The increased amount of traffic combined with higher traffic loads leads to many existing bridges needing strengthening in the future to ensure their expected lifespan. This means the bridge owners will be focusing more on strengthening projects and smart solutions will be crucial for preserving a healthy bridge stock. When strengthening existing non-composite bridges (with steel girder and concrete deck) one potential method is to achieve composite action by installing shear connectors. The post-installed shear connectors prevent slip between the steel girders and the concrete. The composite action will reduce bending stresses and deflection of the bridge, due to the increase in moment of inertia and relocation of the neutral axis. Different types of shear connectors can be used for achieving composite action and each type of connector has its own installation method. The biggest distinction between the methods is how the connectors gain access to the steel girder for installation and what technique is used when installing them. This thesis presents the theory behind composite action, the current methods used for achieving composite action on existing bridges and to what extent a bridge can be strengthened by composite action. The thesis also provides a status of the existing road bridge stock around the world. The four case studies examined in this thesis have used different post-installed shear connectors to manage different strengthening problems like weight restriction, fatigue life of shear connectors and a unique problem on the Pitsund Bridge where loud bangs appeared from the bridge when truck passed in the morning. For the case study on the Pitsund Bridge an interview was conducted that explains the entire procedure of the project, from the noise problem to how the installation of coiled spring pins was performed. The bridge over Lule River at Akkatsfallen consists of two steel girders and a concrete deck. This bridge is chosen as a real case study to determine to what extent a bridge can increase its capacity by achieving composite action. The calculations are performed in accordance with the Eurocodes on both non- and full-composite action and the result is compared to the other case studies.

For bridges to cope with increased requirements such as increased loads, strengthening work can be carried out. In cases where older steel-concrete bridges do not have a composite action, an alternative is to create composite-action to achieve a higher flexural strength. It is introduced by post-installing shear connectors. There are many different alternatives of shear connectors that can be used, hence a number that can be installed from below the bridge to minimize the impact on the traffic. Coiled Spring Pins are of the interference fit type connector and are put in place from below the bridge by first drilling a hole upward through the upper steel flange and then into the concrete slab. Then, the spiral bolt is pushed up into the drilled hole by means of a hydraulic hammer. Using data from push-out tests and non-linear material models for steel and concrete, a non-linear finite element analysis was created using the commercial finite element software Abaqus. The analysis is based on dimensions and load cases that will mimic a planned full-scale beam test that will be carried out later in 2019. To verify that the material and the model behave in a realistic manner, an analysis was initially performed on a beam without composite-action, and a full-composite action beam with infinitely rigid connectors. These were then compared with hand calculations according to Eurocode. When the material models were verified, it is seen that the materials steel and concrete work for themselves in the analysis without composite-action and together in the analysis with full composite-action. The data for the spiral bolts is than defined instead of infinitely rigid connectors and new analyzes were performed to see the effect of the coiled spring pins properties. The results show that a significant increase in the point load in the middle of the beam can take place before failure occurs after installation of this type of shear connector. Already at a low number of connectors and a low shear connection-ratio, a significant increase in the flexural strength is seen in the beam. By using partial-composite action, with a lower number of spiral bolts, a significant higher flexural strength can be achieved in an economical way. / När kraven på att broar ska klara av ökade laster, kan förstärkningsarbeten utföras. I de fall där äldre stål-betongbroar saknar samverkanseffekt, är det ett alternativ att inför samverkan för att uppnå en högre böj-hållfastighet. Det införs genom att man installerar skjuvförbindare i efterhand. Det finns många olika alternativ av skjuvförbindare som kan användas, därav ett antal som går att installera underifrån bron för att minimera påverkan på trafiken. Spiralbultar (Coiled Spring Pins) är av typen presspassnings-förbindare och sätts på plats underifrån bron genom att det först borras ett hål uppåt genom övre stålflänsen och sedan upp i betongplattan. Därefter pressas spiralbulten upp i det borrade hålet med hjälp av en hydraulisk hammare. Med hjälp av data ifrån push-out-tester samt icke-linjära material modeller för stål och betong, skapades en icke-linjär analys i det finita element metods programmet Abaqus. Analysen är uppbyggd med dimensioner och lastfall som ska efterlikna ett planerat full-skaligt balktest som kommer utföras under 2019. För att verifiera att materialet och modellen beter sig realistiskt, utförs en analys på en balk utan samverkan, samt en full-samverkans balk med oändligt styva förbindare. Dessa jämförs sedan med handberäkningar enligt Eurokod. När materialmodellerna var verifierade sågs det att materialen stål och betong arbetar för sig själva i analysen utan samverkan och tillsammans i analysen med full-samverkan. Data för spiralbultarna lades sedan in istället för oändligt styva förbindare och nya analyser utförs för att se påverkan av spiralbultarnas egenskaper. Resultaten visade att en betydande ökning av punklasten i mitten av balken kan ske innan brott uppstår vid installation i efterhand av denna typen skjuvförbindare. Redan vid ett lågt antal förbindare och ett lågt skjuv-förhållande ses en betydande ökning av böj-hållfastigheten i balken. Genom att använda delvis-samverkan med ett lägre antal spiralbultar kan man på ett ekonomiskt sätt uppnå en betydligt högre böj-hållfasthet.