Elsevier

Structures

Volume 27, October 2020, Pages 894-902
Structures

An experimental study on post-punching behavior of flat slabs

https://doi.org/10.1016/j.istruc.2020.06.036Get rights and content

Highlights

  • The increase of the concrete cover enhances post-punching strength.

  • Decrease of the diameter of tensile reinforcement, the post-punching strength increases.

  • Truss reinforcement increases punching and post-punching strengths.

  • Bent-up integrity reinforcement increases punching and post-punching strength.

  • Additional bent-up reinforcement was not effective on the post-punching strength.

Abstract

If unpredictable loads are applied to flat slab-column connections, punching shear failure occurs with almost no warning signs. To prevent progressive collapse of flat slab-column connections, it is necessary to provide a secondary load carrying mechanism after punching shear failure. In this research, some suggestions for establishing an alternative mechanism in flat slab connections after punching failure are proposed. For this purpose, an experimental program was conducted to investigate the post-punching behavior of 17 slabs with various reinforcement layouts and concrete covers. The effects of integrity, compressive, shear, truss, and bent-up reinforcements, diameter of tensile reinforcement, and concrete cover of tensile reinforcement on the post-punching behavior of slab-column connections were studied. The results of the experiments indicate that the integrity reinforcement significantly improves the post-punching strength. The increase of the concrete cover of the tensile reinforcement and decrease of the diameter of the tensile reinforcement result in an increase of the post-punching strength. Bent-up integrity reinforcement increases the punching and post-punching strengths, simultaneously. Truss reinforcement significantly improves the punching and post-punching behavior.

Introduction

Punching shear failure is usually the critical failure mechanism for flat slab reinforced concrete structures [1]. Flat slabs structures are highly vulnerable to collapse [2], [3]. After a punching shear failure, the load carried by the slab-column connection redistributes the load to adjacent connections, and hence, the load increases rapidly in these connections. Consequently, they fail in punching shear due to overloading. This could result in the fall of the slab onto the slab below, thereby propagating the collapse both horizontally and vertically throughout the structure and could lead to progressive collapse of the structure [4]. The key to avoiding these failures is to provide a secondary load carrying mechanism after a slab-column connection has failed in punching shear [2], [3], [4], [5], [6], [7]. Over the past decades, several failures have occurred that resulted in progressive collapses [8], [9], [10], [11]. Under increasing shear load in flat slab-column connections, tangential and diagonal micro cracks form around column [12], [13]. Therefore, reinforcements passing directly over the cracks play a significant role in transferring of shear forces (Fig. 1). Once the shear crack crosses the longitudinal reinforcements, reinforcing bars contribute to the shear transfer by dowel action [7]. As Fig. 2 shows, there are two main modes of failure for dowel action. The first mode (Mode I) is yielding of the reinforcing bar and crushing of the concrete supporting the bar, simultaneously (Fig. 2-a). As Fig. 2-b shows, the second mode (Mode II) is splitting or spalling of concrete [14], [15], [16]. Experimental studies have indicated that if the concrete thickness in the direction of shear force is larger than six to seven times the bar diameter, the mode of failure is mode I. Failure mode II occurs for smaller concrete thickness [14], [17]. Owing to low concrete thickness of the tensile reinforcement, the failure mode of this reinforcement is mode II; therefore, the contribution of tensile reinforcement to the post-punching strength is small [18]. The failure mode of integrity reinforcement (Slab bottom reinforcement passing directly over the column) is mode I; therefore, the main contribution to the post-punching shear transfer is provided by the integrity reinforcement [4], [18]. The integrity reinforcement can prevent the slab from falling down and progressive collapse [19]. As Fig. 1 shows, the column punches through the slab, the tensile reinforcement tears out of the concrete slab in zone 1 causing concrete spalling, and the integrity reinforcing bars breakout concrete in zones 2 [20], [21].

Hawkins and Mitchell [18] recommended the provision of effectively continuous bottom reinforcement passing through the columns, later termed “structural integrity reinforcement”. Mitchell and Cook [6] stated that “Resisting mechanisms developed after the occurrence of initial failures are described along with simple design and detailing recommendations necessary to enable the damaged slab to hang from its supports”. These design and detailing requirements result in effectively continuous bottom reinforcements along column lines that are well anchored into the column or support regions”. Melo and Regan [1] indicated that bottom reinforcements passing through a column and anchored in the slab to either side of it can be highly effective in increasing the post-punching resistance of a slab-column connection. They reported that the angle of inclination of the integrity reinforcements at failure in the vicinity of the column face varied from 22° to 26°. Mirzai and Muttoni [20] concluded that tensile reinforcement provided a limited post-punching capacity when it was not suitably anchored on the soffit of the slab and the integrity reinforcement could increase the residual strength of flat slabs. They advised the use of high-ductility reinforcement for the integrity reinforcements [20]. Habibi et al. [4] showed that the increase of the slab thickness increased the post-punching resistance of slab–column connections.

In this paper, an extensive experimental program was performed to investigate the post-punching behaviour of 17 slabs with various reinforcement layouts and cover concrete. The purpose of the current study is to conduct research on the effects of integrity, compressive, shear, truss, and bent-up reinforcements, diameter of tensile reinforcements, and concrete cover of tensile reinforcements on the post-punching behaviour of slab-column connections. It should be noted that these reinforcements pass through the shear punching crack.

Section snippets

Setup of experimental work and details specimens

Design assumptions included a concrete type C30, reinforcing steel yielding strength of 400 MPa, and reinforcing steel ultimate strength of 600 MPa. In concrete mixture, the maximum aggregate size was 25 mm, and water-cement ratio was approximately 0.45. Table 1 shows the concrete mixture used for the specimens. Table 2, Table 3 show the properties of reinforcing bars and concrete in specimens, respectively. Tensile and compressive reinforcements were designed according to ACI-318-14 [22]. The

Experimental results

In this part, the experimental results of 17 specimens are investigated. The results included shear-displacement curves for all specimens. Table 5 shows the summarized results of these experiments. The displacement is the central deflection of the slab specimens.

Conclusions

In this study, 17 half-scale slabs were tested to study the behaviour of flat slab-column connections after the punching shear failure. The effects of integrity reinforcement, compressive reinforcement, diameter of tensile reinforcement, concrete cover of tensile reinforcement, shear reinforcement, truss reinforcement and bent-up integrity reinforcement on post-punching strength have been investigated. Based on the test results, the following conclusions are drawn:

  • 1.

    The ratio of post-punching

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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