Analysis of Flexural Behavior of Steel-UHPC Composite Girders Based on Plastic Damage Model

A steel-UHPC composite girder is a new girder type whose steel girder is connected to a UHPC slab by shear connectors. Compared to the steel-conventional composite girder, the slab thickness of steel-UHPC composite girder can be greatly reduced due to the ultrahigh strength mechanical properties of UHPC, which decreases significantly the structural weight and enhances the spanning ability. As a result of high tensile strength and strong self-healing ability of micro cracks of UHPC, the steel-UHPC composite girder improves the flaws of the bridge slab existing in steel-conventional composite girders to some extent, such as being prone to crack under external loads in the negative bending moment area and insufficient durability for concrete slab, which greatly improve the safety performance and reduce the maintenance cost for steel-concrete composite girders on the basis of ensuring the good durability of structure. At present, only few studies on the refined mechanical model and numerical analysis for steel-UHPC composite girders have been reported. Additionally, there is still no such an unified theoretical model that can describe compressively the constitutive relationship of UHPC because of the complexity of UHPC materials.
To conduct a refined numerical model and to investigate the flexural behavior of steel-UHPC composite girders, the damage factor was deduced based on the selected uniaxial tension and compression constitutive relation for UHPC. A numerical model of damage mechanics corresponding to a tested steel-UHPC composite girder failed by flexure was established by ABAQUS finite element program, whose applicability is analyzed by comparing the mechanical properties to the test girder. Taking the UHPC slab thickness, the web slenderness and the tension flange thickness as the main structural parameters, the mechanical properties in the whole process of bending failures for 36 numerical model steel-UHPC composite girders were analyzed.
The model validation analysis showed that the response trend of load-displacement curve from numerical calculation was in good agreement with that of the test curve before the failure stage. After the failure stage different from the rapid failure of the test girder, the model girder obtained a more complete load-displacement curve including failure stage and descending stage, showing a good ductility performance. The damage evolution characteristics of the UHPC slab from the numerical calculations were in good agreement with the test results. In General, the load-displacement curve and damage evolution characteristics of the UHPC slab from the numerical calculations were in good agreement with the test results, and therefore the established numerical model can simulate accurately the mechanical behavior of the whole failure process of steel-UHPC composite girders, and reveal truly the stress and strain field transfusions and the damage evolution characteristics of cracking and collapse of the UHPC slab. The flexural strength of steel-UHPC composite girders under unit steel consumption increases greater by increasing the web slenderness than that by increasing the tension flange thickness. However, increasing the UHPC slab thickness has a relatively small effect on the improvement of flexural strength for steel-UHPC composite girders. As the tension flange thickness increases, the ratio of elastic flexural strength to ultimate flexural strength for steel-UHPC composite girders increases significantly, but at the same time, the ductility ability of girders reduces to some extent. On the premise of meeting the ductility demand, the distribution of flexural strength in different working stages of composite girders can be effectively adjusted by changing the tension flange thickness to meet different flexural strength demands of structure.