CIVIL ENGINEERING 365 ALL ABOUT CIVIL ENGINEERING



Experimental SpecimensTo verify the seismic performance of the ASCBRBs, four specimens with different energy-dissipation ratios were tested under cyclic loading. The specimens had the same dimensions but different arrangements of the prestressed disk-spring groups and core plates. The layouts of the assembled BRB (ABRB), the assembled SC brace (ASCB), and two ASCBRBs that were tested herein are shown in Fig. 9.The braces were identical in length (2,380 mm), and the gap between the outer tube and the inner twin screws, which determined the deformation capacity during the test, was 60 mm. End plate I was 190×520  mm, and the thickness of the plate was 40 mm. End plate II was 650×650  mm, and the thickness of the plate, which prevented the relative displacement between the two End plates I, was 50 mm. Two 180×16-mm steel angles with bolt holes were welded onto End plates I to enable the installation of the core plate. The channel steel in the outer tube was Type 14a made of Q345 steel, which was also used for the stiffening ribs that were welded to it. The M16 blocking screws in the outer tube were made of high-strength steel (M8.8) and fixed using double nuts.The guide and cover plates had the same cross section (340×8  mm) and were also made of Q345 steel. The disk springs were made of 60Si2MnA steel, and the twin screw was made of high-strength steel (M8.8). Extra hardening treatment was conducted on the twin screw to prevent partial crush failure due to the movement of the disk springs. The inner twin screws were M48 and the blocking members of the inner screw were nuts with identical dimensions.The core plates were made using Q235 steel, whose yield stress was 242 MPa, with an ultimate elongation ratio δ=18%. The lengths of the elastic, transfer, and yielding segments were 445 mm (495 mm), 275 mm, and 675 mm, respectively. The segment lengths of the core plates in the BRB, ASCBRB_1, and ASCBRB_2 specimens were identical. The cross section of the elastic segment of all the core plates was 22×8  mm. The cross section of the yielding segment in the ABRB and ASCBRB_2 was 130×8  mm, and that in the ASCBRB_2 was 90×8  mm.The filler and guide plates, which prevent the buckling of the core plate in the width and thickness directions, were made of Q345 steel. The shim plates (1-mm in thickness), which separate the core plate from the guide plates and eliminate the effects of friction, were made of stainless steel. The guide, shim, and filler plates are assumed to have no contribution to the stiffness of the energy-dissipation system.The working diagram of the braced frame is shown in Fig. 10. According to the Chinese seismic design code [Code for Seismic Design of Building, GB 50011-2010 (Ministry of Housing and Urban-Rural Development of China 2010)], the maximum drift ratio of the frame during a rare earthquake is 2%. The axial deformation demand of the brace, which is related to the installed angle θ to the floor has a maximum value of 1% when θ=45° [Eq. (4)]. The axial deformation of the belt column is disregarded herein. The design deformation demand of the brace was Δd=1%×2,380=23.8  mm. The axial deformation capacity of the ASCBRB was decided by the arrangement and prestress of the disk-spring group. Fig. 11 illustrates a single disk spring and the disk-spring group adopted herein. The disk spring had an outer diameter of D=100  mm, an inner diameter of d=49  mm, a thickness of t=7  mm, and a height of H=9.2  mm. For (H−t)/t=0.314<0.4, the force-displacement relationship of the disk-spring group can be simplified as a linear relationship with a stiffness k1=46.7  kN/mm for a single piece, in accordance with the Chinese code (Code for Disc Spring, GB/T 1992-2005). When two disk springs are placed in the same direction, the load-carrying capacity Fd doubles. When two disk springs are placed in opposite directions, the deformation capacity f doubles (Fig. 11).The prestress in each group was T0/2=50.8  kN with an initial deformation of 0.55 mm in each disk spring. The elastic deformation range of each disk spring was Δ1=0.75×(H−t)=1.65  mm. The deformation capacity of each prestressed disk-spring group was Δc=(Δ1−0.55)×36=39.6  mm>Δd. The stiffness of each prestressed disk-spring group was 0.5kd=k1×2/36=2.59  kN/mm. The