Experimental study on punching shear behavior of hollow floor slab-column reinforced connection

Abstract

This article presents an experimental study on hollow floor slab-column-reinforced connections, which are enhanced by installing locally solid zone of slab around the column and hidden beam in the floor. To investigate the punching-shear behavior of hollow floor slab-column-reinforced connections, six hollow floor slab-column-reinforced connections under vertical load were conducted on three types of connections with different thickness, namely, two hollow floor slab-column-reinforced connections without punching component, two hollow floor slab-column-reinforced connections with bent-up steel bars, and two hollow floor slab-column-reinforced connections with welding section steel cross bridging. Meanwhile, the strength, stiffness, failure mode, and ductility of hollow floor slab-column-reinforced connections with punching components were obtained and compared with the hollow floor slab-column-reinforced connections without punching component. The results showed that hollow floor slab-column-reinforced connections had the double failure characteristics including punching shear and flexural failure, and flexural failure was the main failure mode as a result of installing hidden beam. The hollow floor slab-column-reinforced connections with punching components exhibited higher initial stiffness and higher loading capacity than hollow floor slab-column-reinforced connections without punching components, but welding section steel cross bridging have a better on improving the connections’ punching-shear capacity than bent-up steel bars.

Keywords

failure mode hollow floor punching component punching-shear behavior slab-column connection

Introduction

Reinforced concrete slab–column structure systems, which have no beams, are competitive structural system in buildings. This structural system has been used widely in the office buildings, parking garages, and apartments because of its advantages, such as conserved material, reduced formwork, easer construction, and flexible partition. In addition, slab–column structure systems have more stories in comparison to beam–column frame or bearing wall systems (Rha et al., 2014). However, when exposed to large vertical forces or horizontal deformations, especially during an earthquake, this type of structure is vulnerable to punching-shear failure at the slab–column connections.

This type of failure is usually brittle, particularly if no shear reinforcement is used in the slab around the column. In some cases, the failure of a joint may cause the failure of the adjacent joints, triggering a progressive collapse of part or even the entire building (Meli and Rodriguez, 1988). Even if punching-shear failure is not observed, this type of structural system is not very efficient for energy dissipation, a highly important structural property, especially for seismic performance. The most typical methods of mitigating punching-shear failure at a slab–column connection are the following: (1) increasing the area of concrete resisting shear stresses, which can be achieved by increasing the thickness of the slab, providing a drop panel or column cap, as shown in Figure 1; (2) providing concrete and reinforcement of higher strength; (3) providing punching components resisting shear stresses; (4) providing high-performance material, including concrete and reinforcement, such as carbon fiber–reinforced polymer (CFRP) sheets and glass fiber–reinforced polymer (GFRP) reinforcing bars; and (5) decreasing the deadweight of floor, which can be achieved by applying tube filler in floor to form hollow floor.

Figure 1.

Slab–column structure with drop panel or column cap in Chinese code GB50010-2010 (code for design of concrete structures): (a) drop panel and (b) column cap.

Increasing thickness is an effective method to improve the ultimate bearing capacity and stiffness (Birkle and Dilger, 2008); however, slab–column structure tends to emerge brittle failure because of the thick plate, which results in bringing burden to bearing capacity of connection. Therefore, drop panel or column capital is a good means to resist shear stresses in order to avoid thick plate (Megally and Ghali, 2000).

In contrast to thick plate and column capital, increasing the strength of concrete and reinforcement are easier to be realized at slab–column structure, which is more liable to be decorated without drop panel or column capital. Many studies have been conducted to analyze the influence of reinforcement ratio on the punching-shear behavior; the results showed that the ultimate capacity and stiffness increased as reinforcement ratio increased (Theodorakopoulos and Swamy, 2002).

The first two methods are effective in increasing punching strength but not ductility (Megally and Ghali, 2000). Adding shear reinforcement increases both strength and ductility, and it is also the most practical way of retrofitting existing slab–column connections, which is the primary motivation of this research. Research on including shear reinforcement in new slabs was initiated by Wheeler (1936). However, over the past decades, a significant amount of researches have been dealt with various types of shear reinforcement for concrete slab–column connections in order to prevent punching-shear failure, for example, post-installed shear bolts (Adetifa and Polak, 2005; Bu and Polak, 2009; Fernández Ruiz et al., 2010; Harajli et al., 2006); stirrups (Glikman et al., 2017; Robertson et al., 2002; Song et al., 2012); continuous cages, two-leg stirrups, and so on (Einpaul et al., 2016); shear studs (Fernández Ruiz and Muttoni, 2009; Simões et al., 2018; Song et al., 2012); ductility reinforcement (Ghali, 2007); shear bands (Kang et al., 2017; Song et al., 2012); lattice shear reinforcement (Park et al., 2007); bend bar (Mirzaei, 2008; Swamy and Ali, 1982); post-tensioned bar (Kang and Wallace, 2006); double hooks (Lian et al., 2000); and thin plate stirrups (Kang and Wallace, 2008).

