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This paper deals with the experimental and theoretical evaluation of punching shear capacity of steel fiber reinforced concrete (SFRC) slab-column connections. Five experimental specimens with a thickness of 160 mm, different fiber volume contents (0, 1.0, and 1.5%) and different flexural reinforcement ratios (0.75 and 1.5%) have been tested. The experimental results were evaluated using a physicalmechanical model based on the critical shear crack theory (CSCT). The model has given a good approximation of experimental punching shear strengths. In general, tests have highlighted a significant increase in load and deformation capacity of fiber reinforced concrete slab-column connections in comparison with reinforced concrete connections.Punching failure mechanism has been the issue of several studies since the early twentieth century until today. In the early 1960s, Kinnunen and Nylander 1 developed a landmark mechanical model for subsequent theories. This model is based on the assumption that punching shear strength is attained at a given critical rotation of the slab. In 1989, Shehata and Regan, 2 starting from Kinnunen and Nylander approach, proposed a new model where they hypothesized three different types of failure. In 1990, Broms 3 improved Kinnunen and Nylander model, including unsymmetrical punching and size effect.In 1991, Muttoni and Schwartz 4 presented the basis of a new mechanical model, whose first draft was published in 2003 5 ; finally, in 2008, the final version of the critical shear crack theory (CSCT) was published. 6 CSCT describes the relationship between punching strength and the width of the critical shear crack accounting for its roughness. Two curves are defined, the first curve describing the failure criterion, and the second representing the load-rotation relationship of the flat slab. The critical state is set at the intersection between these two curves (Figure 1).The CSCT, thanks to its versatility and consistency, can be used in many different cases, like flat slabs with shear reinforcement 7,8 , SFRC slabs 9,10 , and prestressed concrete slabs. [11][12][13][14] Furthermore, CSCT can be applied to slabs strengthened using different techniques, like externally glued fiber reinforced polymers 15 or post-installed shear reinforcement. [16][17][18] For this reason, CSCT has been included in codes of practice and it is the basis for fib Model Code 2010 19 punching design and verification.When fibers are added to concrete, the mechanical properties of concrete change. The post-cracking tensile behavior improves, as fibers delay crack propagation. The behavior of the composite material (SFRC) is highly affected by the fiber reinforcement ratio. The softening behavior, typical of uniaxial test, can be significantly modified by the presence of fibers (Figure 2).

This paper presents a study of the behaviour and load capacity of steel-fibre-reinforced concrete (SFRC) flat slabs under monotonically increased concentrated vertical loads. The SFRC was used only in the local region of the slab–column connection, as the rest of the slab was cast using normal concrete without fibres. The six experimental test specimens had a thickness of 150 mm with different longitudinal reinforcement ratios, using a non-uniform distribution over the slab width. The concretes used were made with different Dramix 4D 65/60 BG steel fibre contents (0, 0·5, 0·75 and 1·0% volume content). The slab tests were complemented by flexural tests on notched beams. This made it possible to determine the tension behaviour of the different concretes used, through a linear post-cracking behaviour and inverse analysis. The inverse analysis made it possible to define the stress–crack opening relationship that characterises the tension behaviour of SFRC and to relate it to the observed behaviour and load capacity of the tested slabs. The tests results show that the tensile behaviour of the SFRC plays an important role in the behavioural and load capacity of the slabs and that it can be considered relevant to physically based models.

This paper presents an experimental study of four flat slab specimens subjected to combined vertical and horizontal cyclic loading. Steel fibre-reinforced concrete (SFRC) was used only in the local region of the slab–column connection, while the rest of the slabs were cast using normal concrete. The specimens measured 4·15 m × 1·85 m × 0·15 m and were connected to two steel half columns by 0·25 m × 0·25 m rigid steel plates, prestressed against the slab using steel bolts, to ensure monolithic behaviour. The specimens were tested using an innovative test setup system that accounted for important factors, such as the ability of bending moment redistribution, line of inflection mobility and assured equal vertical displacements at the opposite slab borders, and symmetrical shear forces. Results show that the presence of SFRC in the slab–column connection region is effective in increasing the deformation capacity of slab–column connections, allowing the increase of horizontal drift ratios.