Punching shear strength of steel fibre reinforced concrete slabs

The ultimate strength of reinforced concrete slabs is frequently governed by the punching shear capacity, which may be increased with addition of traditional fitments such as reinforcing steel, headed studs or shear heads. In addition to these traditional methods of strengthening against punching, steel fibre reinforcement has proved to be an effective and viable alternative. The addition of fibres into the concrete improves not only the shear behaviour but also the deformation capacity of reinforced concrete slabs. This paper presents a mechanical model for predicting the punching strength and behaviour of concrete slabs reinforced with steel fibres as well as conventional reinforcement. The proposed model is validated against a wide number of available experimental data and its accuracy is verified. On this basis, a simple design equation for the punching shear capacity of steel fibre reinforced concrete (SFRC) slabs is proposed.

The Critical Shear Crack Theory for punching design: from a mechanical model to closed-form design expressions

Miguel Fernández Ruiz, Aurelio Muttoni

The Critical Shear Crack Theory (CSCT) is a consistent approach used for shear design of one- and two-way slabs failing in shear and punching shear respectively. The theory is based on a mechanical model allowing to determine the amount of shear force that can be carried by cracked concrete accounting for the opening and roughness of a critical shear crack leading to failure. The theory was first developed for punching design of slab-column connections without shear reinforcement. Its principles were later extended to other cases such as slabs with shear reinforcement, fibre-reinforced concrete or slabs strengthened with CFRP strips and one-way slabs without shear reinforcement. The generality, accuracy and ease-of-use of this theory led to its implementation into design codes (such as the fib Model Code 2010 or the Swiss Code for concrete structures). The design expressions of the CSCT consist of a failure criterion and a load-deformation relationship, whose intersection defines the load and the deformation capacity at punching failure. They are clear and physically understandable, and can be written in a compact manner to be used for design of new structures. With respect to the assessment of the maximum punching capacity, the conventional design expressions of the CSCT can also be used, although they required to be solved iteratively. In order to enhance the usability of the design equations of the CSCT, particularly for the punching assessment of existing structures, this paper presents closed-form design expressions developed within the frame of the CSCT. These expressions allow for direct design and assessment of the failure load. The closed-form expressions keep the generality and advantages of the CSCT approach, but they allow for a faster and more convenient use in practice. In this paper, the derivation of these expressions on the basis of the CSCT principles is presented as well as its benefits and comparison to experimental results and the original design formulation.

2016

Considerations on the partial safety factor format for reinforced concrete structures accounting for multiple failure modes

Miguel Fernández Ruiz, Aurelio Muttoni, Qianhui Yu

The increasing usage of nonlinear analyses for the design of reinforced concrete structures and the necessity of codes of practice to provide a consistent safety format for them is one of the challenges that new generations of codes of practice are facing. Suitable safety formats shall thus account for the peculiarities of nonlinear analysis, such as the possibility of having multiple potential failure modes. In this paper, the applicability of the classical Partial Safety Factor Format (PSFF) for the resistance of reinforced concrete structures (composed of two safety factors: gamma C for concrete compressive strength and gamma S for reinforcement yield strength) is investigated accounting for the possibility of multiple failure modes in nonlinear analysis. In addition, the similarities between nonlinear analysis and typical simple cases in the design of structural concrete are shown. Reliability analysis is performed for the design resistance of concrete structures according to PSFF under different design situations (crosssectional resistance or load-bearing capacity of structural elements and of simple structural systems). The results show that the PSFF applied to material strength variables leads to a satisfactory level of reliability, independently of the development of different failure modes induced by material uncertainties in nonlinear analysis. In addition, it is also observed that the simplification of integrating geometrical and model uncertainties into the partial safety factors for material strength variables can potentially underestimate their influence on the structural reliability in some cases. The case studies shows that occurrence of multiple failure modes can result into significantly different distribution characteristics between the tail and most probable region of the resistance of concrete structures. Attention should also be paid to a proper tail approximation of the probability distribution of the resistance when calibrating safety formats for concrete structures.

ELSEVIER SCI LTD2022

Considerations on the partial safety factor format for reinforced concrete structures accounting for multiple failure modes

Miguel Fernández Ruiz, Aurelio Muttoni, Qianhui Yu

ELSEVIER SCI LTD2022

The Critical Shear Crack Theory for punching design: from a mechanical model to closed-form design expressions

Miguel Fernández Ruiz, Aurelio Muttoni

2016