Design And Experimental Study On Steel Reinforced Concrete Frame-core Wall Structure With Transfer Floor

The steel-concrete hybrid-structure is one of the most widely used structural systems for high-rise buildings in China. Among the tall buildings built by the end of 2004, 22.3 % were hybrid structures for 150 m to 200 m tall buildings, 43.8% for 200 m to 300 m tall buildings, and 66.7% for 300 m or higher buildings. It is expected that the hybrid structure will become the first choice for tall buildings. The seismic design methodology of hybrid structures in the building codes of China were developed mainly based on experimental results, evaluation of earthquake damage, and the engineering experience. Due to the lack of the actual earthquake damage records for hybrid structures, the experimental studies become extremely important. Research on hybrid structures generally involves pseudo-static experimental tests and shaking table tests on structural elements and joints. Small scale tests of hybrid structures for some actual building projects have been performed in China. It is obvious that actual seismic behavior of the hybrid structures such as, structural failure modes under earthquake, story shear distribution, the structural characters of hybrid structures based on the standard design methods, the transfer layer design, and limits of elastic-plastic drift and rotation of the weak story, etc., however, can not be appropriately reflected by those small-scale experimental results It is objective of this dissertation to investigate pseudo-static experimental behavior of a large scale frame core-tube hybrid structure model.Literature survey and review on the design methods of frame core-tube hybrid structures are performed. For the frame core-tube structure, the Technical Specifications for Concrete Structures of Tall Building (JGJ3-2002) provides the general structural design guidelines such as structure configuration, suitable height, width-height ratio and seismic design requirement, and specifies the detailed structural element design methods. The bar finite element method is used for structural analysis and JGJ3-2002 also stipulates the details of computing parameter values. A portal 30 story frame core-tube hybrid structure with steel-encased concrete columns, steel beams, and steel-encased concrete shear wall core, is designed to meet seismic requirement of the Earthquake Intensity 8 in accordance with JGJ3-2002.A pseudo-static experimental test of a 1:10 scale model of the above 30 story frame core-tube hybrid structure prototype was performed. The materials used in the model are generally the same as the prototype structure with very similar strength and modulus of elasticity. Gravel aggregate concrete was used to simulate the prototype structure concrete. Welded Q235 thin steel plates was used to represent structural shapes encased in concrete, and steel wires was used for steel rebar. In order to achieve the equal strain and stress status, a 157 Ton iron block was evenly applied to the model based on the similarity relation. The lateral loads applied to the model were the same pattern as the shear distribution among the structural stories under a minor earthquake.The experimental results indicate that the final failure mode of the frame core-tube hybrid structure is overturn, the bottom part of the concrete core-tube is pulled out and the frame columns are fractured. Many types of cracking during the process of applying lateral load were observed. Shear-flexural inclined cracks appeared on the walls parallel to the loading direction and the cracks were gradually extending to the compression part of the walls. Horizontal tensile cracks were observed on the walls perpendicular to the lateral loading direction. Flexural cracks and shear inclined cracks were observed on the wall link beams. Flexural cracks appeared on the bearings and the middle span of the steel-encased concrete transfer beams and a few inclined cracks were also observed at the bearing pads. Horizontal tensile cracks appeared on the frame columns of the bottom stories and flexural cracks occurred at the column ends of the top stories. The joints of the frames parallel to the loading direction had crossing inclined cracks.During the test, the frame core-tube hybrid structure model has performed well with essential t elastic behavior under minor and moderate earthquakes. The maximum base shear of test model is larger than the elastic base shear of the structure under a strong earthquake. The hysterics loop of the frame core-tube hybrid structure is a reverse narrow "S" which means that the capability of hysterics energy absorption was not limited. The damping ratio of the model was 3.16% at beginning of the test and become 20.81% when the structure failed. The maximum tested drift angle the structure was 1/50 and the maximum inter-story drift angle of 1/48 occurred at the 15-20 stories. Although the inter-story drift angle was relatively big, there were only a few small cracks on those stories. Severe damage including many wide cracks was observed on the first two stories. The maximum drift angles were 1/95 and 1/60 for the first and second stories, respectively. It is rational to use the inter-story elastic-plastic drift angle as the limit value to prevent failure of the bottom stories, but not to the top stories.Various static elastic-plastic