In the past decades, the high-performance material has been continuously applied to slab–column structure, such as CFRP sheets (El-Enein et al., 2014; Harajli and Soudki, 2003; Soudki et al., 2012) and GFRP reinforcing bars (Nguyen-Minh and Rovňák, 2013). Comparing with the punching component, fiber-reinforced polymer (FRP) is not only a noncorrosive and nonmagnetic material with a high strength-to-weight ratio, but it also provides the possibility of embedding microwire sensors into the matrix, thus increasing the attractiveness of usage of FRP bars in concrete structures as a kind of “smart” reinforcement. Adding FRP reinforcement to the tension face of concrete slabs will increase the punching capacity just as increasing the bottom steel reinforcement ratio.

Due to the special constructional features of slab-column, the higher weight of floor makes the slab–column structure easier to brittle punching failure. Thus, in recent years, hollow floor has been popular in slab–column structure on account of the advantage of lightweight. Hollow floor slab–column structure system can not only have the advantages of slab–column structure but also have lightweight, effective sound insulation, energy conservation, smaller earthquakes effects, and so on (Araujo et al., 2011). In 2008, Wang et al. (2008) tested nine simply supported interior hollow floor slab–column connections subjected to vertical loading only. They concluded that punching-shear strength of reinforced concrete hollow slab is affected by effective thickness of slab, the ratio of hollowness, the concrete strength, the strength and reinforcement ratio, and loading area. Gong et al. (2013) tested nine hollow slab–column connections under punching-shear force in which thickness, rib width, and hollow ratio were taken in account, concluding that the punching-shear capacity of hollow floor decreased as hollow ratio increased, as shown in Figure 2.

Figure 2.

Load—deflection curves of specimens with different hollow ratio.

In the research presented in this article, it is noticed that many research works have been conducted to improve the punching-shear capacity and deformation capacity, which prevent the punching failure of slab–column connection. However, there were few reports on the punching-shear behavior of slab-column which consider punching component and locally solid zone of slab around the column simultaneously, available in the literature.

The main aim of this article is to study the punching-shear behavior of hollow floor slab-column-reinforced connection (HFSC) with punching component and locally solid zone of slab around the column. Experiments were carried out on three pairs of half-scale HFSCs with or without punching components, subjected to vertical capacity only. The punching-shear behavior of connection without punching component was compared with that of bent-up steel bar (BSB) and welding section steel cross bridging (WSSCB). Research results can provide reference and basis for analytical and engineering meanings of hollow floor slab–column structure system.

Experimental program

Specimen designs

In this study, the half-scale specimens can be regarded as taken from a prototype structure in which the flat slab spans 7200 mm between columns. The slabs were supported on the bottom surfaces at the lines of contraflexure under static gravity loads, located at 1280 mm × 1280 mm perimeter. The slabs were square in plan, 1400 mm × 1400 mm, to allow for proper anchorage of the flexural bars past the simple supports. The slabs had top and bottom 300 mm × 300 mm column stubs extending 700 and 400 mm from the center of the slab (Figure 3).

Figure 3.

Details of the specimens (dimensions in mm): (a) plane of hollow floor for 120 mm thickness and (b) plane of hollow floor for 140 mm thickness.

Experiments were carried out on three pairs of HFSCs subjected to vertical capacity only, the first one made with hidden beam only, the second one made with hidden beam and BSBs, and the last one made with hidden beam and WSSCB. Every pair has 120- and 140-mm thick slabs with 10-mm concrete cover, as shown in Figures 4 and 5. Hidden beam is arranged along the vertical and horizontal axis of the column as the same wide with the column (Figure 5(d)). In order to mitigate the bending load-carrying capacity and seismic behavior in-plane two directions, tube filler is arranged orthogonally for all of specimens. The 120 and 140 mm thickness of slab have the hollow ratio of 25.73% and 32.96%, respectively. Table 1 and Figures 3 to 5 provide the main information of six specimens.

Figure 4.

Details of punching component (dimensions in mm): (a) sketch map of BSBs, (b) photo of BSBs, (c) detail of construction for BSBs, (d) sketch map of WSSCB, and (e) photo of WSSCB with symmetrical configuration.

