Bridges face increasing demands due to higher traffic loads and more load cycles, which might require strengthening actions. In the Nordic countries, many existing steel girder bridges have non- composite concrete decks. A rational strengthening approach is to install shear connectors, enabling composite action between the deck and the girders for a more efficient use of the structural components. Installing shear connectors in a new steel structure is inexpensive but installing them in existing bridges can be costly. Instead of placing connectors along the entire bridge, they can be strategically installed where they give the most impact, using only as many as needed to get an adequate composite action. Case studies show that selective placement of shear connectors can significantly improve load-bearing capacity while reducing installation efforts. This paper describes two case studies, one single span bridge and one continuous steel-concrete bridge, where the impact from the position of the shear connectors has been evaluated.

Something as simple as the use of lateral bracing for a steel I-girder bridge should be consistent across the globe. However, an investigation into the bridge standards of different countries in Europe and North America revealed little to no consistency in how and why lateral bracing is specified or detailed. Most design codes recognize that adding lateral bracing—the diagonal bracing between the bottom flanges of a typical I-girder bridge superstructure—changes the load path of an I-girder bridge sufficiently to mimic the increased stiffness and fatigue life performance of a much more expensive steel box beam superstructure. Still, lateral bracing is rarely used or, if used, not accounted for in the structural capacity of the structure. This is often because the maximum benefit is found with superstructures with few deep girders in the cross section, and far less benefit is seen for shallower multigirder cross sections. Additionally, accounting for the structural benefit of the lateral bracing increases the complexity of the bridge analysis model and precludes the use of simplified line-girder methods. For these reasons, the investigation showed that even when lateral bracing is used for reasons such as construction stability, it is rarely accounted for as a primary load carrying member in new structures. Since the inclusion of lateral bracing provides no structural benefit but also adds dead load and structural analysis complexity, most agencies attempt to eliminate lateral bracing from their structures and simply increase the capacity of the I-girders. However, for existing two-girder composite bridges, an approach is presented in which the careful addition of a new or structural consideration of existing bottom lateral bracing on an existing two I-girder superstructure could improve the live load distribution and reduce fatigue live load stress ranges sufficiently enough for the structure to remain in service without further structural strengthening.

Today, steel girder bridges with an overlaying concrete deck are typically designed and constructed as composite structures. This approach optimizes the utilization of materials and structural components. However, many existing steel–concrete bridges were originally designed without shear connectors at the steel–concrete interface. With the increasing demands of heavier traffic loads and a greater number of heavy load cycles, these bridges may sometimes require strengthening. One effective method to enhance their bending moment capacity is to develop composite action by installing shear connectors. One type of connector that is well-suited for postinstallation is the coiled spring pin. These coiled spring pins are inserted from beneath the bridge, passing through holes drilled in both the top flange of the steel structure and the concrete deck. This installation process can be executed seamlessly during ongoing traffic without causing traffic disruptions. This stands in contrast to postinstallation of welded-headed studs, where the pavement, water insulation, and concrete must be removed before the connectors are welded to the steel flange, followed by the application of new concrete and surface protection. This paper presents a monitoring project on a bridge strengthened with postinstalled shear connectors. The measurements were done both before and after the strengthening, which made it possible to evaluate the behavior of the nonstrengthened and strengthened structures. A verification with an finite-element model of the bridge with and without the shear connectors was made. The result from the measurements indicates that the steel girder and the concrete deck act as a composite section both before and after the strengthening at the tested load levels. The evaluation of the results also includes the vertical and longitudinal displacements between the concrete slab and the steel top flange, where the latter displacement is also denoted slip. The slip for the nonstrengthened bridge also indicates a full composite behavior. Due to the lack of shear connectors, other interlocking phenomena are most likely sufficient to achieve composite behavior at the tested load levels. Therefore, after the installation of the shear connectors, only a small reduction of the slip is noticed. Nevertheless, shear connectors significantly enhance the connection at the steel–concrete interface and are more reliable and robust, especially for heavier loads in the ultimate limit state.

Building new infrastructure is a crucial part for developing society. Bridges are one part of the infrastructure that has large impact on both cost and climate. Such impact depends on the bridge design, which should be carefully chosen early in the project with several possible alternatives. In Sweden, it has become quite common to evaluate the Life Cycle Cost (LCC) and Life Cycle Assessment (LCA) for different bridge alternatives. This study investigates how three different steel materials in a steel-concrete composite bridge have different impact in a Cost-Benefit Analysis (CBA). In a CBA, all consequences for society are addressed and not only project specific aspects, in comparison to a LCC and LCA. The results show that the alternative with the lowest investment cost during construction is not the best alternative from a CBA point of view. This tells us that society and future generations will need to pay the extra cost if that alternative is chosen.

