Investigation of structural behavior of composite beams with prestressing strand

In recent years, the use of steel-concrete composite beams in high-rise buildings and bridge type structures has increased due to its economic cost. Concrete is considered by designers as a material with high compressive strength and low tensile strength. Structural steel is thought to show similar behavior under tensile and compressive effects. However, local buckling problems can be observed in steel beams under compression. These reasons lead designers to composite section beams where concrete and steel materials are used together in design. Composite section beams allow the design of lighter structures compared to reinforced concrete structures by operating the reinforced concrete slab in compression and the structural steel beam in tension. Thanks to this lightness, wider spans can be crossed with less beam height. However, load transfer between the concrete deck and the steel beam must be ensured in order for the composite section formed by using two different materials such as concrete and steel to work efficiently. In composite beams, the load transfer between the concrete deck and the steel beam is provided by shear nails. The number of shear nails to be used along the beam changes the shear connection degree of the composite beam and accordingly its structural behavior. Different collapse conditions such as concrete crushing, longitudinal shear, shear nail rupture can occur in composite beams. These failure modes vary according to the selection of element dimensions and material properties during the design stages. Steel-concrete composite beams allow economical sections to be obtained by combining different materials. By adding prestressing strands to these economical sections, more efficient sections can be obtained. However, when prestressing is added to steel-concrete composite beams, which already have different collapse conditions due to the combination of many elements, changes occur in the collapse conditions of the composite beams. In order to determine these changes in the structural behavior of prestressed steel-concrete composite beams compared to conventional steel-concrete composite beams, a detailed study based on finite element analysis was carried out in this thesis. The material damage parameters were considered while constructing the finite element models and nonlinear static analyses were performed. The modeling technique used in the creation of finite element models has been validated with 2 experimental studies accepted in the literature. With the validated modeling technique, finite element models were created to determine the effective dimensional parameters (rope placement, transverse reinforcement placement, connection degree, post-tensioning ratio) that can affect the structural behavior and capacity of prestressed composite beams. The finite element models were analyzed for 4-point bending. In the analysis results, it was found that the longitudinal shear behavior can be improved if the transverse reinforcement is located behind the shear nails. It was found that the prestressing strand placed in the tension zone in the center of the span created a compression effect on the concrete deck and caused concrete crushing damage earlier. Effective rope placement, reinforcement placement and connection degree were determined for prestressed composite beams. After determining the effective dimensions, 600 finite element models were analyzed for parametric study. In the analysis results, material damage parameters were analyzed in detail and data on collapse conditions were obtained. Using the obtained data, an equation is proposed to calculate the capacity of prestressed composite beams without the need for 3D modeling.