Shear cracks in reinforced concrete in serviceability limit state
Shear cracks are formed when high oblique tensile stresses, e.g. in thin webs, exceed the tensile strength. A known example of this phenomenon is the extensive shear cracks that were found on the box-girder bridges Gröndal and Alvik, which were mainly caused by insuﬃcient amount of shear reinforcement. In order to avoid this incident (inadequate amount of shear reinforcement), the reinforcement stress is often being assumed as a ultimate limit load in order to fulﬁll requirements regarding crack control in the service-ability limit state (SLS). This method has led to a overestimation of the reinforcement amount in bridge-design. The aim of this master thesis is therefor to study the shear crack phenomenon and investigate if the amount of shear reinforcement in bridges can be reduced. The ﬁrst part of this thesis studies the shear cracking behavior in concrete in a plane stress state, while the second part focus how design standards as well as manuals treats shear cracks.
Shear cracking in the reinforced concrete panels has been studied with non-linear ﬁnite element analysis and compared to experimental testings performed by the University of Toronto. Three diﬀerent loading conditions for the panels has been analyzed: pureshear, compression or tension combined with shear. The panels are to represent parts of a web in a box-girder bridge that are subjected to in-plane stresses. The non-linear ﬁnite element analysis was performed in the FE-program Atena where mainly the crack propagation and crack pattern were studied. The material model in Atena is a smeared crack model with either ﬁxed or rotated crack direction. The panel analysis, in SLS, gave various results. For loading conditions pure shear and tension/shear, the response of the FE-analysis gave a similar result regarding crack pattern but diﬀered in size of crack width. For compression/shear, only micro-cracks developed and did not reﬂect the result from the real panel tests. This may be the consequence of a too stiﬀ FE-model and the fact that, in the real tests, some cracks occurred due to out-of-plane bending.
With methods described in Eurocode 2 and the Swedish handbook for EC2, a shear crack calculation model was created in order to determine the reinforcement stress and crack width. As a reference for the shear crack calculations, a wing structure (1 m strip) has been used which is part of a railway bridge located in Abisko. These calculations were done in order to investigate if the amount of shear reinforcement could be reduced and at the same time fulﬁll crack control demands in SLS. The bridge department at Tyréns AB concluded, according to a truss model, that the wing section should be reinforced with a amount of 14.1 cm2/m2 while our model showed that the crack width demand could be fulﬁlled with a equivalent amount of 9.82 cm2/m2, i.e. a reduction around 30%.
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