It is not a simple task to assess the fatigue capacity of steel bridges in order to prolong their lifespan. The traffic load history for existing structures is rarely known in detail, constraints within the static system can often be difficult to estimate as well as the choice of the fatigue detail category. Too conservative assumptions may result in replacement of existing bridges, since minor overestimations of stresses have significant effects on the lifespan of the structure, due to the log-log relationship of the well-established S-N curves. One way of reducing uncertainties, related to loads and load-effects as well as to the dynamic effects, is to perform measurements on existing structures exposed to live loads. This paper presents the fatigue evaluation of derived fatigue stress-cycles based on rainflow counting (RFC) on measured data from two different, but identically built, steel bridges in Sweden. The Rautasjokk Bridge located along the Iron Ore line (Malmbanan) were found to be exposed to more and higher stress cycles compared to the Åby Bridge, located along the Main line (Stambanan). For the Rautasjokk Bridge, the trains loaded with iron ore were responsible for about 80% of the fatigue damage on the stringer beams. Presented in this paper is also a comparison of theoretical methods for fatigue assessment, prescribed in the Swedish bridge assessment code. Furthermore, suggestions for improvement of theoretical assessment methods are also presented.

This paper treats the use of horizontal trusses between the bottom flanges of new I-girder bridges, to create a box-like behaviour. In contrast to the general vertical cross frames of an I-girder bridge, the horizontal trusses bring along substantial torsional stiffness of the cross section of a bridge. The concept gives large advantages when it comes to fatigue caused by eccentric loading, since the I-girders will share the load more equally. The concept is exemplified by bridges in Finland, Guatemala and France, and some design aspects as well as practical aspects are discussed. 

The increased amount of traffic and the increasingly heavier loads on the road network push the existing infrastructure to its design limit. Bridges, which are an important part of the road network, need to be adopted to the new traffic demands regarding both the load capacity and the fatigue limit state, FLS. Steel-concrete composite bridges, with twin steel I-girders, is a common bridge type in the Nordic countries. These bridges are often designed using beam models, assuming that the concrete deck is a statically determinate structure supported on the two steel girders in the transversal direction. This assumption implies that the most loaded girder can sometimes be subjected to even more than 100% of an eccentric load, e.g., the traffic loads in the design codes. If the girders are strengthened to be able to share the eccentric loads more equally, it would have a significant impact on the load capacity, especially for the fatigue limit state. By introducing horizontal trusses between the bottom flanges of the girders, making the cross-section act more like a box-girder, the torsional stiffness will increase so that the girders will share the eccentric loads more equally. The bracing system of the trusses can be designed in different shapes, each of them with pros and cons for the existing bridge structure. In this paper, the effects from different shapes of the bracing are evaluated of a single span I-girder composite bridge. The increased torsional stiffness and the change of the internal shear flow will increase the load capacity of the steel girders.

Traffic density and vehicle weight have been increasing over time, which implies that many existing road bridges were not designed for the high service loads and the increased number of load cycles that they are exposed to today. One way to increase the traffic load capacity of non-composite steel-concrete bridges is to use post-install shear connectors and one type of shear connector is the coiled spring pin. This type of connector has advantages for strengthening of existing bridges, since it enables an installation from below while the bridge is still in service and does not bring along removal of concrete and pavement, nor welding to the top flange.

This paper describes one ~50 years old Norwegian single span steel-concrete bridge that was strengthened with post-installed coiled spring pins. The strengthening method and the design procedure are presented, along with the results from a field monitoring on Sagstu bridge, performed to evaluate the behaviour of the strengthened structure. The results show that the coiled spring pins counteract the slip and bring along a very good degree of composite action.

Today, steel girder bridges with concrete deck slabs are generally constructed as steel-concrete composite structures, to utilize the material and the structural parts in an efficient way. However, many existing bridges constructed before the early 1980´s were designed without shear connectors at the steel-concrete interface. With increasing traffics loads and higher amount of load cycles, there is sometimes a need to strengthen these bridges. One way to increase the bending moment capacity is to create composite action by post-installation of shear connectors. The authors have studied the concept of strengthening by post-installed shear connectors, with a focus on a connector called coiled spring pin. This paper presents the results from the first beam tests performed with this kind of shear connector. In line with the previous push-out tests, the test results indicate a very ductile shear connection, with a potential to be a material- and cost-efficient strengthening alternative.

The overall traffic density and the allowed vehicle weights have been increasing over the last decades. As a response, new roads and railway lines are continuously planned and built. On the other hand, the existing bridges must also be capable of dealing with the increasing traffic volume and the increasing vehicle weights. Many existing bridges were not originally designed for the high traffic loads and todays traffic volume. To cope with the increased loads and load frequencies, the aged structures need repair and strengthening. To avoid the replacement of existing structures, the overall objective is to improve the structural performance of existing steel and steel-concrete bridges by providing innovative and well documented repair and strengthening solutions to bridge designers and road/railway-administrations in Europe. In this paper the use of UHPFRC as a solution for bridge and viaduct repair and strengthening has been presented.