Behavior of steel–concrete composite bolted connector in precast reinforced concrete shear wall

Abstract

An assembling method of precast shear walls was previously proposed using steel–concrete composite bolted connectors. To further investigate the effectiveness and mechanical behavior of the proposed composite connector, 11 specimens were fabricated and tested under monotonic tensile loading. The test results provided comprehensive data (e.g. load, deformation, failure mode) on the effects of variation in the thickness of steel cap plate, concrete strength, bolt tension, and bolt diameter. Two typical failure modes were observed in the test: bearing failure and bolt shear failure. Finally, the equations for calculating the ultimate strength and yield strength of steel–concrete composite bolted connector are proposed in this article by reference to those of conventional bolted connection. The proposed calculations are demonstrated to be accurate enough through verification with the experimental data.

Keywords

composite structures curling high-strength bolts monotonic tensile test precast construction shear walls

Introduction

High-strength bolt connectors are widely used in steel structures. The advantages of bolted connection include convenient construction, better-quality control, as well as less labor cost. A series of experimental and theoretical studies were performed on the application of using the suitable steel structure connection method to novel structure systems, such as composite beam (Ataei et al., 2016), steel–concrete composite bridge (Suwaed and Karavasilis, 2017), and precast wall (Guo et al., 2019; Sun et al., 2016). Conventional “wet” methods are widely used in connecting adjacent precast shear wall panels, for instance, connecting reinforcing bars with lap splices and cast-in-situ concrete or with mechanical couplers and fresh cementitious grout (Aaleti et al., 2013). However, the two common types of assembling procedures both require cast-in-situ work that partly reduces the construction efficiency.

Sun et al. (2016) proposed a reinforced concrete precast shear wall utilizing high-strength bolts to connect the adjacent shear wall panels and form an integrated lateral load-resisting structural system. The horizontal connection in the proposed shear wall comes down to the steel–concrete composite bolted (SCCB) connector, which is made of three components: horizontal steel connector (H-connector), top/bottom steel units in shear wall panel, and high-strength bolts. To form a reliable load transfer route, steel units are cast onto the shear wall panel, and vertical steel bars are welded to the inner side of units before casting. The concept is illustrated in Figure 1.

Figure 1.

Schematic of the system concept.

Different from conventional bolted connections in steel structures, the SCCB connector has longer bolts and a sandwiched concrete part. Preliminary test results of walls (Sun et al., 2015, 2019) have confirmed the feasibility of the bolted assembling method and the influence of slippage in horizontal connection. Further finite element and theoretical analysis (Sun et al., 2016) showed that the external load was mainly transferred through three ways: friction, bearing action, and contact action. Furthermore, the finite element analysis also confirmed that the connector slippage had an impact on the deformation of the precast shear wall. Moreover, as a “dry” method, the SCCB connector used in the precast shear wall serves to transfer gravity load, lateral earthquake, and wind load through the shear wall segments at different heights. It is necessary to carry out in-depth studies on the mechanical behavior of the SCCB connector.

The behavior of the conventional bolted connection in steel structures has been extensively studied, both experimentally and analytically. Some of the studies on the performance of bolted connection are briefly summarized below. Kitipornchai et al. (1994) proposed an instantaneous and continuous bolt-slippage model to study the influence of joint slippage: two idealized bolt-slippage models were successfully introduced in the lattice steel-tower analysis. Ungkurapinan et al. (2003) conducted experiments on bolted joints in steel electric transmission towers and developed mathematical expressions to take the load–deformation behavior of bolted joint into account. Liu et al. (2015) conducted numerical studies of the high-strength friction-grip bolt shear connectors in sustainable composite beams, and the ultimate strength was investigated by finite element analysis. Teh and Uz (2015, 2016) proposed a series of design equations to determine the ultimate capacity of steel bolted connections. However, the physical reality of bolted connection is a complex problem involving material nonlinearity, especially for the long-shank SCCB connector.

