| Up to now, a large number of researches on the relationship between seismic action level R and ductility demandμfor the single plastic hinge single-degree-of-freedom (SDOF) systems have been done by researchers both domestic and abroad, and consistent conclusions have been gotten. Meanwhile, the R-μrelationships of SDOF systems have been directly applied to the relevant provisions in most national seismic design codes, many researchers have tried to directly apply the approximate linear relationships between R andμof the single plastic hinge SDOF systems into hyperstatic multi-degree-of-freedom (MDOF) systems since most structures in engineering practice are hyperstatic MDOF systems. However, influenced by factors such as plastic hinge mechanisms, structural overstrength and multi-modes reaction characters, the inelastic dynamic response of hyperstatic MDOF systems are much more complicated. The basic definitions of seismic action level and ductility, and the quantitative relationship between them may be essentially different from the single plastic hinge SDOF systems, make the corresponding relationships between them for hyperstatic MDOF systems are hard to define. Though a few research results on this have been reported in recent years, the analysis methods still lingered in modifying the research results of the single plastic hinge SDOF systems, and no profound analysis results on ductility demandμfor the deformations of structures, the inter-storey drifts and the plastic hinges of components under different R has been gotten. Since seismic checking analysis under rare earthquakes gradually become the dominate method in the new seismic design generation, it is necessary to study the inelastic dynamic response patterns of real structures which are designed with different seismic action levels under rare earthquakes. Accordingly, not only the relationships between the seismic actions and the structural deformations mentioned above can be observed, but also the quantitative relationships between the design seismic action levels for hyperstatic MDOF or SDOF systems and the inelastic deformation demands under rare earthquakes may be concluded. Finally, designing suggestions can be supplied for these types of structures.Based on this background, main research work in this thesis as following:①Summarize and comment on the representative research findings of R-μrelationships both for SDOF and MDOF systems and overstrength characteristics of hyperstatic structures. ②A single-bay single-storey RC frame, a two-bay three-storey RC frame and a three-bay six-storey RC frame locate in intensity zone 8(0.3g) are set as typical structures and each is separately designed under five different R values, in which the coefficient R still represents the seismic action level, butμonly reflects the plastic hinge rotation ductility of the components. Then inelastic dynamic response analyses under certain ground motions for these RC frames are taken to check the relationship between R and the overall structural deformations, the inter-storey drifts and the plastic hinges of components for hyperstatic MDOF systems.③Based on the results of analyses, the reasons why the structures designed with relative small seismic action level can withstand the resistance earthquake and rare earthquake level are explained.④Through the dynamic pushover analyses of typical frame-wall structures which separately adopt with different stiffness of coupling beams and different storeys, locate in different intensity zones and are strictly designed according to Chinese current codes, the influences of intensity zones, storey numbers and the stiffness of coupling beams on overstrength of frame-wall structures are studied.From the research works stated above, the main conclusions can be preliminarily obtained as followings:①The analysis results indicate that, for both types of structures, though the overall reactions, storey reactions, ductility demand of elements and plastic energy-dissipation forms of hyperstatic MDOF systems vary with the R value changes, the changing magnitude is not that obvious as the single plastic hinge SDOF does. It is reasonable to suggest a specific R-μrelationship which is different from the one for the single plastic hinge SDOF system should be provided for a certain type of hyperstatic MDOF systems based on its specific performances under rare earthquakes. Take the moment frame in this thesis as an example, on one hand, it is suggested to choose an optimized force reduction factor R neither too big nor too small for bearing capacity design, on the other hand, in this R value interval, though all the structures will get into the inelastic state, the rotation magnitude of all the components may still in some reasonable scope. Hence, on that the basis of the structure is rational and its overall stability is favorable, reasonable seismic fortified measures should be taken to ensure the components have enough ductility capacity to withstand the resistance earthquake and rare earthquake levels.②The main three reasons the structures designed with relative low seismic action levels can withstand the resistance earthquake and rare earthquake levels are stiffness degradation, hysteretic energy-dissipation and overstrength, among which the key reason is the stiffness degradation with its severity related to R value.③The results of dynamic pushover analyses indicate that, "seismic margin" of moment frames between the rare earthquake level point and the first peak point of dynamic pushover curve( i.e. failure point) is around 1.4 ~ 2.5, and this value for frame-wall structures has approximately reached 3.5, which means that a considerable potential level exists between the rare earthquake level and collapse.④Frame-wall structures designed strictly according to Chinese current codes show different overstrength degrees, which are significantly larger than the existing research results of moment frames. The stiffness of coupling beams has remarkable influences on overstrength coefficients for both 0.2g and 0.3g intensity zones. Generally, the overstrength values decreased if increase the stiffness of coupling beams in the structures, and the bottom shear forces are relatively big for the structures with stiffness reduced coupling beams, especially this trend is more evident in 0.3g intensity zone. But the storey numbers has few effects on the overstrength values. When the structures with same storey number, the ones locate in low intensity zone get larger overstrength value. |
PDF Full Text Request