analysis methods for high-rise hybrid structures are discussed. The hybrid structure experimental model was analyzed using SAP2000. Concentrated plastic elements were used for frame beams and columns. Shear walls were considered as equivalent beams combination model. The horizontal loads of story distribution pattern were the same as the experiments. The calculated skeleton curve of top displacement - bottom-shear agrees well with the experimental curve prior to the structure failure. The plastic hinges distributions under minor, moderate, and major earthquakes are generally the same as the experiment results.How to calculate the damping is important for the static elastic-plastic analysis. Based on the idealized elastic-plastic hysteretic loop, the methods of computing the added damping for hybrid structures in elastic-plastic stage were investigated. . Comparing with the experimental results, an added damping modification factor of 0.3 for a hybrid structure is proposed for practical use. A ductility factor of 3.54 for was obtained by using bi-linear method and equaling initial stiffness and a framed core-tube hybrid structureThe model experimental test indicated that hybrid structures do not have good ductility. Relying only on the structure ductility may not be able to achieve the no collapse objective during a major earthquake. Performance-based seismic design may be a better way to solve the problems. According to the "over limits design review " experience, the seismic design of tall hybrid structures can be performed by using the principle of no-yielding under a moderate earthquake or essential elastic performance under moderate earthquakes. Based on the fundamental principles of seismic design, it is proposed that the seismic force used to design the structure load-carrying capacity is taken as the structure elastic seismic force under a major earthquake divided by the hybrid structure's ductility factor . This seismic force is between the elastic seismic force under a moderate earthquake and the seismic force under a minor earthquake. This will ensure a no-collapse structure under a major earthquake. The structure load-carrying capacity will be enhanced after amplifying the seismic force without modifying the internal forces. In order to ensure the structure to have better ductility, however, some structural measures in the current design codes, such as the steel reinforcement ratio, steel stirrups ratio, axial compression ratio, etc., should remain. Comparison of several performance-based design methods was made. It is concluded that the elastic internal forces under a moderate earthquake are the largest, the seismic forces resulted from the method of no-yielding under a moderate earthquake are the second largest, and the seismic forces obtained from the method proposed i.e., elastic seismic forces under a major earthquake divided by the hybrid structure's ductility factor coefficient, are the third one. The smallest forces was obtained by the minor earthquake design method. While using the minor earthquake design method, the adjustment factor of the structure internal forces is complicated, for example, the adjusted column shear of the super or first grade seismic design will exceed the column shear under a moderate earthquake. This makes it difficult to understand structural seismic behavior during earthquakes.The distributed shear gradually increases along the height of the framed core-tube hybrid structure. The shear carried by the bottom frame is the smallest portion of the entire story shear. The ratio of the shear carried by a upper story frame to its story shear is larger than 20%, it is, therefore not properly to adjust the shear carried by a frame by 20% of the story shear. During the elastic-plastic stage, the shear carried by each story frame tends toward the same. It is reasonable to use the maximum frame story shear to adjust each story shear in the building codes.There is a potential overturning failure mode for a high-rise hybrid structure. The overturning moment carrying by the frame is almost the same as the moment carrying by the tube. For the upper stories, the outside frames carried larger overturning moment . This implies that the frame is not just a second line of defense, and it basically has the same functions as the core-tube to resist overturning.The shear forces are re-distributed after the wall getting into elastic-plastic stage. The shear carried by the tensile part of the wall will gradually decrease and the shear carried by the compression part of the wall will gradually increase. It should be pointed out that a shear wall may lose its shear-carrying capability when a small eccentric tension occurs.Transfer beams carry the concentrated forces from the columns of upper story frames. The concentrated forces are required to be amplified in the design by the building codes, to ensure they are strong enough to be still in elastic stage or have enough ductility under a major earthquake so that the global structure ductility under a major earthquake can be achieved. The steel shape transfer beams used in the experimental test have such characteristics. The computing analysis indicates that the steel shape transfer beams are still in elastic stage under a major earthquake and have high shear-carrying capacity. There is no shear failure in either the computing analysis or the experimental test.