A comprehensive experimental and analytical study on the behavior of the SCCB connector is presented in this article. The general working conditions of the connector are illustrated first. A total of 11 specimens with different parameters were tested for failure under monotonic tensile loading. The experimental results concerning testing phenomena, failure mode, and load–deformation relationship are then summarized. This is followed by the derivation of calculation equation for ultimate strength and yield strength referring to existing conventional bolted connection. The proposed equations were verified by testing strength and testing failure modes.

General working conditions of SCCB connector in precast shear wall

Under horizontal load coupled with vertical load, horizontal connection of the precast shear wall is in tension and compression simultaneously. Figure 2 depicts the different working conditions of the horizontal connection. Only one kind of working condition exists in the tensile zone. The compressive zones are subjected to three kinds of working conditions due to the different initial clearance between $δ$ (i.e. clearance between steel unit and H-connector) and $[δ]$ (i.e. clearance between bolt shank and bolt hole). It should be noted that the working condition in Figure 2(d) is a critical condition that may be too idealized to exist. In general, friction and bearing action are the two main load-transferring ways for the tensile zone, and the compressive zone has contact action in an additional way.

Figure 2.

Working conditions of horizontal connection. (a) In tension. (b) In compression—Type I ( $δ < [δ]$ ). (c) In compression—Type II ( $δ > [δ]$ ). (d) In compression—Type III ( $δ = [δ]$ ).

In consideration of the construction-related errors, a reasonably large clearance between steel unit and H-connector is indispensable to wall panel assembly. In this case, the condition $δ > [δ]$ is a real existent fact, which leads to residual clearance $(δ - [δ])$ after the closure of initial clearance in the compressive zone, as shown in Figure 2(c). Thus, the transfer of the load is similarly achieved via friction before sliding and subsequently via a combination of friction and bearing action. Eventually, the tensile zone and the compressive zone of horizontal connections will experience a similar working condition, namely, friction-bearing condition, as shown in Figure 2(a) and (c). On this basis, 11 specimens were designed by “friction-bearing condition” philosophy and tested under tensile loading.

Experimental investigation

Specimens design

Shown in Figure 3 is a schematic of the specimen, the prototype of which was extracted from the horizontal connection in the above-mentioned precast shear wall. Given the symmetry of the horizontal connection, the specimen was designed as half of the prototype. A whole connector specimen was made of three components: high-strength bolts, steel cap plate (i.e. H-connector in the precast shear wall), and steel inner plate with internal concrete (i.e. steel unit and internal concrete wall part in the precast shear wall). The bolt holes were fabricated by bench drilling machine, which was identical to the manufacturing method of the previous wall test (Sun et al., 2016).

Figure 3.

Configuration of horizontal connection.

Table 1 summarizes the details of the 11 specimens. The parameters were systematically varied and included bolt diameter (G1 Series), bolt tension (G2 Series), concrete strength (G3 Series), and thickness of cap plate (G4 Series). BASE Series was set as a control group.

Table 1.

Specimen details.

Test specimen		Bolt diameter $d_{b}$ (mm)	Bolt tension P (kN)	Diameter of bolt holes $d_{0}$ (mm)	Concrete strength grade	Thickness of sandwiched concrete $t_{c}$ (mm)	Thickness of inner plate $t_{1}$ (mm)	Thickness of cap plate $t_{2}$ (mm)
BASE-1		20	85	22	C30	90	6	6
BASE-2		20	85	22	C30	90	6	6
BASE-3		20	85	22	C30	90	6	6
G1	G1-1	16	85	18	C30	90	6	6
G1	G1-2	24	85	26	C30	90	6	6
G2	G2-1	20	70	22	C30	90	6	6
G2	G2-2	20	100	22	C30	90	6	6
G3	G3-1	20	85	22	C50	90	6	6
G3	G3-2	20	85	22	C50	90	6	6
G4	G4-1	20	85	22	C30	90	6	10
G4	G4-2	20	85	22	C30	90	6	14

The measured material properties of the steel plates are summarized in Table 2. Grade 10.9s bolts were used in the specimens. According to the specification (GB/T 1231-2006, 2006), the ultimate strength ( $f_{bu}$ ) and yield strength ( $f_{by}$ ) of Grade 10.9s bolts are 1240 and $0.9 f_{bu}$ MPa, respectively. Standard concrete cubes (150 × 150 × 150 mm) were prepared during casting and tested during the experimental period. The average compressive strength of cubes was tested to be 24.3 MPa for Grade C30 and 54.9 MPa for Grade C50.

