In order to resolve the strength decrease due to the buckling of the steel brace, a buckling-restrained brace system(hereinafter “BRB”) was originally developed in Japan in 1990, followed by the development mainly in the U.S. and Japan of various shapes. The BRB system enables a large amount of ener...
In order to resolve the strength decrease due to the buckling of the steel brace, a buckling-restrained brace system(hereinafter “BRB”) was originally developed in Japan in 1990, followed by the development mainly in the U.S. and Japan of various shapes. The BRB system enables a large amount of energy to dissipate through stable cyclic behaviors by constraining buckling, using a BRB such as a steel tube around a steel core. Under a large earthquake, the system allows a large, inelastic deformation to occur while avoiding local and overall buckling in the steel core. As BRBs have a high cumulative ductility under a large earthquake, and since they were originally mentioned in NEHRP 2003 as a seismic-force resisting system, they are being used in the United States’ ASCE 7-05 as a seismic-force-resisting system (R=8) with a high response modification factor, and are being used in Japan as hysteretic dampers. Also they will be reflected on the Korea Building Code 2009 which has been modifying.
Recently, many high-rise buildings have been constructed in Korea. As a result, there have been many cases where wind loads, rather than earthquake loads, are the dominant lateral forces. Consequently, serviceability due to wind has become a problem. A BRBs have a fine structural performance when seismic load is the dominant load. However, when the braces only stay in the elastic range under such conditions as a minor earthquake or a wind load, energy cannot be dissipated since the BRBs are hysteretic dampers. Thus, to apply the BRB to high-rise buildings, the development of the Hybrid Buckling-Restrained Brace system (hereinafter “H-BRB”), which is a BRB with good seismic performance and with improved serviceability against wind loads, is needed.
The H-BRB system is a hybrid damper system created by adding dampers to the ends of the previous BRBs (which have a good seismic performance), in order to improve wind performance.
In this paper, to evaluate the structural performance of the H-BRB, testing and analysis were performed on a wind and earthquake load.
The experiment was performed in the elastic and inelastic ranges of the steel core. In the elastic range of steel core, control on wind-induced vibration was estimated by cyclic tests to analyze the load transfer mechanism and damping of the H-BRB. Moreover, in the inelastic range of steel core, the seismic performance of the H-BRB was evaluated by cyclic tests according to the loading protocol of AISC seismic provison(2005).
The analytical study was performed in a seismic-governed building model and a wind-governed building model. DRAIN-2DX, a time-history analysis program, was used. Three three-story and three-span buildings were designed by test results and according to the Korean Building Code 2005 and governed by an earthquake load. They were analyzed using three recorded earthquakes Taft (1952, PGA=0.179g), El Centro (1940, PGA=0.348g), Northridge (1994, PGA= 0.843g). Also, three 40-story buildings were designed by test results and according to the Korean Building Code 2005 and governed by a wind load. They were analyzed using wind load transformed wind tunnel test result considering Seoul’s wind environment.
The Hybrid Buckling-Restrained Brace has been developed to improve the control of the wind-induced vibration of BRB, and the results lead to the following conclusions.
Wind Performance of H-BRB
(1) The load transfer mechanism is acquired if the deformation of the steel core (the primary resisting component) is equal to that of the damper and tube (the secondary resisting component) under cyclic loading. In the elastic range test of H-BRB, when the damper was under a double-layer condition, the deformations of the primary and secondary resisting components that were connected parallel to each other were very similar (H-BRB-125 test specimen=0.967~0.977; H-BRB-150 test specimen=0.980), thus, confirming that the load transfer mechanism of the H-BRB system was secured under a double-layer condition.
(2) When comparing the equivalent damping ratios of the H-BRB specimens in the elastic range, the H-BRB-125 test specimen showed a 1.873% damping ratio, which was 0.985 of the theoretical value of 1.900% when the damper was under a double-layer condition. Thus, the expected damping ratio was secured. Moreover, when the H-BRB system under a double-layer condition is applied to high-rise buildings, a decrease can be expected in the acceleration and displacement of high-rise buildings.
(3) When time-history analysis was performed on three 40-story buildings, the roof acceleration of the H-BRB model is 10.11 milli-g and is 29% less than that of the BRB model. The building serviceability could be improved if H-BRB was applied to high-rise buildings.
(4) The time history analysis, for evaluating the appropriateness of the practical solution which is proposed to easily apply the H-BRB, was compared with the refined analytical model. The roof displacement was about the same but the roof acceleration came out to be 1.107 times greater. Therefore, the H-BRB's practical solution that is proposed in this paper may be applied in the structural design and the evaluation process of the roof acceleration.
Seismic Performance of H-BRB
(1) The H-BRB test specimen, in which a damper was operated under a double-layer condition, secured stable hysteretic behaviors in the steel core and the secondary resisting component under cyclic loading, before the secondary resisting component separated (steel core compression force/tensile force = 1.12). Moreover, as the contribution made by the secondary resisting component diminished after the separation of the welding of the stopper, the H-BRB system showed changes in its behavior mechanism. However, the steel core of H-BRB showed the same cyclic behavior as that of the BRB, without a sudden decrease in its strength or stiffness.
(2) In the inelastic range of H-BRB for the H-BRB test specimen, in which the damper was operated under a single layer condition, the contribution rate of the damper varied according to tension and compression, thus showing a distribution that differed from the strength pattern of the BRB’s steel core.
(3) The H-BRB is classified by two types according to a secondary resisting component, which can function or not after the steel core’s yielding. A H-BRBⅠ behaves with only a steel core after a core yielding. However, a H-BRBⅡ behaves with a steel core and a damper together after that. Based on the result from the nonlinear time-history analysis on three 3-story buildings, In the earthquake load, the H-BRB's excellence in the seismic performance can be secured when a secondary resisting component doesn't function after the steel core’s yielding.
