Year: 2010
Type: dissertation
Abstract: Shear failure is an important failure mode for pile caps, civil engineering structures in reinforced concrete, often used as substructures for bridges.However, while relatively thin slabs, such as flat slabs for office buildings, have been subjected to intense research in the past, there is a lack of generic models for thicker structures today and building codes are still based on less appropriate empirical or semi-empirical models.For this reason, the design of pile caps for shear failures, and punching failure in particular, often results in dense reinforced structures.A rational approach to shear failures in three-dimensional structures is needed to provide a safe and efficient design of pile caps.In order to comprehend the complex cracking and failure process in pile caps, the different shear transfer mechanisms of forces in structural concrete, as well as shear and punching failures of flexural elements are described in this thesis.A review of the design procedures for shear and punching proposed by the Swedish design handbook (BBK04), the European standard (Eurocode 2) and the American building code (ACI 318-08) is conducted.The models of BBK and Eurocode are applied to the analysis of four-pile caps without shear reinforcement.The comparison with the experimental results indicates that the analysis with Eurocode predicts failure loads more accurately than with BBK, however both standards result in significant variations between similar cases, mainly because they accord too much importance to some parameters, while neglecting others.In light of these facts, strut-and-tie models appear to represent a suitable alternative method to enhance the design of pile caps.Strut-and-tie models have been developed and used successfully in the last two decades, and present a rational and consistent approach for the design of discontinuity regions in reinforced concrete structures.Though, the guidelines for strut-and-tie modelling in the literature are mainly intended to study structures in plane, and it is questionable to apply them in the case of pile caps, structures with large proportions in the three dimensions.Adaptations seem required for the geometry and the strength of the components.A strut-and-tie model adapted to the design and analysis of pile caps has been developed in this project.The model is based on consistent three-dimensional nodal zone geometry, which is suitable for all types of nodes.An iterative procedure is used to find the optimal position of the members by refining nodal zones dimensions with respect to II the strength of concrete under triaxial state of stress.Away from nodal regions, a strength criterion is formulated for combined splitting and crushing of struts confined by plain concrete.In addition, the specificities of shear transfer mechanisms in pile caps are considered and a combination of truss action and direct arch action for loads applied close to the supports is taken into account, hence reducing the required amount of shear reinforcement.The method developed is compared to the design codes predictions for the analysis of four-pile caps.The results obtained by the strut-and-tie model are more reliable, both for assessing the failure loads and the failure modes.The iterative procedure is presented in some design examples and guidelines are given to apply the method to pile caps with large number of piles.
Authors: Gautier Chantelot, Alexandre Mathern
Language: English
Subfield: Building and Construction
Field: Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W21200719