Table 2.

Measured material properties of steel plate.

Thickness (mm)	Yield strength (MPa)	Ultimate strength (MPa)
6	293.2	430.2
10	287.8	444.0
14	328.8	477.1

Test procedure

The preparation of specimens and experimental setup are depicted in Figures 4 and 5, respectively. To get the bolt tension precisely, a preliminary test was carried out to acquire the torque-pretension coefficient of high-strength bolt (JGJ 82-2011, 2011). In order to reach the designed tension during working, 10% additional tension was applied to consider the loss of tension according to the specification (JGJ 82-2011, 2011). A 10% frictional force was loaded before loading to ensure that universal testing machine and instrumentation work well. The displacement-controlled procedure was performed at a constant rate of 1 mm/min (Kong, 2017). Slippage between the inner plate and the cap plate was measured by displacement gauges, and the applied load was measured by force transducer of the testing machine.

Figure 4.

Preparation of specimens.

Figure 5.

Experimental setup.

Experimental results

Testing phenomena and failure modes

The general phenomena from a typical specimen are as follows. Elastic deformation had been the major deformation before the slippage happened. Then, a hiccup sound occurred with slippage, which is due to abrasion between the cap plate and the inner plate. After that, the sound muted slowly while the bolt shank appeared to flex. With further increase displacement, the cap plates began to curl with obvious flexural deformation in bolt. Two typical failure modes were observed at the end of test: G1-1, G4-1, and G4-2 ended up with bolt shear failure, while the rest of the specimens faced bearing failure. The typical failures are shown in Figures 6 and 7.

Figure 6.

Representative bearing failure (BASE-3).

Figure 7.

Representative bolt shear failure without curling (G1-1).

It is interesting to note that all the specimens except G1-1 and G4-2 failed with plate curling as seen in Figures 7 and 8. However, this study of curling is not sufficient (Chung and Ip, 2000; Kim et al., 2008; Rex and Easterling, 2003); some recent research has shown that curling reduces the load capacity of bolted connection (Kim et al., 2008, 2015). In this article, the calculation of ultimate strength covered the curling reduction. In actual design, the curling is commonly precluded by limiting the maximum end distance according to the latest Chinese specification (GB 50017-2017, 2018).

Figure 8.

Failure with plate curling (BASE-3).

Load–deformation relationship and strength

In the test, the relative position of bolts to their individual host holes in the actual specimens is understandably random. When slipping started, individual bolts would have come into contact with the bolt holes in a progressive manner (Sun et al., 2016). Therefore, data modification was performed before usage: the original curves before the end of slippage stage were removed to the left side of the original point. Figure 9 compares the load versus modified deformation curves of the 11 specimens.

Figure 9.

Load versus modified deformation curves of 11 specimens.

As depicted in Figure 9, two kinds of deformation processes can be summarized. The specimens with bolt shear failure were in poor ductility and the load dropped suddenly during loading, while the specimens with bearing failure behaved more ductile.

Based on the simplified relationship proposed by Ungkurapinan et al. (2003), a simplified model has been utilized in this study (refer to Figure 10). The load–deformation response can be divided into four regions: elasticity stage (Region 1), slippage stage (Region 2), post-slippage yield stage (Region 3), and post-slippage hardening stage (Region 4). Note that all the curling happened at the post-slippage hardening stage for the specimens. Four key points are identified on the simplified curve: slippage starting (point A), bolt bearing (point B), connector yielding (point C), and peak (point D). The slippage load, yield load, and ultimate load are denoted as $V_{A}$ , $V_{C}$ , and $V_{D}$ , respectively. The SCCB connector exhibited evident stiffness change at point C, resulting in a bilinear process.

Figure 10.

Simplified load–deformation relationship.

