Effect of transverse bending on the shear capacity of concrete bridges
Author: Dimosthenis Karagiannis
Language: English
external pageDOI: 10.3929/ethz-b-000485497call_made
Abstract
This thesis aims at contributing to a better understanding of the load-carrying behaviour of concrete-box girder webs against the combined loading by in-plane shear and transverse bending. With traffic intensity as well as traffic loads steadily increasing, a large number of existing bridges needs to be assessed for structural safety. This combined loading is an important load case that needs to be assessed for. While the transfer of in-plane shear is the principal function of the webs, their monolithic connection to the deck
gives rise to simultaneously acting transverse bending moments. These bending moments are mainly caused by local bending of the deck and distortion of the box-girder section due to the introduction of torques which in turn are caused by eccentric loads. Through an experimental series carried out in the frame of this work, the governing parameters influencing the load-deformation response were identified and are used for the development of different assessment methods at varying levels of approximation.
The first part of the thesis examines the causes of transverse bending in webs, summarises the relevant parameters influencing the response of webs against the combined loading and outlines the knowledge gaps. These observations were the starting point for the definition of a new test series specifically designed to address the behaviour of concrete box-girder-webs.
The second part of the thesis covers the experimental campaign. A total of 10 tests on orthogonally reinforced concrete shell elements were conducted in the Large Universal Shell Element Tester (LUSET), a new testing facility at ETH Zurich. The shell elements, representing a part of a box-girder web, were subjected to combinations of homogeneous loading and imposed deformations, defined such that the boundary conditions imposed to the web by the overall part of the bridge structure were appropriately considered. The tested parameters, included the reinforcement ratio, the kinematic restraints and the loading sequence. In order to study the effect of the web’s stiffness degradation on the transverse bending moment distribution in a transverse section of the bridge, a so-called hybrid test was also conducted. In this test, a finite element model, which was continuously updated with the actual bending stiffness of the physical specimen installed in the LUSET, reflected the global behaviour of the transverse bridge section and accordingly determined the load to be applied. A discussion of the influence of the investigated parameters concludes this part of the thesis.
In the third part of the thesis, a layered shell element is defined based on the work of Seelhofer and Kaufmann and extended in order to enable the consideration of applied kinematic restraints as well as fixed and interlocked cracks. The overall capability of the layered shell formulation to predict experimentally obtained load-deformation responses is checked against the data obtained in the second part of the thesis. Following the validation of the proposed shell formulation against experimental data, the
applicability of rigid-ideally plastic models on members with very low reinforcement ratios is discussed. Making use of the observations on the element level, experimental as well as analytical, the modelling approach of Marti is revisited with the purpose to extend its applicability to cases with very low stirrup reinforcement ratios. Finally, a two-dimensional hybrid frame/shell analysis method for the study of the system behaviour of a transverse section of a box-girder bridge is defined. This method enables a more realistic determination of the transverse bending moment distribution, accounting for the stiffness development of the relevant elements with increasing load. The experimental and theoretical findings are used for the development of assessment methods that incorporate the favourable effect of considering the system behaviour in the definition of the governing transverse bending moment acting on the web at the ultimate limit state.
The thesis concludes with a summary of the results obtained in the first three parts of the thesis and a set of recommendations for future research.