Experimental and analytical investigation of punching shear capacity of biaxial voided slabs
Introduction
Reinforced concrete (RC) voided slabs are adopted in modern construction as alternate for conventional solid slab systems in flat slab buildings. These types of slabs are constructed with plastic void formers having spherical, donut, oval, or cuboid shapes [[1], [2], [3], [4], [5]] and reduce their self-weight up to 50% in comparison with RC solid slab without any significant change in its flexural capacity [[6], [7], [8]]. However, a considerable reduction in the shear capacity of the voided slab is reported. One-way shear capacity of the biaxial hollow slab with rounded box and donut type hollow void formers showed 40% and 27% reduction in comparison with that of the solid slab, respectively [9]. Similarly, the reduction in punching shear capacity (two-way shear) was studied by researchers with various shape of void formers [1,[10], [11], [12], [13], [14], [15], [16], [17]] and reported that it decreases up to 40% in comparison with conventional RC solid slab [1]. The reduction in punching shear capacity of cylinder voided slab with 10% volume void ratio was observed to up 30% in comparison with reference solid slab [18]. The punching shear capacity of slab with donut type void shape was 87% of that of solid slab and the critical failure section was observed to be in the range of 0.5–2.5 times of effective depth of slab (d) from face of the column which depends on the number of void formers in that section [14,19]. The slab specimens with cylinder shape voids carried 50–70% of the punching shear capacity of solid slab [17]. The reduction of punching shear capacity of the slab with sphere shape void was about 4.41–18% and 14.7–29.4% for slabs with voids at sections located at 2d and d from face of the column, respectively [16]. Further, in the same study it was observed that the perimeter of the critical failure section in voided slabs was 4.2–41.7% higher than that of solid slabs. The punching shear capacity of the specimens with plastic units of box type voids (hexahedron with rounded edges) and solid cross-shaped parts was 43% and 18% lower than that of the specimens without voids [13]. These studies explore that the punching shear capacity of voided slab is highly dependent on the shape and location (from face of the column) of voids.
In this paper, punching shear capacity of the biaxial voided slab is calculated based on the conventional method or equations given in standards such as ACI 318-14 [20], EN 1992-1-1 2004 [21] and IS 456 2000 [22]. The presence of voids in the prediction of actual punching shear capacity of biaxial voided slabs is considered by adopting the effective concrete area available to resist the punching shear. In addition, experiments were carried out on eight full-scale specimens to understand the effect of shape, location and size of void formers on the punching shear capacity of the biaxial voided slab. The experimental results based on present study and the test data available in the literature (33 specimens) were compared with the estimations by provisions in the building standards (specified for solid slabs), and a modified equation is proposed (accounting for reduced concrete area).
Section snippets
Experimental program
Punching shear test helps to investigate the behaviour of slab-column connection. Eight full-scale specimens were tested – one RC solid slab, four sphere and three cuboid voided slabs. The behaviour of slab systems is expressed in terms of load versus displacement plot and, the results are compared using load carrying capacity corresponding to ultimate failure.
Load displacement behaviour
All slab specimens showed typical punching shear failure. The load versus mid-span displacement (LVDT 3) for all the eight tested specimens is shown in Fig. 7. By comparing the plots for voided slab specimen V1 and reference solid slab S, it can be observed that both the specimens show nearly identical load – displacement behaviour. Such an observation can also be made from the experimental test data (PS and PD-N-4) of Chung et al. [19]. This is reasonable because, after cracking, a little
Punching shear capacity prediction of solid slabs by various standards
The punching shear capacity of the solid slab can be predicted by various standards, such as ACI 318 2014 [20], EN 1992-1-1 2004 [21] and IS 456 2000 [22]. It varies for each standards based on three key parameters such as (i) critical section for punching shear which governs the control perimeter, (ii) permissible shear strength of concrete, and (iii) effect of flexural reinforcement. The equations to predict the punching shear capacity of solid slab with these parameters are summarised below.
Experimental test data and prediction of punching shear capacity
The experimental results based on present study (7 voided slab) and the test data available in the literature (33 specimens) were compared with the estimations by provisions in the building standards (specified for solid slabs). The details and experimental results of voided slab specimens (in total 40 test specimens) are summarised in Table 4. These test specimens covers wide range of void former shapes (sphere, cylinder, donut, hexahedron and cuboid), void locations from the column face (0.07d
Conclusions
The following observations are drawn based on the experimental investigation of biaxial voided slab under punching shear and comparing the results with the predictions by relevant code provisions (ACI 318-14, EN 1992-1-1 2004 and IS 456 2000).
- 1.
The load – displacement behaviour of voided slab (voids located beyond 0.5d distance from face of column) shows insignificant difference in comparison with reference solid slab.
- 2.
The estimation of punching shear capacity of biaxial voided slab by existing
Acknowledgement
This work was supported by Department of Science and Technology, Ministry of Science and Technology, India (SR/S3/MERC/0040/2012) and M/s Post Tension Services India Pvt. Ltd. (PTSI), Vadodara, Gujarat, India (WO/GEN/0001/16-17). The authors wish to acknowledge the assistance and facilities offered by Technical Staff, Structural Engineering Laboratory, IIT Madras.
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