The kinds of methods frequently used in finding the yield point of load–deformation curve are the geometric graphic method and the equivalent elastic-plastic energy method. Feng et al. (2015, 2017) proposed that conventional yield point defined methods may define a higher point than the inflection point for the curves with noticeable stiffness changes, and the shortcoming could be precluded by the farthest point method. Figure 11 shows connecting the original point with the peak point on the backbone curve, while searching the point whose perpendicular distance to the connecting line is farthest on the curve; the farthest point is regarded as the yield point. As previously mentioned, for bolted connection, there are two stages before the post-slippage yield stage. To obtain the yield point with the method, it is reasonable to choose the bolt bearing point (i.e. point B in Figure 10) as the nominal original point. The results of the ultimate strength and yield strength are summarized in Table 3.

Figure 11.

Farthest point method for determination of yield points of members (Feng et al., 2015).

Table 3.

General results for specimens.

Test specimen	Failure mode			Ultimate strength			Yield strength
Test specimen	Test	Predicted	Curling	${V_{u}}^{test}$ (kN)	${V_{u}}^{cal}$ (kN)	${V_{u}}^{test} / {V_{u}}^{cal}$	${V_{y}}^{test}$ (kN)	${V_{y}}^{cal}$ (kN)	${V_{y}}^{test} / {V_{y}}^{cal}$
BASE-1	–	B	–	–	430.0	–	–	314.3	–
BASE-2	B	B	Y	411.1	430.0	0.96	305.5	314.3	0.97
BASE-3	B	B	Y	363.7	430.0	0.85	347.4	314.3	1.11
G1-1	S	S	N	305.0	270.6	1.13	245.7	265.0	0.93
G1-2	B	B	Y	463.5	455.3	1.02	315.9	363.5	0.87
G2-1	B	B	Y	416.6	430.0	0.97	321.9	302.3	1.06
G2-2	B	B	Y	430.0	430.0	1.00	335.5	326.3	1.03
G3-1	B	B	Y	478.5	430.0	1.11	326.7	314.3	1.04
G3-2	B	B	Y	480.1	430.0	1.12	331.1	314.3	1.05
G4-1	S	S	Y	490.1	439.0	1.12	399.1	460.8	0.87
G4-2	S	S	N	451.4	439.0	1.03	374.6	460.8	0.81
					M	1.03		M	0.97
					SD	0.092		SD	0.099
					Coefficient of variation	0.089		Coefficient of variation	0.103

BASE-1 was not tested to failure due to the limitation of test equipment.

Bearing failure is denoted as B; bolt shear failure is denoted as S.

In the test, load cell under the bolt head was used to measure bolt tension. For ultimate strength, the cell thickened the diameter of bolts, therefore, calculation of bearing capacity employed the diameters of load cell when curling happened (30, 34, and 36 mm cell diameter were corresponding to 16, 20, and 24 mm bolt, respectively).

Effect of connector details

Considering the testing phenomena and load–deformation curves, the following effect of parameters can be summarized.

Figure 9 shows the different bolt diameter and thickness of cap plate designed in Series BASE, G1, and G4. The bolt diameter and the thickness of cap plate will significantly influence the failure mode. Given that G1-1 and Series G4 all failed with bolt shear, G1-1 exhibited the lowest ultimate strength with the slenderest bolt shank, while the Series G4 exhibited the highest ultimate strength with the thickest cap plate. This implies that a slender bolt and a relatively thicker cap plate will lead to bolt shear failure.

The specimens with bolt tension of 85, 70, and 100 kN were denoted as Series BASE and Series G2. With increasing bolt tension, the ultimate strength increased slightly. The phenomenon demonstrated that curling influenced the ultimate strength controlled by bearing failure.

Series BASE and Series G3 had different concrete compressive strengths. As seen in Figure 9 and Table 3, when the concrete compressive strength increases by 126%, the ultimate strength only increases by 16%. This means that the influence of the concrete compressive strength was negligible on the ultimate strength of connector, but distinct on stiffness. The reason was that the change in concrete strength affected the elastic modulus of the concrete, but the failure of the SCCB connector was not controlled by the concrete to some extent. It was similar to the high-strength friction-grip bolt shear connector (Liu et al., 2015), which was a kind of shear connector and also involved concrete.

