Inelastic Dynamic Response Analysis of Frame Structures Reinforced with Steel Bars in Different Strength Grades and RC Shear Wall Structures
|Keywords||Moment Frames Steel Bars Strength Grades HRB500 Steel Bars Shear-wall Structures Inelastic Dynamic Response Analysis|
HRB400 steel bars have been treated as the leading reinforcements in the 2010 edition Code for Design of Concrete Strutures;HRB335 steel bars will be gradually eliminated in engineering practices;and HRB500 steel bars have been introduced into the new code first time for saving mineral resources and the strategy of national economy sustainable development. Though a certain amount of RC components in different type reinforced with HRB500 steel bars have been tested, effective analysis results are still lacked for seismic performance of RC structures reinforced with HRB500 steel bars. To fill up this research vacancy, preliminary comparisons of inelastic dynamic response analysis of RC moment frames separately reinforced with HRB335 and HRB400 and HRB500 steel bars under rare ground motions have been done by graduates from Shaoliang Bai’s academic team before. But these results need further verifications, as well as the response characteristics of these three RC structures need to be discussed and explained. Therefore, in the first part of this thesis, three Seismic Class I RC moment frames with three spans and eight storeies reinforced with different strength steel bars (HRB500, HRB400 and HRB335) locate in Intensity Region 8(0.3g)are designed as typical structures. Then the nonlinear dynamic response analyses of these structures under rare earthquake have been done for further comparison and research. And dynamic pushover history response analyses and comparisons of single freedom systems which are separately reinforced with HRB335 and HRB500 steel bars under rare earthquake are done.Main conclusions from the works above as following: For fixed cross section dimensions of beams and columns, the reinforcement ratios and the second stiffness of the components would decrease if the strength grade of longitudinal reinforcements increased. During inelastic dynamic response analyses, after the components successfully get into the second stiffness stage, the higher strength steel bars the structures reinforced with, the faster the lateral stiffness decreased, the longer the vibrate periods get, the bigger the displacements at yielding and the more the earthquake response decreased. These factors balanced the displacements of the three typical frames after yield under rare earthquake. Since components reinforced with higher strength grade steel bars yield with higher reaction forces, the plastic hinges at the beam ends and column ends appear slower and the rotations are smaller, which means ductility demand is also lower.These analyses also indicate that, the dynamic response characteristics of structures changes when they get into the second stiffness stage after cracking. Hence, to appropriately determine crack moment and the second stiffness of components is important for getting reliable inelastic dynamic response analysis results. This characteristic should be paid enough attention in inelastic dynamic response analysis method of all parts of components in future.Apart from this, a series of inelastic dynamic response analysis of RC frames and frame-shear structures and frame-tube structures under rare earthquakes have been strictly designed according to Chinese codes by domestic academic teams especially the teams from civil engineering college, Chongqing University ,but it still lacks analyses and studies on RC shear wall structures which are widely used in residential high-rise building around China. To fill up this research vacancy, in the second part of this thesis, the modeling typical structural units are extracted from these widely spread structures, and a typical Seismic Class II 20-floor shear-wall structure in which the walls are connected by slender coupling beams (span-height ratios are larger than five) locates in Intensity Region 8(0.2g)is designed according to current codes. The inelastic dynamic response analysis of the structure under rare earthquake has been done. Relative comprehensive evaluations are made according to structure’s performances. The analyses results indicate the responses of the structure under several rare ground motions are relatively stable, and seismic performance can be controlled in an expected range. The plastic deformation parts under most ground motions mainly distribute at the ends of slender coupling beams below the middle floors and the stiffness of these components accompany severe degradations. The bottoms of the walls don’t yield, and their stiffness degradations are not severe, hence the overall stiffness of the structure is in control and response of lateral displacement is fine. It should be noted that under some ground motions, the rotation ductility demand of coupling beams locate around 2 to 3 floors above the bottom strengthened region are quite high, and they may be damaged due to exceeding their limited deformation capacity. In addition, under some of the ground motions, effect shear forces of walls at bottom or below the middle floors are higher than the combined shear forces which are increased by the measures of“strong shear and weak moment’’and it can leads to shear failure.In addition, the main seismic performances of the shear-wall structure are specified as following: ①Lateral displacements on each floor together with their heights show flexure-shear shape distribution. The distribution of inter-storey drift with structure height shows that inter-storey drift is the biggest below the middle floors and smaller at the bottom floors and the smallest at the top floors. The distribution of inelastic harmful inter-story drift along structure height indicates that it is big at the low floors and gradually get smaller at the up floors. The harmful inter-story drift is around 0.1% to 0.3%, and the biggest harmful inter-story drift locates at the second floor.②The distribution of internal forces of coupling beams along structure’s height indicates that the biggest internal forces appears at middling and lower floors and get smaller at the bottom floors and the smallest one appears at the top floor. This characteristic consistent with the distribution of hinges and rotation ductility demand at the beam ends. The distribution of shear forces of shear walls along structure’s height indicates that the biggest one appears at middling and lower floors and get smaller at the bottom floors and smallest one appears at the top floors.③The shear wall at the floor above the bottom strengthened region yields under some ground motions. This indicates the method that design moment of shear walls should amplify 1.2 for the floors above the bottom strengthened region only applied for the shear wall structures of Seismic Class I in the 6.2.7 provision of the code for Seismic Design of Building may not enough. It is suggested that the range expands to the Seismic Class II shear walls.④As the stiffness degradation is not too severe , shear forces of the bottom walls are quite high under rare ground motions. The amplify coefficient 1.4 of forces for the Seismic Class II shear-walls in the current code doesn’t cover the high shear effect. It should be concerned by the design codes.