SC system consists of four identical prestressed disk-spring groups that provide a SC force of 2T0=203  kN(4) ΔaxialLb=Δd·cosθH/sinθ=2%cosθsinθThe SC ratio αSC, proposed by Eatherton et al. (2014b) (Qiu and Zhu 2016), is defined as the ratio of the resilient force divided by the strain-hardened BRB strength (5) where β = compression strength adjustment factor; and ω = strain-hardening adjustment factor for the buckling-restrained core plate, where β=1.2 and ω=1.6 (Bruneau et al. 2011). The SC ratios of the specimens are listed in Table 1. A lower value of αSC indicates a higher energy-dissipation capacity and a poorer resilience.Table 1. Self-centering capacity of different specimensTable 1. Self-centering capacity of different specimensNo.Sum of prestress (kN)Area of core plate (mm2)αscABRB—1,040—ASCB203——ASCBRB_12037200.64ASCBRB_22031,0400.44Quasi-Static Experiment and ResultsThe experiment was conducted at the State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University. The load-carrying capacity of the servohydraulic actuator was 2,000 kN with a deformation capacity of ±200  mm. To prevent the brace from moving in directions other than the axial direction, the specimen was placed on a steel platform that only permitted axial deformation, as shown in Fig. 12.Considering the conditions of the loading devices, pin connections were adopted for both ends of the novel brace in the test. High machining accuracy is required for pin connections to avoid uneven stress in the gusset plate. Generally, welding of the ribs and removal of the beam ends from the rigid region are necessary when the novel braces are used in buildings. In the absence of a loading rate in the code, we formulated a low-speed loading protocol to demonstrate the hysteretic behavior while disregarding the influence of damping for the displacement-dependent ASCBRB. The loading protocols for different specimens are shown in Fig. 13.The loading protocol in the Chinese code recommends that the loading displacement for each level be a multiple of the yield displacement Δy. However, Erochko et al. (2015a) reported that the connection at the ends of the brace and length tolerance influence the brace stiffness as well as the yield displacement Δy. The ASCBRB mechanism described in the “Mechanical Properties” section does not yield a precise estimation of the stiffness, and a detailed model is required for further study. The same loading protocol was employed for the specimens, and the results were used to calibrate the detailed model.The Chinese code loading protocol was followed for the full-length braces after the ASCBRB mechanism had been thoroughly studied. The connections, torsion mode of the core plate, and out-of-plane deformation have been observed to influence the stability of full-length BRBs (Black et al. 2004; Palmer et al. 2013). With regard to the loading condition of the devices, the specimens were considerably shorter than actual braces used in buildings, and only axial deformation was considered in this study.The hysteretic curves of the specimens are shown in Fig. 14.The ABRB specimen had satisfactory energy-dissipating capacity, but its ultimate ductility (μ=Δu/Δy=6.2) can be improved. The yield force of the ABRB specimen (fsy=250  kN) was close to the value calculated based on the section and the material test result (fsy=1,040×0.242=251.7  kN). The initial stiffness of the brace based on the test result was 96  kN/mm, which was 10% lower than the calculated stiffness (128.3  kN/mm). This discrepancy is probably on account of the connections at both ends, which were in series with the brace members. The obvious decline in the restoring force of the ABRB indicated that the core plate ruptured during the 12th cycle of the tension test, as shown in Fig. 14(a). The maximum displacement of the specimen was Δm=16.2  mm, and the residual displacement was Δr=12.7  mm after the 11th cycle. The residual displacement ratio was Δr/Δm=78.4%, which indicates that residual interstory drift in the ABRB frame is inevitable after a severe earthquake.Fig. 14(b) indicates that the ASCB possesses fairly good resilience and relatively low energy-dissipating capacity. The restoring force of the ASCB at the end of Phase II (230 kN) was higher than the designed prestress value (203 kN) obtained from the