Cracking in R/C members is a complex phenomenon characterized by high scattering, mainly affected by concrete in tensile zone. The aim of this paper is to evaluate the effectiveness of formulas for crack width prediction developed in last decades. First, an historical review of available literature models is presented and discussed. Second, the progress and missteps, which have characterized the development of international code provisions, are shown. Finally, a comprehensive comparison between predicted crack widths and experimental data is performed. Some formulas more than others provide mean values of crack width in good agreement with experimental data; nevertheless, predictions are always very scattered, as ratios between theoretical and experimental values have a coefficient of variation not lower than 30%. Results show that the prediction capacity does not necessarily increase as the cracking model is refined.

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).

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The European seismic code 8 (Eurocode 8) classifies buildings as planwise regular according to four criteria which are mostly qualitative and a fifth one, which is based on parameters such as stiffness, eccentricity, and torsional radius, that can be only approximately defined for multistory buildings. Therefore, such plan-regularity criteria are in need of improvement. ASCE seismic code, according to a different criterion, considers plan (or “torsional”) irregularity in a building when the maximum story drift, at one end of the structure, exceeds more than 1.2 times the average of the story drifts at the two ends of the structure under equivalent static analysis. Nevertheless, both the ASCE approach and the threshold value of 1.2 need to be supported by adequate background studies, based also on nonlinear seismic analysis. In this paper, a numerical analysis is carried out, by studying the seismic response of an existing R/C school building taken as the reference structure. Linear static analysis is developed by progressively shifting the centre of mass, until the ratio between the maximum lateral displacement of the floor at the level is considered and the average of the horizontal displacements at extreme positions of the floor at the same level matches and even exceeds the value of 1.2. Then, nonlinear dynamic analyses are carried out to check the corresponding level of response irregularity in terms of uneven plan distribution of deformation and displacement demands and performance parameters. The above comparison leads to check the suitability of the ASCE approach and, in particular, of the threshold value of 1.2 for identifying buildings plan irregularity.

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