Punching strength of continuous flat slabs

Reinforced concrete slabs with uniform thickness are common in residential and commercial buildings but can also be found in other types of structures. Such slabs are susceptible to punching shear failures, where a supporting column penetrates through the slab and leads to an immediate local failure that may trigger a progressive collapse of the building. Provisions for punching shear in most codes of practice are still mainly empirical, calibrated on the basis of experiments on test specimens that traditionally model only an isolated part of the slab within the points of contraflexure around the column. However, the punching behavior of actual continuous slabs may be influenced by effects that cannot occur in isolated specimens, such as moment redistribution between hogging and sagging moments, which changes the location of the points of contraflexure, and compressive membrane action. These effects can lead to higher punching strengths of actual continuous slabs compared to isolated specimens. The first part of the thesis introduces an axisymmetric model to analyze the influence of these effects on the flexural deformations of continuous flat slabs. Combined with the failure criterion of the Critical Shear Crack Theory, the model can be used to predict the punching capacities of such slabs. Good agreement was found between the model predictions and the results of some unconventional punching tests from the literature. A simplified method, sufficiently straightforward to be used in design or assessment and given in a format compatible with the punching provisions of the Model Code 2010, is also proposed for calculating the load-rotation curves of continuous slabs. The second part of the thesis contains the results of a test campaign comprising 13 isolated symmetric punching specimens. The study focuses on the influence of the size of the supported area and the slenderness of the slab. Other investigated parameters are the flexural reinforcement ratio and the presence of shear reinforcement. A novel experimental approach is used for tracking the formation and development of internal cracks. Measurement points were installed inside small holes drilled on the slab soffit on two sides of the column in the regions were punching cracks were expected to appear. Displacements of these points at various stages of loading were followed with a high-precision coordinate measuring arm. In most cases, the punching failure cracks were seen to develop independently of the flexural cracks, either appearing at the moment of failure or, in some cases, already at earlier stages of loading. Although the slabs were nominally axis-symmetric, different crack development patterns could be observed on the two monitored sides of the columns. On the basis of the experimental evidence, a new punching model is proposed for slabs without shear reinforcement. Punching failures are assumed to occur due to reaching a critical triaxial stress state below the flexural cracks in the compression strut and a consequent formation and propagation of a failure crack. The proposed model uses the theory of plasticity with a general triaxial yield criterion together with an effectiveness factor based on fracture mechanics that is a function of the depth of the compression zone and the size of the column. The influence of membrane forces in continuous slabs on their punching strength is taken into account by adjusting the depth of the compression zone.

Experimental investigation of reinforced concrete slabs with punching shear reinforcement

Stefan Lips, Aurelio Muttoni

Over the past century, the use of flat slabs in buildings and especially in parking garages has been growing as it is an economic and efficient solution. Flat slabs are easy to build and have, through their smaller depth, an economical and architectural advantage compared to slabs on girders. Because of their limited depth, flat slabs are especially sensitive to deflections and to punching shear, which are their main design criteria. Furthermore, flat slabs without punching shear reinforcement have a rather brittle failure mode with little deformation capacity. This behavior potentially limits the redistribution of the internal forces in flat slabs in the case of punching of an isolated column and can thus lead to progressive collapse of the entire structure. To increase both the strength and the deformation capacity of flat slabs, punching shear reinforcement can be provided in the vicinity of the columns. Although the influence of punching shear reinforcement on the strength of flat slabs has been intensively investigated, there are still fundamental uncertainties such as the contribution of the shear reinforcement and the strength of the concrete struts close to the column. The paper presents the results of an extensive experimental campaign performed at the Ecole Polytechnique Fédérale de Lausanne (EPFL). Sixteen full-scale slab specimens (3.0 x 3.0 m in plane) with varying parameters such as the column size, the slab thickness, the shear reinforcement ratio, and the shear reinforcing system have been investigated. The performance of these specimens is analyzed and compared to modern design codes and to the critical shear crack theory.

