Strut-and-tie modelling of reinforced concrete pile caps

[parhyang]

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 [/align]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.

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[joker™]

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[gulilero]

Strut-and-tie modelling of reinforced concrete pile caps
Master of Science Thesis in the Master’s Programme Structural Engineering and
Building Performance Design
GAUTIER CHANTELOT
ALEXANDRE MATHERN
Department of Civil and Environmental Engineering
Division of Structural Engineering
Concrete Structures
Chalmers University of Technology

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
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.

Code:

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