Figure 5.

Details of slab–column connections (dimensions in mm): (a) connections without punching component (A-A),(b) connections with BSBs (A-A), (c) connections with WSSCB (A-A), (d) details of hidden beam, and (e) details of column.

Table 1.

Main parameters of specimens.

Specimen	h (mm)	Pipe diameter (mm)	Rib width (mm)	Hollow ratio (%)	Details of reinforcement	Reinforcement ratio (%)	Punching component
HFSC1	120	63	35	25.73	C8@100	0.55	–
HFSC2	140	90	35	32.96	C8@100	0.55	–
HFSC3	120	63	35	25.73	C8@100	0.55	BSBs
HFSC4	140	90	35	32.96	C8@100	0.55	BSBs
HFSC5	120	63	35	25.73	C8@100	0.55	WSSCB
HFSC6	140	90	35	32.96	C8@100	0.55	WSSCB

HFSC: hollow floor slab-column-reinforced connection; BSB: bent-up steel bar; WSSCB: welding section steel cross bridging.

Note: HFSC1 and HFSC2 served as basic specimens.

Material properties

All specimens were cast using ready-mixed concrete. Concrete cubes (150 × 150 × 150 mm³) were prepared and tested for its cube-compressive strength when tests were carried out. The mean concrete cube strength for six specimens is 31.15 MPa. Yield stress f_y and ultimate tensile strength f_u for reinforcing bars and channel steel were obtained by testing both the test coupons and actual bars according to Chinese Standard GB/T228.1-2010 (2010). Table 2 shows the average f_y, f_u, yield strain ε_u, and elongation.

Table 2.

Tensile test results of steel.

Material	Steel size (mm)	f_y (MPa)	f_u (MPa)	ε_u (με)	Elongation (%)
HRB400	6	530.00	656.67	2650.00	16.32
	8	535.00	666.67	2675.00	19.6
	16	463.33	643.33	2316.67	17.27
Q235B (web)	4.8	296.67	355.00	1483.33	28.34

Test setup

The specimens were tested in a testing frame as shown in Figure 6. The slabs were simply supported by four steel frames that were all 940-mm tall (Figure 6(a)). Four round steel bars of 60-mm diameter were used to support the slab in each direction, two were 1400 mm in length, the others were 1200 mm in length. Moreover, the round steel was connected by a π-type steel frame welding with equal angle steel, which was anchored by the means of oblong holes at the steel girder (Figure 6(b)). Oblong holes were used to provide free rotation and negligible in-plane restraint at the edges of the slabs. To prevent shear failure in plate edges, steel plates measured 14-mm thick, 120-mm wide, and equal length with round steel were inserted between slab bottom and round steel (Figure 6(c)). Figure 6(d) shows the test setup under the loading frame.

Figure 6.

Test setup: (a) photo of the test setup, (b) hinged support, (c) plate bottom supporting plate, and (d) sketch map.

Test content

To ensure full contact between the bottom surface of the concrete slab and the top surface of the steel plate, preload was conducted at 10 kN each step with three steps before formal load. Following the verification of instrumentation functionality, a hydraulic jack with a capacity of 1000 kN and a ±100-mm displacement meter was used to apply the monotonic load up to failure at a load control rate of 10 kN/min. The load was continuously increased until slab failed as specified in Chinese Standard GB/T 50152-2012 (2012). The formation of cracks on the sides and bottom surface of the slabs and corresponding loads were marked and recorded during the test.

For each specimen, five linear variable differential transducers (LVDT) were arranged to the deformability of the connection. One LVDT at the center of bottom column was installed to measure the central slab–column connections’ deflection, and four LVDTs were placed at the corner to assess the warping performance, as shown in Figure 6(a). According to mechanical characteristics of slab–column connection under the vertical load, several electrical gauges were arranged at the HFSCs. Figure 7 shows the location of the gauges. Eight electrical strain gauges were attached to the top surface of slab to measure the concrete compressive strains, as shown in Figure 7(a). Figure 7(b) and (c) shows the measuring point of top reinforcement bars and bottom reinforcement bars. Some electrical gauges were placed to measure strains at the center of oblique section of BSBs (Figure 7(d)). A number of electrical gauges were installed at the different positions on the WSSCB, including top flange (150 mm from the cylinder edge), bottom flange (250 mm from the cylinder edge), and the web (200 mm from the cylinder edge), as shown in Figure 7(e) and (f).

Figure 7.