In order to resolve the strength decrease due to the buckling of the steel brace, a buckling-restrained brace system(hereinafter “BRB”) was originally developed in Japan in 1990, followed by the development mainly in the U.S. and Japan of various shapes. The BRB system enables a large amount of energy to dissipate through stable cyclic behaviors by constraining buckling, using a BRB such as a steel tube around a steel core. Under a large earthquake, the system allows a large, inelastic deformation to occur while avoiding local and overall buckling in the steel core. As BRBs have a high cumulative ductility under a large earthquake, and since they were originally mentioned in NEHRP 2003 as a seismic-force resisting system, they are being used in the United States’ ASCE 7-05 as a seismic-force-resisting system (R=8) with a high response modification factor, and are being used in Japan as hysteretic dampers. Also they will be reflected on the Korea Building Code 2009 which has been modifying.
Recently, many high-rise buildings have been constructed in Korea. As a result, there have been many cases where wind loads, rather than earthquake loads, are the dominant lateral forces. Consequently, serviceability due to wind has become a problem. A BRBs have a fine structural performance when seismic load is the dominant load. However, when the braces only stay in the elastic range under such conditions as a minor earthquake or a wind load, energy cannot be dissipated since the BRBs are hysteretic dampers. Thus, to apply the BRB to high-rise buildings, the development of the Hybrid Buckling-Restrained Brace system (hereinafter “H-BRB”), which is a BRB with good seismic performance and with improved serviceability against wind loads, is needed.
The H-BRB system is a hybrid damper system created by adding dampers to the ends of the previous BRBs (which have a good seismic performance), in order to improve wind performance.
In this paper, to evaluate the structural performance of the H-BRB, testing and analysis were performed on a wind and earthquake load.
The experiment was performed in the elastic and inelastic ranges of the steel core. In the elastic range of steel core, control on wind-induced vibration was estimated by cyclic tests to analyze the load transfer mechanism and damping of the H-BRB. Moreover, in the inelastic range of steel core, the seismic performance of the H-BRB was evaluated by cyclic tests according to the loading protocol of AISC seismic provison(2005).
The analytical study was performed in a seismic-governed building model and a wind-governed building model. DRAIN-2DX, a time-history analysis program, was used. Three three-story and three-span buildings were designed by test results and according to the Korean Building Code 2005 and governed by an earthquake load. They were analyzed using three recorded earthquakes Taft (1952, PGA=0.179g), El Centro (1940, PGA=0.348g), Northridge (1994, PGA= 0.843g). Also, three 40-story buildings were designed by test results and according to the Korean Building Code 2005 and governed by a wind load. They were analyzed using wind load transformed wind tunnel test result considering Seoul’s wind environment.
The Hybrid Buckling-Restrained Brace has been developed to improve the control of the wind-induced vibration of BRB, and the results lead to the following conclusions.
Wind Performance of H-BRB
(1) The load transfer mechanism is acquired if the deformation of the steel core (the primary resisting component) is equal to that of the damper and tube (the secondary resisting component) under cyclic loading. In the elastic range test of H-BRB, when the damper was under a double-layer condition, the deformations of the primary and secondary resisting components that were connected parallel to each other were very similar (H-BRB-125 test specimen=0.967~0.977; H-BRB-150 test specimen=0.980), thus, confirming that the load transfer mechanism of the H-BRB system was secured under a double-layer condition.
(2) When comparing the equivalent damping ratios of the H-BRB specimens in the elastic range, the H-BRB-125 test specimen showed a 1.873% damping ratio, which was 0.985 of the theoretical value of 1.900% when the damper was under a double-layer condition. Thus, the expected damping ratio was secured. Moreover, when the H-BRB system under a double-layer condition is applied to high-rise buildings, a decrease can be expected in the acceleration and displacement of high-rise buildings.
(3) When time-history analysis was performed on three 40-story buildings, the roof acceleration of the H-BRB model is 10.11 milli-g and is 29% less than that of the BRB model. The building serviceability could be improved if H-BRB was applied to high-rise buildings.
(4) The time history analysis, for evaluating the appropriateness of the practical solution which is proposed to easily apply the H-BRB, was compared with the refined analytical model. The roof displacement was about the same but the roof acceleration came out to be 1.107 times greater. Therefore, the H-BRB's practical solution that is proposed in this paper may be applied in the structural design and the evaluation process of the roof acceleration.
Seismic Performance of H-BRB
(1) The H-BRB test specimen, in which a damper was operated under a double-layer condition, secured stable hysteretic behaviors in the steel core and the secondary resisting component under cyclic loading, before the secondary resisting component separated (steel core compression force/tensile force = 1.12). Moreover, as the contribution made by the secondary resisting component diminished after the separation of the welding of the stopper, the H-BRB system showed changes in its behavior mechanism. However, the steel core of H-BRB showed the same cyclic behavior as that of the BRB, without a sudden decrease in its strength or stiffness.
(2) In the inelastic range of H-BRB for the H-BRB test specimen, in which the damper was operated under a single layer condition, the contribution rate of the damper varied according to tension and compression, thus showing a distribution that differed from the strength pattern of the BRB’s steel core.
(3) The H-BRB is classified by two types according to a secondary resisting component, which can function or not after the steel core’s yielding. A H-BRBⅠ behaves with only a steel core after a core yielding. However, a H-BRBⅡ behaves with a steel core and a damper together after that. Based on the result from the nonlinear time-history analysis on three 3-story buildings, In the earthquake load, the H-BRB's excellence in the seismic performance can be secured when a secondary resisting component doesn't function after the steel core’s yielding.
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