Composite Bridges : Innovative ways of achieving composite action

Abstract: The topic of this thesis is steel-concrete composite bridges and innovative ways of achieving composite action. The typical superstructure consists of three main components: the steel girders, the concrete deck slab and the shear connectors. The latter connects the steel and the concrete parts to each other, which enables a design where the parts are assumed to act as one structural member, the composite beam.The research presented in this thesis is primarily focused on different construction- and strengthening-methods, developed to reduce the impact on the road users, mainly by reducing the time spent on the construction site and the need of traffic restrictions.The prefabricated steel girders give composite bridges some advantages in the construction stage, in comparison to the more common in-situ cast concrete bridges, since the girders can be launched or lifted into their final positions. Such an installation procedure is often favourable in case of crossings over roads, railways, rivers etc., since it minimizes the impact on the citizens using the infrastructure below the bridges and the need of temporary supports. In order to shorten the time spent on the construction site and to reduce the impact on the road users even more, prefabrication of the concrete deck can also be considered.In this thesis, a review of different prefabrication techniques for composite bridges is presented, along with a study of one specific prefabrication concept that reduces the need of in-situ cast deck joints. This concept, with prefabricated concrete deck elements with dry joints, utilizes concrete shear keys to transfer shear forces over the transverse deck joints, while in-situ cast joints are used for the longitudinal connection between the steel girders and the concrete deck slab.The structural behaviour of composite bridges with dry deck joints has been investigated by large scale beam tests, along with field measurements on a composite bridge built with this prefabrication concept. The load capacity of the shear keys has also been investigated by laboratory tests. The test results have been compared to numerical analyses and different design models, with the aim of developing design recommendations.The results indicate that this type of bridges do not behave as conventional composite bridges with in-situ cast deck slabs. For single span bridges, which only experience positive bending moments, the structural behaviour in the ultimate limit state is close to the structural behaviour of conventional composite bridges. However, the degree of composite action is strongly reduced at lower load levels. This should be taken into account in the design in the fatigue- and the serviceability-limit states. Sections under negative bending moments behave in general as non-composite sections, which was expected due to the dry deck joints.Based on the evaluation of the test results and the state -of-the-art review, design recommendations and design criteria are presented, along with production and execution recommendations for this type of prefabricated bridges.Strengthening of existing bridges is another activity that often leads to traffic restrictions, which causes costs and troubles for the road users and the society. One method for strengthening non-composite steel-concrete bridges is post-installation of shear connectors, to create composite action. The composite cross-section has a larger stiffness and bending capacity, implying that a larger traffic load often can be allowed. It must however also be assured that other structural parts do not limit the load capacity of the structure.There are several different types of shear connectors that can be used for post-installation, and some are more suitable than others. This thesis presents a state-of-the-art review on post-installed shear connectors in general and Coiled Spring Pins in particular. The latter is an interference fit connector that can be installed from below the bridge, with no or minor impact on the traffic on the bridge.The behaviour of Coiled Spring Pins, used as shear connectors in composite bridges, has been investigated by experimental methods. Push-out tests have been used to study the static strength and the fatigue lifetime, while field monitoring of a real bridge structure has been used to study the behaviour on a structural level. The tests results have been evaluated and design criteria and design recommendations have been suggested.The static tests and the following analysis show that Coiled Spring Pins are a very ductile type of shear connector, with a slightly different load-deformation behaviour than headed shear studs. The static strength of the shear connection shows a quite small spread even when different parameters are varied quite a lot. The performed fatigue tests in dicate a fatigue strength that are somewhat lower than headed studs, in terms of detail category, while previous test series by other researchers indicate a higher fatigue strength than headed studs. It can be noted that there is a large scatter between the results from different test series, performed by different researchers. The reasons to this scatter are discussed in the thesis and a conservative fatigue design criterion is presented.The results from the field monitoring indicate that a bridge strengthened with Coiled Spring Pins behaves as a composite structure and that the Coiled Spring Pins reduce the slip significantly. The analysis of the test results shows that a design assuming full composite action, with rigid shear connection, describes the measured behaviour in a good way.Based on the state-of-the-art review and the different tests performed, design recommendations and criteria are presented, along with production and execution recommendations for post-installation of Coiled Spring Pins.