Strain measuring of specimens: (a) concrete of top plate, (b) reinforcement bars of bottom plate, (c) reinforcement bars of top plate, (d) BSBs, (e) top flange of WSSCB, and (f) bottom flange of WSSCB.

Test results and discussion

The test results are presented in terms of the cracking pattern, deflections, strains in concrete, reinforcing bars and punching components, ultimate capacity, and mode of failure. Table 3 summarizes the test results.

Table 3.

Mechanical behaviors of specimens and failure mode.

Specimen	P_cr (kN)	P (kN)	K_i (kN mm^–1)	K_u (kN mm^–1)	K_u/K_i	S_Δu (kN m)	Δ_u (mm)	Δ_y (mm)	μ
HFSC1	80	395	29.89	9.78	0.33	13.02	41.72	8.44	4.94
HFSC2	90	426	41.11	17.78	0.43	13.28	38.63	7.32	5.28
HFSC3	100	414	44.8	11.57	0.26	12.75	39.69	7.97	4.98
HFSC4	120	439	61.68	17.56	0.28	11.12	31.24	5.62	5.59
HFSC5	100	460	50.44	24.18	0.48	12.36	32.83	5.98	5.49
HFSC6	110	568	60.50	24.75	0.41	16.61	33.83	6.81	4.97

HFSC: hollow floor slab-column-reinforced connection.

Cracking pattern and failure mode

The first load cracks observed during testing are given in Table 3. These cracking loads to peak load can be seen to range from about 19.36% for HFSC6 to about 27.33% for HFSC4. Cracking developments were also tracked up to failure and photographs were taken at the end of test. Figure 8 shows the schematic diagram of test specimens’ fracture distributes, where the blue lines represent the initial cracks and the red lines represent the main cracks.

Figure 8.

Schematic diagram of fracture distribute: (a) HFSC1, (b) HFSC2, (c) HFSC2, (d) HFSC4, (e) HFSC5, and (f) HFSC6.

Generally, failure mode of slab–column connections can be classified as punching-shear failure and flexural failure according to the cracking pattern. Punching-shear failure includes a strip of circular main crack around the column, forming a punching vertebral body during the formation of main crack. Flexural failure consists of some diagonal main cracks from the corner of column to the corner of slab, which can divide the slab into several rigid bodies. However, under vertical load, a number of slab–column connections have double failure characteristics including punching-shear failure and flexural failure, which can be called flexural shear failure.

For specimens HFSC1 and HFSC2 tested under vertical load, the cracks were first formed tangentially around the edge of the column. As the applied load increased, more cracks subsequently were observed at the slab bottom surface, and then propagated diagonally to the corner of slab from the corner of column. At applied load of 140 kN for HFSC1, and 150 kN for HFSC2, four diagonal cracks were all appeared before the tangential cracks outside the circumference of the column. Simultaneously, the tangential crack outside the circumference of the column was first observed. The number of cracks and crack width increased with increasing loads. Subsequently, when the applied load increased to 160 kN for HFSC1 and 210 kN HFSC2, four tangential cracks outside the circumference of the column were formed after four diagonal cracks. This implied that flexural failure occurred before punching-shear failure. Figure 9(a) shows the average values of four tangential cracks to the circumference of the column. It can be found that the average value of circular main crack to column were 51.50 mm for HFSC1 and 50.50 mm for HFSC2, which were approximately equal to 0.5h₀ (effective thickness). Soon afterwards, the widths of initial cracks continued to increase to form the main cracks. Moreover, the first cracks at the top surface of slabs started to be noticed at the yield load. As the applied load increased to the peak load, it should be mentioned that the main crack of HFSC1 consists of annular crack near column and diagonal “X” shape crack, which got larger constantly and developed into the plastic hinge line. The slabs were divided into several pieces by plastic hinge line, which indicate that the connections have the characteristic of flexural failure.

For specimens HFSC3 and HFSC4, constructed with BSBs, the crack patterns were similar to specimens HFSC1 and HFSC2 at the end of test. This indicated that bend steel bars could not change crack pattern of the HFSCs. Nevertheless, it can be noticed from Figure 9(a) that the average value of circular main crack to column for specimens HFSC3 and HFSC4 appears to be larger than that for the same thickness specimens HFSC1 and HFSC2 by about 44.67% and 21.29%, respectively. This suggested that the circular main crack around the column was transferred outwards by bend steel bars.

Figure 9.

Circular main cracks: (a) the first cracks for all specimens and (b) the second cracks for specimen HFSC5 and HFSC6.