theoretical model owing to the friction between the twin screws and the disk springs. The residual displacement after the brace reached the peak displacement of Δm=31.1  mm was Δr=0.3  mm; the residual displacement ratio was Δr/Δm=0.96%. Furthermore, the restoring force did not reduce even after 17 cycles, and the deformation capacity of the ASCB was significantly higher than that of the ABRB. The initial stiffness of the ASCB was 347  kN/mm, which is higher than that of the ABRB owing to the contribution of the outer and inner members.The hysteretic curves and the theoretical models of ASCBRB_1 and ASCBRB_2 [Figs. 14(c and d), respectively] fitted well. The experimental yield forces of ASCBRB_1 and ASCBRB_2 were 400 and 475 kN, respectively. The core plate in ASCBRB_2 was identical with that in ABRB (theoretical fsy=251.7  kN) and its theoretical yield force in ASCBRB_2 was fsy=174.2  kN.The prestressed disk-spring groups in ASCBRB_1 and ASCBRB_2 were identical with the arrangement used in the ASCB specimen (theoretical fsc=203  kN). The experimental yield force of the core plate fsy and the experimental activation force of the SC system fsc were recognized from the test result and marked in Figs. 14(c and d). The initial stiffness did not change significantly from one cycle to the next, and the restoring force reduced when the core plate ruptured during the 12th cycle. The ductility of ASCBRB_1 and ASCBRB_2 was the same as that of ABRB. The residual displacements of the two specimens were Δr=0.61  mm (Δm=17.1  mm) and Δr=0.75  mm (Δm=17.5  mm), which indicates that the braces had fairly good resilience (Δr/Δm=3.6% for ASCBRB_1, and Δr/Δm=4.3% for ASCBRB_2) compared with the ABRB specimen.The calculated initial stiffness of the ED system was 105.5  kN/mm, and the initial stiffness of the entire brace was 431  kN/mm. For specimen ASCBRB_2, the initial stiffness of the ED system was 128.3  kN/mm, and initial stiffness of the brace was 360  kN/mm. The test results indicate that the novel SCB can reduce the residual displacement by 73% at the same peak displacement. The initial stiffness of the two novel braces also improved by 180%–300% compared with the stiffness of the ED system.Rupturing and local buckling were observed at the free end of the core plate when the specimens were disassembled after the experiment, as shown in Fig. 15(a). Local buckling was observed in the core plate at the free end; this was caused by the straight-line laser-cut shape of the core plate, which may have resulted from a manufacturing fault.Permanent deformation was also observed in the guide plates near the free end, as shown in Fig. 15(b); this indicates that the thickness of the guide plate near the free end should be increased. Plastic torsional buckling was observed in the cruciform-section BRB ends, which extended out of the confinement tube (Black et al. 2004). However, the thick L-shaped steel and bolt groups at the free end prevented torsional buckling at the free end of the ASCBRB. At the fixed end, because of the clamping bolts, the core plates were protected from damage. For all the tested specimens, there were no buckling failures in the outer tube, blocking members, twin screws, or cover plates, which indicates that these elements can be reused.To provide resilience, unlike existing ASCBRBs, the proposed ASCBRBs use parallel disk-spring groups, which are less expensive and are commercially available. Unlike in previous studies (Miller et al. 2012; Eatherton et al. 2014a), in this study, the damage to the proposed ASCBRBs was concentrated at the core plate; this enables replacement of the ASCBRB components. Moreover, complete bolt connections also allow for easy assembly and maintenance of ASCBRBs. Notably, the bolts were adopted in the blocking members and connections for both ends. Moreover, out-of-plane deformation is unlikely to occur in shortened specimens, which is limited by the condition of the loading device. Moreover, manufacturing imperfections in the core plate would also result in premature failure of the ASCBRB. These factors may negatively affect the repeatability of the tests conducted on the proposed brace. Thus, tests involving multiple cycles should be conducted on full-length specimens in future studies.



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