Shear strength analysis of slabs without transverse reinforcement under concentrated loads according to ABNT NBR 6118:2014

Abstract

Concentrated loads in slabs without transverse reinforcement, usual in highway bridges, result in the horizontal spreading of the shear force towards the supports, situation in which not all the slab width contributes in the shear strength. Based on this, the analytical models of shear strength and punching capacity in slabs may not be suitable to deal with this loading. Since this topic is not widely discussed in the national technical literature, the paper aims to present contributions to these analyses with a focus on the accuracy level of the shear strength analytical models recommended by ABNT NBR 6118:2014. Therefore, the models available in the Brazilian code were applied to an experimental database with 118 test results and the results obtained by the Brazilian and European codes were compared. The results demonstrated that, as presented in the Brazilian code, shear strength model in one-way slabs can lead to unsafe resistance predictions while the punching capacity model can lead to very conservative predictions. From the analysis, it is concluded that considering the reduction of the shear force, in the case of loads distributed in small areas close to the support in slabs, and the use of more suitable procedures to define the effective width, it is possible to improve the level of accuracy of relations between experimental and theoretical values, but this still leads to high percentages of unsafe predictions of resistance (> 40%).

Shear and Punching Capacity Predictions for One-Way Slabs under Concentrated Loads Considering the Transition between Failure Mechanisms

“…Nowadays, the most widespread approach to defining the effective shear width b eff [3][4][5]12], also referred to as the French approach, is based on the horizontal load spreading toward the supports from the back sides of the load (Figure 7a). In the French approach, the angle of this horizontal spreading is fixed at 45 degrees.…”

“…Consequently, enhanced recommendations to assess the punching capacity of such slabs using analytical code expressions are scarce [20]. Until now, most publications showed a large scatter between tested and predicted resistances of one-way slabs under concentrated loads when using punching shear expressions [12,26]. Since such slabs may fail either by one-way shear or two-way shear (punching), this study addressed both shear failure mechanisms.…”

“…In the literature, a large scatter between tested and predicted resistances is commonly reported for punching capacity predictions of one-way slabs under concentrated loads [12,26]. In practice, this occurs mainly due to two reasons: (i) most design codes do not discuss how the enhanced shear capacity for loads close to the support could be considered in punching capacity predictions; and (ii) the effect of the free edges disturbing the contribution of the control perimeter lateral sides is not considered in the calculations (Figure 2c).…”

“…In the last approach, the effective shear width beff is frequently multiplied by the unitary shear resistance (vR) in comparing tested and predicted resistances. In some studies [5,12], different models of the effective shear width were tested with the current Eurocode shear expressions. Between the main results, it was observed that the French effective shear width provides good predictions of one-way shear capacity when the load is placed close to the support (av/dl ≤ 2) or when the slabs fail in one-way shear as wide beams [13].…”

Recommendations to assess the shear and punching capacity of one-way slabs under concentrated loads according to the ABNT NBR 6118:2014

“…Nevertheless, until now, most publications addressed only how to check the one-way shear capacity of such slabs [4], [6], [7], [9], sometimes neglecting that the evaluated slabs failed by punching. Additionally, most analytical approaches to evaluate the one-way shear capacity or punching capacity of these slabs with design code expressions show a large scatter between tested and predicted resistances [3], [10]. In practice, most available approaches to evaluate the one-way shear capacity of these slabs overestimate the contributing slab strip to the shear capacity (frequently named the effective shear width beff) when the distance from the load to the support increases [3], [9].…”