Specimens HFSC5 and HFSC6 constructed with WSSCB show the different crack pattern in comparing to specimens HFSC1, HFSC2, HFSC3, and HFSC4. Similarly, the first circular main crack was noticed sooner than diagonal main cracks. However, at applied load of 370 and 380 kN for HFSC5 and HFSC6, the second circular main crack appeared about 30 mm outside of WSSCB. In addition, cracks of specimens HFSC5 and HFSC6 were observed to be more in number and narrower in width than other specimens. From Figure 9, the average value of first circular main crack to column for specimens HFSC5 and HFSC6 were almost same as specimens HFSC1 and HFSC2. The average values of second circular main crack to column were 278.25 and 275.00 mm for specimens HFSC5 and HFSC6, respectively. The reason why specimens HFSC5 and HFSC6 had the second circular main cracks is that WSSCB enhanced the stiffness of concrete and formed the stiffness mutation at the boom end of WSSCB. Unfortunately, the crack pattern of specimen HFSC5 was not symmetrical due to the damage of western WSSCB at the load 456 kN (Figure 8(e)).

Ultimate capacity

Figure 10 provides the applied load versus the maximum deflection at lower columns for all specimens. It is clear from Figure 10 that all specimens have the similar curves up to failure and the load-deflection curves are smooth. In contrast to HFSC1 and HFSC2, the punching components can increase the stiffness and ultimate bearing capacity, moreover, the WSSCB has a better effect. In addition, all specimens had still excellent work performance, high strength, and good deformation capacity at failure stage. It can be concluded that hidden beam can delay stiffness degradation, improve the deformation ability and shift the failure mode from the punching failure to flexural failure.

Figure 10.

Load—deflection curves of specimens.

Table 3 provides the punching-shear capacities for all tested slabs. It is noticed that the cracking load (P_cr) and peaking load (P) ranged from 80 to 120 kN and 395 to 568 kN, respectively. As evidenced in Table 3, HFSC6 showed the highest punching-shear capacity (568 kN), a result of constructing with larger thickness and WSSCB. Comparing with specimens HFSC1 and HFSC3 with 120-mm thickness of slab, as one might expect, it can be seen that increasing thickness can improve the punching-shear capacity of slab connections: HFSC2 and HFSC4 increased by 7.30% and 6.81%, respectively. In addition, the punching-shear capacity of specimen HFSC6 was larger than specimen HFSC5 by about 23.48%. However, the calculated value (23.48%) is not accurate and larger than true value due to specimen HFSC5 with earlier failure of WSSCB.

For the same thickness of specimens, it can be seen that the punching-shear capacity of HFSC3 and HFSC4, constructed with BSBs, were higher than those of HFSC1 and HFSC2 by about 4.8% and 3.1%, respectively. Specimens HFSC5 and HFSC6 containing the same thickness but constructing with WSSCB, the punching-shear capacity were higher than the basic specimens HFSC1 and HFSC2 by about16.4% and 33.3%, respectively. Based on the above analysis, it can be concluded that WSSCB is a more effective punching component than BSBs.

Ductility factor

The ductility factor (µ) can be used as a measure of deformation capability of the specimens. It is calculated by the method proposed by Pan and Moehle (1989), which can be proved to be suitable for slab–column connections. In the method, the ductility factor (μ) is defined as the ratio of ultimate displacement (Δ_u) to yield displacement (Δ_y), where ultimate displacement is defined by points corresponding to the applied load declined to 85% of the maximum load, and yield displacement is defined by points corresponding to 2/3 maximum load. The yield displacement, ultimate displacement, and ductility factor are shown in Table 3. It can be seen that the ductility factor ranged from 4.94 to 5.59. As for the reinforced concrete members subjected to shear, the failure mode can be referred to as the ductile failure when the ductility factor is higher than 3. This implies that the failure mode of all specimens has excellent deformation capacity.

Stiffness degradation and energy dissipation

Stiffness degradation is the reaction of crack propagation and plastic deformation of specimens, which is defined as the ratio of ultimate stiffness to post-cracking stiffness. The ultimate stiffness is the secant stiffness corresponding to the applied load declined to 85% of the maximum load and post-cracking stiffness is the secant stiffness corresponding to cracking load (Osman et al., 2000). The energy dissipation (S_Δ_u ), which is used to evaluate the energy dissipation capacity of the specimen, denotes the area between the curves and abscissa from 0 to Δ_u. As expected, it can be noticed from Table 3 that the BSB can increase ductility factor but has an adverse impact on the stiffness degradation and energy dissipation. However, the WSSCB has a better effect on improving the ultimate capacity and stiffness degradation than the BSBs.