Calculation of ultimate strength and yield strength

General calculation of frictional resistance

As mentioned before, for a bolted connection, the load-transferring mechanism is different before and after sliding has occurred. While the load transfer relies solely on friction before sliding, the load mechanism changes to both friction and bolt bearing after sliding has occurred. Based on high-strength bolt design methodologies (JGJ 82-2011, 2011), the first significant slippage occurs after the friction is overcome at shear force $F_{f}$ that is given as

F_{f} = n_{f} μ P

(1)

where $n_{f}$ is number of sliding interfaces (standard SCCB connector has two sliding interfaces); $μ$ is the mean slip coefficient between steel plates, $μ = 0.4$ is adopted considering the untreated clean-rolled steel plates with red rust (JGJ 82-2011, 2011; and P is bolt tension.

With increasing load, the friction will be insufficient to avoid slipping. Later, the bolt shank contacts the bolt hole of cap plate, and the cap plate or bolt shank reaches its material yield strength, then material ultimate strength. According to the testing phenomena, the calculation of ultimate strength involves two failure modes.

Calculation of ultimate strength

Bearing failure

As a result of thicker bolt and thinner cap plate, bearing failure may happen by the severe elongation of bolt hole in the SCCB connector. Large stress and plastic deformation occur in the cap plate so that bearing failure takes place. Compared with cap plate, no apparent damage was observed on the inner plate in the tests. It is believed that good adhesion between the concrete and the inner steel plate strengthen the inner plate. The bearing capacity is expressed as

F_{br}^{u} = KC d_{b} \sum t_{p} f_{u}

(2)

where $d_{b}$ is bolt diameter; $\sum t_{p}$ is total thickness of cap plates; $f_{u}$ is material ultimate strength of cap plate; C is bearing coefficient (AISC, 2016; Kamtekar, 2012; Teh and Uz, 2016), 3.5 is adopted according to the study of Teh and Uz (2016); and K is reduction factor considered the effect of curling. Kim et al. (2015) proposed a regression equation to acquire the factor K, which varies from 0.76 to 1.0. To more accurately predict the ultimate strength of the SCCB connector, K is recommended to be 0.7.

Bolt shear failure

As a result of slenderer bolt and thicker cap plate, bolt shear failure may occur in the SCCB connector. The high-strength bolt is subjected to axial tension while being sheared. In light of von Mises yield criterion, the function considered as shear stress coupled with tensile stress is obtained as

{(\frac{σ}{f_{bu}})}^{2} + {(\frac{τ}{f_{buv}})}^{2} = {(\frac{P}{A_{b} f_{bu}})}^{2} + {(\frac{F_{s}^{u} / n_{f}}{A_{b} f_{buv}})}^{2} = 1

(3)

in which, $A_{b}$ , $f_{bu}$ , $f_{buv}$ is the cross-sectional area, material ultimate strength, and material ultimate shear strength of high-strength bolt, respectively. The ultimate shear strength of high-strength bolt $f_{buv}$ is equal to $\frac{1}{\sqrt{3}} f_{bu}$ . The bolt shear capacity $F_{s}^{u}$ can be solved using equation (3).

Ultimate strength

Thus, the ultimate strength of the SCCB connector is given as

{V_{u}}^{cal} = \min (F_{br}^{u}, F_{s}^{u})

(4)

The predicted failure mode is corresponding to the minimum capacity that controls the ultimate strength.

Verification of calculated ultimate strength

In Table 3, the ultimate strength and failure modes obtained from the calculation equations above have been compared with the test results. Figure 12 plots the scatter diagram of the ratios ${V_{u}}^{test} / {V_{u}}^{cal}$ , and the box charts in Figure 13 summarize the ratios. The comparison demonstrates a generally good agreement between the calculated and the experimental results. The mean value and coefficient of variation of the ratio ${V_{u}}^{test} / {V_{u}}^{cal}$ are 1.03 and 0.089, respectively. The predicted failure modes are consistent with the testing ones.