Proceeding of the 8th fib-PhD Symposium2010

Connections by Adhesion, Interlocking and Friction for Steel-Concrete Composite Bridges under Static and Cyclic Loading

Dimitrios Papastergiou

Steel-concrete composite bridges with twin-I steel girders and prefabricated slabs constitute an economical and competitive solution for small and medium bridge spans. In recent years they have started to gain their share in the construction market. However, the current slab-to-girder connection with groups of headed shear stud connectors is not well adapted for prefabrication and fast erection, and does not respond successfully to the durability of the construction. New types of connections by adhesion, interlocking and friction constitute a conception that aims to answer these demands. The resistance of these connections is based on the development of longitudinal shear stresses in the interfaces which form the connection. The goals of this study are to complete the existing research in this field by proposing scientific tools in order to predict the connection's structural performance, i.e. prediction of the connection's ultimate resistance and deformation capacity, including the post-failure behaviour. In addition, the behaviour of the connection under cyclic loading is also investigated to assess the connection's resistance to fatigue and to propose a design method for composite bridges under fatigue loading. To fulfill theses goals a combinational methodology was applied which included: study of the state of the art: a) in composite bridges so as to define the requirements and b) in interface behaviour in order to emphasize the parameters which are responsible for the resistance and obtain information for the nature of cyclic loading damage mechanisms, experimental investigations: a) to reveal the laws describing the interfaces and the connection's behaviour for static and cyclic loading, and provide data for development of analytical expressions and model validation and b) demonstrate the capacity of a composite beam under cyclic loading and the extent of its remaining resistance at ultimate limit state, analytical studies: a) to produce the laws describing the interface behaviour under static and cyclic loading and b) to evaluate the impact of the cyclic loading to the slip in the interfaces and in the connection and thus propose a damage factor for cyclic loading, finite element analysis in order to enable an analytical description of the confinement effect, caused by the reinforced concrete slab, which contributes to the resistance, the development of a numerical model to predict the connection's structural performance and to propose a design method for composite bridges fabricated with the new connection. Application of the mentioned methodology resulted in several useful conclusions: The interface behaviour can be described by analytical laws. The post failure behaviour of the interfaces was explained by accounting for the kinematics of the failed surface and basic notions of fracture mechanics. The damage due to cyclic loading on the interfaces and on the connection is expressed by the accumulation of a residual slip. Provided that the shear stresses remain lower than an elastic limit and the accumulated slip does not exceed the failure slip for static loading, fatigue failure is avoided. The confinement effect was incorporated, analytically, together with the laws of the interface behaviour in a numerical model to predict the connection's structural performance and allow a parametrical study. A design method for the verification of composite beams under static and cyclic loading was developed.

EPFL2012

Punching strength of actual two-way slabs

Jürgen Einpaul, Miguel Fernández Ruiz, Aurelio Muttoni

Many building codes rely on empirical formulations with respect to the punching shear design provisions. Such empirical equations are typically calibrated on the basis of laboratory tests performed on simplified geometries (usually isolated specimens) with usually statically-determined loading conditions. On the contrary, actual structures, particularly two-way slabs, are continuous and highly redundant systems. The isolated test specimens are thus unfortunately not always suitable to reproduce the actual behaviour of flat slabs and empirical models calibrated on their basis may not be applicable for their design. In this paper, the application of the Critical Shear Crack Theory (theoretical support of Model Code 2010 provisions) is shown for some unusual test specimens aiming at investigating actual inner bays of continuous flat slabs. This analysis shows that moment redistribution between the column region and mid-span develop after concrete cracking and that accounting for this behaviour is required to obtain good predictions of the actual flat slab response. It is also shown that the Critical Shear Crack Theory leads to consistent results in terms of failure mode, punching shear strength and deformation capacity as it allows accounting for this behaviour. On the contrary, empirical design formulas fail to predict the influence of continuous flat slabs on the punching strength.

4th fib Congress, Mumbai2014

Connections by Adhesion, Interlocking and Friction for Steel-Concrete Composite Bridges under Static and Cyclic Loading

Dimitrios Papastergiou

EPFL2012