Seismic Performance Assessment of Substituting RC Coupling Beams with Replaceable Perforated Steel Type in Tunnel Form Concrete Building Structures

The coupled-shear wall system has been proven to be an outstanding lateral-load resisting system in the medium- to high-rise buildings. In this system, several individual shear walls are coupled with coupling beams. Thus, the walls show an integrated performance in resisting the earthquake loads by providing control over lateral displacements. In tunnel form buildings, the strength and the lateral stiffness are influenced by the coupling beam. In this system, the coupling beams act as a fuse in this system; they are the first elements to undergo inelastic deformations. The proper design of the concrete coupling beams (CCBs) leads to the use of diagonal reinforcements and high-level of detailing. There is a problem in construction of RC tunnel form buildings related to inserting the diagonal reinforcing shear bars, which is practically difficult and in most cases, this reinforcement is ignored. Therefore, these elements as the structural fuses have a minor contribution in dissipation of seismic input energy. Recent researches showed that the steel coupling beams (SCBs) exhibit better performance than the CCBs in terms of ductility and energy absorption during cyclic loadings. The pinching effect was not observed in the hysteretic loops for SCBs, which implied a more stable post-elastic behavior and a higher energy dissipation capacity than the CCBs. Also, the energy dissipation capacity of SCBs was more than three times larger than that of the CCBs. In this study, a regular symmetric plan seven-story tunnel form building was used. The coupling beams with a length of 1 m and a depth of 0.7 m above the openings were inserted. The building was assumed to be residential and located in Tehran. The story height was 3 m, and the soil was considered type B based on the regulatory seismic code of Iran. The building was designed according to ACI 318-14 by using ETABS software. The thickness of the walls and the slabs were 20 cm and 15 cm, respectively. No. 8 rebar with spacing of 20 cm was designed for the vertical and horizontal reinforcements in two layers. In the first two stories, only the vertical reinforcements were of No. 12 rebar. The compressive strength of concrete and the yield strength of reinforcements for structural members were 25 MPa and 400 MPa, respectively. The diagonal reinforcements of the CCBs were designed to provide the ductility and the improvement of the shear strength. In this research, the CCBs were substituted by SCBs with circular holes. The shear strength of the beam was improved with diagonal stiffeners. The holes in the web of the beam were used to convey installations and to avoid perforating shear walls in tunnel form buildings. The diagonal stiffeners prevented the buckling of the beam’s web. Also, the shear strength and the tension field action were improved. Therefore, the seismic performance of the coupling beam was overall enhanced. As the main design equipment the SCB, the relations related to determining the shear capacity of the steel beam were exploited. Then, the seismic behavior of tunnel-form structures with regular RC coupling beams against the same structures designed with the proposed steel beams has been evaluated and compared in a nonlinear range. Finite element modeling and nonlinear analyses were conducted in PERFORM-3D. The incremental dynamic analysis regarding the probable ground motions was performed on the buildings to consider the effects of amplitude variation, frequency content, and the duration of ground motions on the response. The results showed that the use of the proposed SCB reduces the system stiffness and thus increases story drifts. In addition to decreasing the probability of the walls to attain the first levels of failure slightly, under the design earthquake and the maximum probable earthquake, the reliability of buildings in achieving predetermined performances was increased. Indeed, the use of this proposed coupling beam also increases the ductility of the tunnel form structures. Easy to implement and easy to repair or replacement of this steel coupling beam after a destructive earthquake are its other advantages compared to RC type.