Figure 12.

Ratio ${V_{u}}^{test} / {V_{u}}^{cal}$ and ratio ${V_{y}}^{test} / {V_{y}}^{cal}$ .

Figure 13.

Overall ratio ${V_{u}}^{test} / {V_{u}}^{cal}$ and ratio ${V_{y}}^{test} / {V_{y}}^{cal}$ .

Calculation of yield strength

Most studies focus on the ultimate strength of bolted connection (AISC, 2016; Kamtekar, 2012; Teh and Uz, 2016). In Figure 9, the load–displacement curve had a significant yield point, which was a turning point between the post-slippage yield stage and the post-slippage hardening stage. Therefore, it is of significance to calculate the yield strength of SCCB connector.

Similar to ultimate strength, two modes control yielding of the SCCB connector. However, there are some features about the calculation of yield strength. One is that high-strength bolt or steel cap plate is supposed to yield at the moment, consequently, the $f_{u}$ , $f_{bu}$ , and $f_{buv}$ in equations (2) and (3) could be replaced by $f_{y}$ , $f_{by}$ , and $f_{byv}$ , respectively. $f_{y}$ is the material yield strength of cap plate. The other experimental results revealed that curling happened till the post-slippage hardening stage, resulting in a load transfer containing unreduced bearing capacity and friction action for yield strength; therefore, $K = 1$ is reasonable for calculating the yield strength.

And the equations can be given as

F_{br}^{y} = C d_{b} \sum t_{p} f_{y}

(5)

{(\frac{P}{A_{b} f_{by}})}^{2} + {(\frac{F_{s}^{y} / n_{f}}{A_{b} f_{byv}})}^{2} = 1

(6)

{V_{y}}^{cal} = \min (F_{br}^{y}, F_{s}^{y}) + F_{f}

(7)

The calculated yield strength has also been compared with the testing yield strength, as shown in Table 3, Figure 12, and Figure 13. The comparison demonstrates that the calculated yield strengths are also significantly consistent with the testing ones.

Conclusion

A monotonic tensile test was carried out to investigate the mechanical behavior of SCCB connector in reinforced concrete precast shear wall. A representative SCCB connector consists of three parts: high-strength bolts, steel cap plate, and steel inner plate with internal concrete. This new-type connector is proposed to expedite a more convenient and reliable assembling method for precast shear wall. Based on the experimental results and analysis, more specific conclusions can be drawn.

The failure modes and ultimate strength of all specimens were governed by bearing failure or bolt shear failure in the test, and the bearing failure was more ductile than the bolt shear failure. Curling occurred at the post-slippage hardening stage for most connectors.

Comprehensive parametric studies of concrete strength, thickness of cap plate, bolt diameter, and bolt tension were performed by monotonic tensile tests. The testing results showed that the bolt diameter and thickness of cap plate had significant influences on the behavior of the SCCB connectors. A slenderer bolt and relatively thicker cap plate will lead to bolt shear failure, and a thicker bolt and relatively thinner cap plate will lead to bolt shear failure. Comparatively speaking, the concrete strength and bolt tension impact the strength of connector less.

By referring to the failure modes of SCCB connector and calculation of conventional bolted connection, the calculation equations of ultimate strength and yield strength for connector have been derived. Considering the influence of curling on bearing capacity, a reduction factor of $K = 0.7$ is recommended for calculating ultimate strength. Calculation of bolt shear capacity is derived from von Mises yield criterion to consider the influence of bolt tension. Besides, the calculation equations of yield strength are also derived. The calculation provides a good prediction for ultimate strength, yield strength, and failure mode.

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: This work was supported by the National Natural Science Foundation of China (No. 51708107), the National Natural Science Foundation of Jiangsu Province (No. BK20170668), Postgraduate Research & Practice Innovation Program of Jiangsu Province (No. KYCX18_0109), and the Fundamental Research Funds for the Central Universities.

ORCID iD

Hongbo Jiang

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