Concrete strains

Each specimen was provided with eight top strain gauges in the vicinity of loading area, two orthogonal strain gauges in each direction, as shown in Figure 7(a). The applied load versus the concrete compressive strains for specimens with 140-mm thickness (HFSC2, HFSC4, and HFSC6) is given in Figure 11(a).

Figure 11.

Load–strain curves: (a) top concrete of HFSC2, HFSC4, and HFSC6, (b) the strain versus the distance to cylinder edge of HFSC1, (c) top bar and bottom bars of HFSC1 at the same position, (d) the same reinforcement bars of HFSC1, HFSC3, and HFSC5, (e) the same reinforcement bars of HFSC2, HFSC4, and HFSC6, (f) bent-up steel bars of HFSC4, (g) WSSCB’s flange of HFSC6, (h) WSSCB’s web of HFSC5, and (i) WSSCB’s web of HFSC6.

The measuring points (N-SP, N-CZ) at the north of slabs were selected to study the compression strength of concrete, where N-SP and N-CZ represent the measure point parallel to and perpendicular to corresponding cylinder edge in the north. The above test results show that the slab was divided into several bodies by plastic hinge line, which propagated diagonally to the corner of the slab from the corner of column. At the same load, it can be observed from Figure 11(a) that the measured strains of N-SP were higher than the measured strains of N-CZ on account of intensive compression among several rigid bodies.

Reinforcement strains

The strains in the longitudinal reinforcement were measured at several locations, as shown in Figure 7(b) and (c). Based on the obtained test results, the yield strain of longitudinal reinforcement was 2675 με. As expected, the first yielding of the longitudinal bars occurred under vertical load ranging from 339 to 368 kN for all specimens.

For specimen HFSC1, as shown in Figure 11(b), the strains were typically higher in the middle of the slab subjected to vertical load. The strains proportionally decreased toward the supports of the slabs at the same load. At applied load of 395 kN, the strain of measuring point S6 was far less than S7 (15.34% of S7). In general, with the same distance to the middle section of slab, Figure 11(c) affirms that bottom reinforcement bars showed higher strains, compared to top reinforcement bars. The strains of S22, S23, and S18 at the bottom of slab were 2760, 1476, and 1042 με at a load of 395 kN, that is, 1.58, 3.61, and 2.32 times of S10, S11, and S6, respectively.

Figure 11(d) and (e) provides the applied load versus the measured strains for specimens with 120-and 140-mm thickness of slabs, separately. The effect of the punching components can be seen in HFSC1-HFSC2, HFSC3-HFSC4, and HFSC5-HFSC6, which was arranged without punching component, with bent steel bars and with WSSCB, respectively. For measuring point S15, the strains in the bottom reinforcement were 3815, 3183, and 1879 με for HFSC1, HFSC3, and HFSC5 at a load of 395 kN, respectively. In addition, slab–column connection with BSBs for HFSC3 and with WSSCB for HFSC5 reduced the measuring strains by 16.57% and 50.75% in contrast with HFSC1, respectively. This confirms that punching components can help the reinforcement bars share the punching-shear resistance and hold back the cracking, which was in agreement with the experimental results.

BSBs strains

Six strain gauges were also measured on BSBs, as shown in Figure 7(d). Figure 11(f) shows the applied load versus the measured strains of the BSB. Very small strains on BSBs increased slowly until the cracking load. The strains of BSBs reached the yield strain (2675 με) at loads 340 and 360 kN for specimens HFSC3 and HFSC4, respectively. Most of the measuring points reached the yield strain except S26 for HFSC3. It was concluded from the Table 3 that the BSBs enhanced resistance for punching load with no apparent effects on improving the punching-shear capacity in comparison to WSSCB.

WSSCB strains

For specimen HFSC6, the 10 strain gauges (S31–S40) were arranged on WSSCB, S31–S35 and S36–S40 were placed at top and bottom of flange, as shown in Figure 7(e) and (f). In general, it is evident from Figure 11(g) that the strains of S31, S33, and S35 were less than zero, and S36, S37, and S38 were greater than zero. This confirms that top flange and bottom flange of WSSCB were in compression and in tension, respectively. At the peaking load (560 kN), the minimal strains at top and bottom flange were −788 and 3685 με.

Figure 11(h) and (i) provides the applied load versus the strains of web of the channel steel. The strains consisted of principal tensile strain and principal compressive strain. It was evident from Figure 11(i) that the WSSCB’s web of HFSC6 was mainly in tension. Moreover, the ratio of the tension strains to yield strain (1483 με) ranged from 1.52 to 5.00 for different measured points. For specimen HFSC5, the west weld of WSSCB was fractured at the applied load of 450 kN, the measuring loads fast decreased to 310 kN, then continued to increase to 460 kN. This conforms to the phenomenon that the strains first increased, then decreased and continued to increase with the applied load during the process of experiment.

