1. Introduction
The coupling beam in shear wall structures is a key energy-consuming member that provides earthquake protection for the whole structure. A reasonable structural design scheme is to make the coupling beam yield before the wall stems. Coupling beams should be designed with good ductility to consume seismic energy and reduce damage to the main structure. Traditional reinforced concrete coupling beams have a relatively small span length and poor ductility, which cannot provide effective protection to the shear wall against earthquakes. To solve this problem, scholars all over the world have carried out research on the reinforcement, section design and composite configurations for shear wall coupling beams.
Paulay et al. [
1] proposed a reinforcement design with diagonal rebars concealed in beam-column connections. Through experiments, it was found that this reinforcing method can improve the bearing capacity of the beam and avoid the yield of the steel bars and the partial crushing of concrete. Moretti [
2] conducted a quasi-static test on this new reinforcing method and the results showed that the ductility and energy dissipation capacity of the reinforced beams supported by the diagonal concealed columns were greatly improved. Tegos et al. [
3] proposed a rhomboid reinforcement scheme for the main reinforcement and designed different forms of rhomboid layout of reinforcement to carry out quasi-static tests. The results show that the rhomboid layout of reinforcement can effectively restrain the development of inclined cracks and improve the strength and ductility, with a slight stiffness decrease.
Because steel-concrete composite beams have the advantages of both concrete and steel beams, they have become a new development direction for composite structures. Gong and Shahrooz [
4] compared the failure modes of two groups of sectional steel composite beams, and the results showed that the concrete wrapped around the steel section provides constraint to the steel section. Subedi [
5] proposed to use steel plate composite beams for the first time. It was verified through tests that steel plate concrete composite beams have better shear performance compared with ordinary beams. Lam et al. [
6] studied coupling beams with a vertically embedded steel plate along the whole span, either with or without shear studs. The results showed that embedded steel plates could improve the strength and stiffness of coupling beams. Teng et al. [
7] first proposed to use concrete-filled steel tubular beams. Through an experimental study, it was found that the lower tensile zone of the steel pipe cracked and failed.
For prefabricated structures, the vertical joints of the components have an important effect on the seismic performance of the structures. At present, many scholars have carried out a series of studies on the vertical joint connection of prefabricated concrete shear walls.
Sun et al. [
8] proposed a vertical joint device for prefabricated shear walls, which is composed of a built-in H-type steel frame and high-strength bolts. Through quasi-static tests, it was found that this connection method could effectively transfer forces. Liu et al. [
9] embedded a connector at the joint and connected it by welding. The test results show that the welding joint can fully transfer the shear force at the vertical joint. Twigden [
10] studied a precast concrete shear wall system using a slot-bolted connection; the tests evaluated a design procedure with both the global and local force-displacement response parameters.
Jesper et al. [
11] improved the connection form of stirrup bolts and added buckle-resisting bars to the post-cast belts. The test found that the steel bars in the specimens could effectively transfer stress. Ramin et al. [
12] used a finite element software to compare and analyze two vertical connection forms of stirrup bolts and embedded steel member bolts. The simulation results show that the energy dissipation capacity of the embedded steel bolt connection is 127% higher than that of the stirrup bolt connection. Trevor et al. [
13] used glass fiber (GFRP) composite boards and carbon fiber (CFRP) anchors as connection materials to reinforce vertical joints between prefabricated walls. Through static cyclic load tests, it was shown that this composite connection structure had good performance. Li et al. [
14] designed connections with long and short section steel embedded in wall stems, respectively. Low cycle repeated loading tests were carried out to study the mechanical properties of the steel beam and concrete shear wall connected by the embedded steel sections.
The results of most experimental studies show that the common failure of reinforced concrete coupling beams starts from the ends of the beams. The existing research on the prefabricated coupling beams is still very limited and has not addressed many important issues, such as the preferable joint design for desired cost-effectiveness, constructability and seismic performance. Based on a broad literature review [
15] and the unique characteristics of prefabricated double stem shear walls, a new structure of coupled shear walls is proposed in this paper, in which the two ends of the coupling beam use an arched section to increase the end section area, and the joints between the coupling beam and the wall stems are reinforced with steel connectors and horizontal and inclined steel bars. The new coupled shear wall can improve the shear bearing capacity of the beam-wall connection, optimize the stress distribution, improve the overall ductility, and ensure the convenience for prefabricated construction. The structure of the prefabricated arch beam and its connection with the wall stems are introduced in detail, and the seismic performance of the new arch beam joints are evaluated through experimental research and finite element analyses. As a preliminary but pioneering research, this paper provides reference information for further studies and for practical applications in this field. The paper covers the following sections: first, the structure of the prefabricated arch coupling beam is proposed; then, the test of the coupling beam is introduced and the results are summarized and analyzed; followed by an finite element analysis of the coupling beam; conclusions are made at the end of the paper.
2. Prefabricated Arch Coupling Beam Joint Design Scheme
The joint, as shown in
Figure 1, consists of the following parts: prefabricated wall stems, prefabricated arched coupling beam, cast-in-place composite connection layer and shaped steel connectors (steel sections in the wall and steel plate in the beam).
The structure of the prefabricated wall stems is shown in
Figure 2. The prefabricated upper and lower wall stems are connected in the field by welding the embedded I-shaped steel sections. A gap remains between the upper and lower wall stems that will be filled with concrete cast in place, together with the top portion of the coupling beam (see the composite layer in
Figure 1), to connect the wall and the beam. To further improve the connection, U-shaped rebar hooks are extended out of the precast wall stem and beam, which are connected and cast in concrete (see
Figure 2 and
Figure 3). The prefabricated arch beam is composed of the following two parts: the bottom part of the beam is precast (
Figure 3) and the upper part is cast in place, as shown in
Figure 4. An embedded trapezoidal steel plate at the end of the beam is weld-connected to the flange of the I-shaped steel section in the wall. Reinforcements in the upper part of the beam are connected with the reinforcements in the wall. A mechanical threaded connection sleeve is embedded at the end of the coupling beam to connect the horizontal bars between the wall and the beam, as shown in
Figure 4. After connecting the reinforcements and the steel members between the upper and lower prefabricated walls and between the wall and the beam, concrete is cast in the gap between the upper and lower walls, as well as the top of the prefabricated beam. The cast-in-place concrete, designated as the composite layer in
Figure 1, bonds the wall stems and the beam together. The I-shaped steel sections embedded in the precast wall stems also provide temporary support during the assembling of the walls.
6. Conclusions
In this paper, a new type of prefabricated arch coupling beam is proposed based on the force characteristics of coupling beams and the requirements of prefabricated construction. Pseudo-static tests and finite element analyses were carried out to evaluate the seismic performance of the wall-beam structure. The following conclusions are drawn.
The arch coupling beam provides an effective connection with the wall stem with good shearing resistance and ductile behavior. The cracks of the arch beam are uniformly distributed along the span direction of the beam.
The I-steel section in the wall stem and the steel plate in the beam can effectively connect the wall and the beam; there is no slip at the construction joints. The shear bearing capacity of the connection is improved by the steel connectors between the beam and wall stem.
The oblique bars in the precast beam can effectively prevent the development of oblique cracks. Sufficient stirrups should be provided to restrain the concrete at the wall-beam connection. The amount of stirrups should match other reinforcements, so as to ensure that they work together with the longitudinal bars and the oblique bars before failure.