in the adjacent box sections. This paper will investigate the advantages of including the evaluation of parapet strength, initial load rating and analysis during the life history of the bridge structure.

Multihazard consideration for infrastructure applications is gaining popularity due to the anticipated overall cost reduction it offers while maintaining the needed safety levels. This approach considers increasing the complexity of the structural systems to meet the demands of the current environment and takes advantage of the recent developments and innovations in computing, analytical, and sensing technologies. However, at present, no quantitative methods have been developed to fulfill such a promise. This paper discusses the need for multihazards considerations in infrastructure applications and provides a quantitative approach to multihazards considerations. A general theory of multihazards in infrastructure is introduced and applied to structural analysis, design, life cycle costing, risk assessment, and structural health monitoring. In all, groundwork is laid for quantification of multihazards considerations. Utilization/furthering of such quantitative treatment of multihazards theory in these fields will assist in reducing overall costs while maintaining or increasing safety levels.

Public safety requires that bridges be designed for the entire spectrum of loading conditions that could result in failures. During the past decade, seismic bridge design has become oriented towards specific performance objectives by considering possible rates of earthquake occurrence over the design life of the structure. A geographic information system (GIS) methodology for establishing potential long-term seismic risk for bridges is described in this paper. The GIS graphical and computational capabilities to analyze spatially distributed problems are shown to be an ideal, powerful tool for establishing ground motions within the performance-based criteria of modern seismic bridge design. Sample outputs for the Eastern United States (EUS) are presented as an application. Seismic events in this area are infrequent; however, they can pose a serious threat, especially considering that many of the bridges in EUS were built before seismic design was required. There are many problems associated in determining ground motions in the EUS, including a scarcity of recorded data, incomplete knowledge of seismic wave propagation characteristics, and sometimes insufficient knowledge of geologic conditions. In spite of these difficulties, it is possible to arrive at a rational estimate of the seismic risk potential in a probabilistic manner, combining available information, assumptions, and uncertainties using a GIS environment.

Near-field ground motions cause significant damage to highway bridges because of high peak ground accelerations and high peak ground velocities of long period pulses. To quantitatively assess the influence of near-field ground motions on seismically excited highway bridges, a ground motion model consisting of both pulse-type low frequency (near-field) and broadband frequency (far-field) components is proposed. In this model, the effects of the local site condition and the seismic source are taken into consideration by varying relative contributions of near-field pulse-type and far-field broadband random ground motion components in the synthetic ground motion. Extensive numerical simulations are carried out to quantify effects of pulse-type component and soil-structure interaction through a parametric study. Simulation results demonstrate that pulse-type components in ground motions amplify the response quantities of the highway bridge significantly over those by the broadband component. Nonlinear viscous dampers are effective in reducing all response quantities of the bridge to prevent serious damage to bridge bents and bearings.

The objective of this paper is to present the impact of bridge retrofitting in the San Francisco Bay Area on direct loss reduction. For this purpose, the losses from the characteristic earthquakes on the San Andreas and Hayward faults due to ground shaking, landslide or liquefaction are estimated for pre- and post-retrofitted bridges. Bridge losses are computed using the fragility functions from HAZUS [Earthquake Loss Estimation, Technical Manual, 1999] and the Boore et al. [Seismic Research Letters, 1997, 687(1), 128–154] attenuation relationship for ground motions at bridge sites. Loss from liquefaction and landslides is also estimated using HAZUS. The replacement cost for each bridge is calculated using values obtained from the California Department of Transportation (Caltrans). The loss estimates show that the annual risk reduction due to retrofitting is about 15% for the San Andreas fault and 10% for the Hayward fault events. If all possible events are taken into consideration, these values are expected to further increase, demonstrating the benefit of retrofitting.

This paper discusses the management, maintenance and repair of Croatian bridges. Unfortunately, activities related to bridge maintenance are not carried out regularly, but on an ‘as-needed’ basis with inspection and testing undertaken only when structural damage becomes self-evident. Poor in-service performance and condition of existing bridges in Croatia led to the conclusion that the currently employed approach to solving bridge maintenance issues is not satisfactory, and should be improved. The framework for more efficient bridge management in terms of the bridge management system named HRMOS exists. Unfortunately, the introduction of HRMOS did not ensure that the activities related to bridge maintenance are carried out regularly nor according to the prescribed procedures. To illustrate the grave consequences of neglecting bridge maintenance activities, several case studies are discussed, including world-renowned structures such as the Krk bridges. These clearly show that if we choose to act only when damage of a bridge structure becomes self-evident, the repairs are not only expensive but very difficult to perform.

The transportation infrastructure plays a major role in the economy of the USA. Providing a reliable mobility and serviceability of this infrastructure is extremely important. Bridges are an integral part of this infrastructure and periodic inspection and maintenance plays a major role in keeping these operational. Appropriate maintenance avoids major bridge rehabilitation and replacement, and extends the structural life cost-effectively. Effective maintenance programs involve several aspects such as scheduling a maintenance activity, the activity itself, and the scope of such an activity. The main challenge faced by a bridge manager is to institute maintenance activities with minimum expenditure. The focus of this paper is to illustrate the possible role of Structural Health Monitoring (SHM) in bridge maintenance and management aspects. This paper illustrates a cost – benefit (value) approach for use of SHM in maintenance applications to quantify the decision-making process involved with maintenance activities. Several examples are presented to show the effectiveness of using SHM in maintenance activities, with a detailed illustration.

The use of distributed optical fibre sensors for strain measurements in beams, by means of Brillouin scattering effect, has been proposed recently. Several researchers have stressed the theoretical and practical difficulties related to this kind of measurement. These include the mechanical characterization of optical fibres, the decay of strains in the protective coatings, the spatial resolution of the Brillouin scattering, the brittleness of the glass core, the elastic – plastic response of the coatings, the end effects, and the different effects of strain readings in dilatation or in contraction. A solution to each of the above-cited problems would entail further research effort. However, all the works pointed out that the qualitative strain response of bending beams is clearly accounted for by distributed optical fibre sensors. In spite of the above-mentioned uncertainties, the distributed nature of the sensor makes it very attractive when safety assessment of large structures, such as bridges, tunnels, dams or pipes, is involved. In the present paper, the detection of defects or damage in bending beams by means of distributed optic-fibre sensors is proposed. In particular, the fibre-optics distributed sensor has been used for measurement of the deformation of a steel beam in experimental laboratory tests. Comparison of the experimentally measured strains, carried out on both damaged and undamaged beams, revealed the presence and position of defects in the beam. Quality and accuracy of the measurements carried out with distributed optical-fibre sensors are discussed, focusing on the applicability of the identification method.