Conclusion

To investigate the effects of punching components on punching resistance and identify the punching behavior of hollow floor slab–column connections, six half-scale experiments were conducted subjected to concentrated loading in this study. The used punching component included BSBs and WSSCB. Based on the presented results, few conclusions have been drawn as follows:

Although hollow, the specimens still have a higher punching-shear capacity and ductility factor of 4.94–5.59 with hidden beam and locally solid zone of slab around the column. There was no punching vertebral body appeared in hollow floor due to the fact that hidden beam can delay stiffness degradation, improve the deformation ability and shift the failure mode from the punching failure to flexural shear failure.

Punching behavior of hollow floor slab–column interior connections was improved by the use of either BSBs or WSSCB, including the punching-shear capacity and stiffness. Meanwhile, WSSCB has a remarkable effect on improving the punching strength. The punching-shear capacity was increased by average value of 3.95% for specimens with BSBs, and 24.85% for specimens with WSSCB.

The main crack of HFSCs consists of circular cracks near column and diagonal “X” shape cracks. However, HFSC5 and HFSC6 have the double transverse cracks in each direction. The first transverse cracks have the average distance of approximately 50 mm to column as with HFSC1 and HFSC2. The second transverse cracks, which can be regarded as punching failure surface appeared at the outside of WSSCB, have the average distance of approximately 276.7 mm to column. In comparison to the other specimens, the first transverse crack of HFSC3 and HFSC4 showed the biggest distance to column with the average value of 67.9 mm. This indicates that the BSBs can make the punching failure surfaces transfer outside of 0.5h₀.

Comparing with the specimens with punching components, the normal specimens showed the higher strains for the same position of reinforcement bars. In addition, most measuring points of punching components reached the yield strain. This provides evidence that punching component can help the reinforcement bars share the punching-shear resistance and hold back the cracking.

The HFSC is introduced to avoid brittle failure at the slab–column connections. Comparing with normal hollow slab–column connection, it can be seen that HFSC has a higher ultimate bearing capacity and better deformation capacity. However, the seismic behavior of slab–column structure with HFSC is ambiguous, especially the comparison with solid slab–column structure. As a consequence, the above experimental study can provide the foundation for the future studies, including the ultimate bearing-capacity calculation method, which considers reinforcement ratio, hollow ratio, and punching components simultaneously.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The research described in this article was supported by the National Natural Science Foundation of China (Grant No.: 51778214), the Scientific and Technological Project of Henan Province (Grant No.: 152102210066), and a Projected funded by the Shanghai Key Laboratory of Engineering Structure Safety, SRIBS (Grant No.: 16DZ1201805), which are gratefully acknowledged.

ORCID iD

Longji Dang

References

Adetifa

Polak

(2005) Retrofit of slab column interior connections using shear bolts. ACI Structural Journal 102(2): 268–274.

Araujo

Loriggio

Da Camara

(2011) Anchorage failure and shear design of hollow-core slabs. Structural Concrete 12(2): 109–119.

Birkle

Dilger

(2008) Influence of slab thickness on punching shear strength. ACI Structural Journal 105(2): 180–188.

Polak

(2009) Seismic retrofit of reinforced concrete slab-column connections using shear bolts. ACI Structural Journal 106(4): 514–522.

Einpaul

Brantschen

Fernández Ruiz

et al . (2016) Performance of punching shear reinforcement under gravity loading: influence of type and detailing. ACI Structural Journal 113(4): 827–838.

El-Enein

Azimi

Sennah

et al . (2014) Flexural strengthening of reinforced concrete slab–column connection using CFRP sheets. Construction and Building Materials 57(5): 126–137.

Fernández Ruiz

Muttoni

(2009) Applications of critical shear crack theory to punching of reinforced concrete slabs with transverse reinforcement. ACI Structural Journal 106(46): 485–494.

Fernández Ruiz

Muttoni

Kunz

(2010) Strengthening of flat slabs against punching shear using post-installed shear reinforcement. ACI Structural Journal 107(4): 434–442.

GB/T228.1-2010. Metallic materials—tensile testing—part 1: method of test at room temperature, Beijing, China: China Architecture & Building Press (in Chinese).

10.

GB/T50152-2012. Standard for test method of concrete structures. Beijing, China: China Architecture & Building Press (in Chinese).

11.

