Design and assessment of concrete bridges for longitudinal shear and transverse bending
Author: Rebecca Dürmüller
Language: English
Abstract
Most of the bridges built between 1950 and 1970 are characterized by rather low stirrup reinforcement ratios in their webs. Over time these structures need reassessment, due to increasing traffic load and/or the addition of traffic lanes.
Existing interaction models, based on the rigid-plastic stress field approach are typically conservative for normal reinforcement ratios. However, their application implies sufficient defor-mation capacity, which cannot be assumed a priori for low reinforcement ratios. The Layered Cracked Membrane Model (LCMM) developed at ETH Zürich, is able to model all relevant effects, such as compression softening and tension stiffening and therefore implicitly respects deformation capacity limits. Although this model is capable of capturing realistic behavior, its application requires a time-consuming nonlinear analysis.
The thesis examined the applicability range of different interaction models for webs with very low stirrup reinforcement ratios. Furthermore, a simple assessment method for combined in-plane shear and transverse bending in the web of a box-girder bridge was proposed, which allows for the consideration of a possible plastic redistribution along with a check of the deformation capacity, without the need for time-consuming and complicated nonlinear FE modelling.
The study revealed that Menn's interaction model generally provides good results. However, at high shear forces, the results can be somewhat unconservative, especially when the kc factor proposed by Menn is used in comb-nation with very low compression field inclinations. If the kc factor is defined based on the imposed longitudinal strains, the model pro-vides more consistent results. The Eurocode was found to be too conservative for application to webs of box girder bridges.
The proposed assessment aims at considering the favourable effect of possible plastic redistribution after the yield moment in the web is reached. It enables the comparison of the deformation demand required to allow for the plastic redistribution to the web’s deformation capacity.
For typical geometries of a three-span bridge, it was found that a plastic redistribution will almost always be required. In the case of an insufficient deformation capacity of the web, the most efficient strengthening measure (from the point of view of constructability and costs) is an increase of the deck’s stiffness which in turn leads to a reduction of the deformation-demand in the web.
