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.

Steel-concrete composite bridges are commonly designed with full composite action between the steel girders and the concrete deck to enhance structural efficiency and durability. This composite action is achieved through shear connectors, which transfer shear forces and ensure that both materials act as a single unit. While full composite action is preferred, partial composite action may be sufficient in certain cases, depending on interlocking effects between steel and concrete. One strategy to improve the performance of composite bridges is the implementation of horizontal trusses between the lower flanges of the steel girders, which enhance load distribution and lateral stability.

Although previous research has investigated the effects of horizontal trusses on global load distribution, their influence on shear force distribution, particularly at shear connectors, remains largely unexplored. Studies on monorail track beams indicate that transverse shear forces significantly affect shear connectors, reducing their capacity and altering failure mechanisms. A similar effect may occur in composite bridges with twin girders and horizontal trusses.

This paper presents a case study of a single-span steel-concrete bridge in Sweden, examining the impact of a torsional rigid structure with a semi-closed cross-section on local shear flow distribution. The study also investigates how shear connector rigidity affects force distribution along the steel-concrete interface.

This thesis deals with the subject of lateral bracing between the bottom flanges of I-girder composite bridges. The focus is on the impact of adding lateral bracing on existing bridges, as well as on new bridges. Experience and knowledge from bridge projects around the world are investigated and implemented in the evaluation of the research subject.

Many existing bridges are in need of being strengthened or replaced, due to the increased traffic volume and heavier traffic loads. Different approaches can be used to prolong the lifetime of existing bridges. The approach is different depending on the cause, but for increasing the lifetime regarding fatigue some of the most suitable options are described in this thesis. A proposed concept is presented, in this thesis, along with some research questions to be answered.

The use of lateral bracings in composite bridges varies between different parts of the world. In one country it can be a requirement/common praxis for long span composite bridges with two I-girders, in other countries there are no requirements of using them. Some parts of these regulations and requirements can be traced back to the tradition in both manufacturing and construction of this type of bridges. This thesis investigates how lateral bracing is used around the world to distribute eccentric loads between primary longitudinal structural members, provide resistance to lateral loads, and to permit an existing two-girder structural system to be retrofitted to behave similarly to an often more expensive closed steel box girder.

Furthermore, several case studies have been conducted to investigate the impact on the structural behavior of composite bridges where a lateral bracing is implemented in the structure. The results from these case studies are presented in the thesis and show the advantages of the quasi-box section for which the lateral bracing is closing the composite cross section. By making the I-girder composite cross section acting more like a box-section, the distribution of eccentric loads between the girders is improved. The impact on longitudinal stresses from traffic loads and the additional effects on internal sectional parts are also evaluated and discussed.

Furthermore, proposals of the connection design for lateral bracings in existing bridges are suggested. Finally, conclusions from the results are stated.

The ways of designing and building steel girder bridges with a composite concrete deck vary much between different parts of the world. A bridge system with twin steel I-girders or a concept with multiple (more than two) girders can be used. The design and use of details and secondary systems also vary a lot. In order to give horizontal stabilization and distribute horizontal loads, bracing between the I-girders bracings are often used. Although not so common, the bracing can also be used to distribute vertical loads between the main girders. To describe and to analyze the possible impact from using a lateral bracing, this paper describes the design of a curved bridge in Guatemala City and its challenges. The new Bridge over the Pinula River is designed as a steel-concrete composite bridge with multiple steel girders with a concrete deck on top and has lateral bracing between the girders. The bridge is used as a case study to analyze the impact from a lateral bracing system on the vertical load distribution between the longitudinal girders. The bridge was designed with lateral bracing between the top flanges along the whole bridge and with bracing between the lower flanges near the supports. The case study shows that the distribution of eccentric vertical loads between the longitudinal girders can be improved by using lateral bracing between the lower flanges for multiple girders. In some cases, it may not be beneficial to use lateral bracings along the whole bridge length, like for example this bridge. In this case due to the governing design cases in the construction stages where these bracings influenced the torsional stiffness of the bridge in such way that an unfavorable distribution of the support reactions between the six girders at some supports was achieved.

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.

Some old bridges have a truss between the bottom flanges not intended for torsional effects but for transferring horizontal forces. This paper describes the effects of the truss on torsion for a Norwegian three span bridge from 1967, without composite action. Furthermore, the effects of post-installed shear connectors are investigated. For composite bridges without intended composite action in bending, the effects of the slab preventing the top flanges from moving laterally should not be ignored, since this is important for the deformations of the girders under eccentric loading. Furthermore, the load distribution between the girders for an eccentric load is significantly enhanced if the horizontal truss is considered. The paper also investigates and presents the effects of post-installed shear connectors, with respect to bending stresses in the bottom flanges (moderate effects) and the top flange (large effects). 

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.