GB50010-2010. Code for design of concrete structures. Beijing, China: China Architecture & Building Press (in Chinese).

12.

Ghali

(2007) Flat plates in seismic areas: comparison of shear reinforcement systems. ACI Structural Journal 104(6): 712–724.

13.

Glikman

Polo

Bayrak

et al . (2017) Application of an inclined shear reinforcing assembly for slab-column connections. ACI Journal 321: 7.1–7.20.

14.

Gong

Zhu

Liang

et al . (2013) Experimental study on punching shear behavior of hollow slab-column interior connection. Journal of Southeast University (Natural Science Edition) 43(2): 420–424 (in Chinese).

15.

Harajli

Soudki

(2003) Shear strengthening of interior slab–column connections using carbon fiber-reinforced polymer sheets. Journal of Composites for Construction 7(2): 145–153.

16.

Harajli

Soudki

Kudsi

(2006) Strengthening of interior slab–column connections using a combination of FRP sheets and steel bolts. Journal of Composites for Construction 10(5): 399–409.

17.

Kang

THK

Wallace

(2006) Punching of reinforced and post-tensioned concrete slab-column connections. ACI Structural Journal 103(4): 531–540.

18.

Kang

THK

Wallace

(2008) Seismic performance of reinforced concrete slab-column connections with thin plate stirrups. ACI Structural Journal 105(5): 617–625.

19.

Kang

THK

Lee

et al . (2017) Punching and lateral cyclic behavior of slab-column connections with shear bands. ACI Structural Journal 114(5): 1075–1086.

20.

Lian

Zou

et al . (2000) The experiments of slab-column connections with double stirrups in punching resistance. Journal of Hunan University (Natural Sciences Edition) 27(5): 91–97 (in Chinese).

21.

Megally

Ghali

(2000) Seismic behavior of slab-column connections. Canadian Journal of Civil Engineering 27(1): 84–100.

22.

Meli

Rodriguez

(1988) Seismic behavior of waffle-flat plate buildings. Concrete International 10(7): 33–41.

23.

Mirzaei

(2008) Post punching behavior of reinforced slab-column connections. In: 7th fib PhD Symposium, Stuttgart, 11–13 September, pp. 1–8. Infoscience EPFL scientific publications.

24.

Nguyen-Minh

Rovňák

(2013) Punching shear resistance of interior GFRP reinforced slab-column connections. Journal of Composites for Construction 17(1): 2–13.

25.

Osman

Marzouk

Helmy

(2000) Behavior of high-strength lightweight concrete slabs under punching loads. ACI Structural Journal 97(3): 492–498.

26.

Pan

Moehle

(1989) Lateral displacement ductility of reinforced concrete flat plates. ACI Structural Journal 86(3): 250–258.

27.

Park

Ahn

Choi

et al . (2007) Lattice shear reinforcement for slab-column connections. ACI Structural Journal 104(3): 294–303.

28.

Rha

Kang

THK

Shin

et al . (2014) Gravity and lateral load-carrying capacities of reinforced concrete flat plate systems. ACI Structural Journal 111(4): 753–764.

29.

Robertson

Kawai

Lee

et al . (2002) Cyclic testing of slab-column connections with shear reinforcement. ACI Structural Journal 99(5): 605–613.

30.

Simões

Fernández Ruiz

Muttoni

(2018) Validation of the critical shear crack theory for punching of slabs without transverse reinforcement by means of a refined mechanical model. Structural Concrete 19: 191–216.

31.

Song

Kim

Song

et al . (2012) Effective punching shear and moment capacity of flat plate-column connection with shear reinforcements for lateral loading. International Journal of Concrete Structures and Materials 6(1): 19–29.

32.

Soudki

El-Sayed

Vanzwol

(2012) Strengthening of concrete slab-column connections using CFRP strips. Journal of King Saud University: Engineering Sciences 24(1): 25–33.

33.

Swamy

Ali

SAR

(1982) Punching shear behavior of reinforced slab-column connections made with steel fiber concrete. ACI Journal 79(5): 392–406.

34.

Theodorakopoulos

Swamy

(2002) Ultimate punching shear strength analysis of slab–column connections. Cement and Concrete Composites 24(6): 509–521.

35.

Wang

Sun

Wang

et al . (2008) Punching shear behavior of reinforced concrete hollow slab. In: 11Th ASCE aerospace division international conference earth and space (Earth and Space 2008), Long Beach, CA, 3–6 March, pp. 1–7. Reston, VA: ASCE.

36.

Wheeler

(1936) Thin flat-slab floors prove rigid under test. Engineering News Record 116(2): 49–50.