Research On Parallel Base Isolation Seismic System With Rubber And Sliding Supports

The parallel isolation system is nonlinear and its practical calculation is complex. The corresponding regulations in the seismic code GB50011-2001 are specially for the structures adopting rubber bearings which have been researched adequately. In our country, the calculations of base isolation structures are often offered by the scientific research departments, not like designing the traditional seismic buildings by the design departments, which restricts its development.At the present, the base isolation is used in the low buildings (usually under 40 meters) and its mainly belongs to the masonry or frame structures, which are of shear mode and the horizontal displacement of its superstructure is small. Therefore, the calculating model can be mostly used of the single-degree-of-freedom system in this paper, which is proved reasonable with the multi-degree-of-freedom model.The damping ratio of the isolation layer is different in the different references and therefore its maximum value is analyzed briefly by the means of the damping ratio for the single rubber bearing support in this paper. At the same time, the calculating methods have been researched adequately concerning the vertical and horizontal stiffness of rubber bearing support and the calculating formulas are given out in this paper. Moreover, the friction bearings are researched on the damping mechanism and its simplified curves of the damping force-displacement are given out and that is the basic hypothesis for calculating the base isolation system. For the isolation layer, when its horizontal displacement takes place under the seismic horizontal force, the vertical loads will transfer from the rubber bearings to the friction ones. The more intensive the seism, the more this transfer, the larger the increase of the friction force, and hence the greater the ability of the system to dissipate the seismic energy and to reduce its displacement of response. This is the self-adjustable protective characteristic of the parallel base isolation system.Meanwhile, the calculating methods of the parallel base isolation system are studied on three aspects: the time history analysis with fortran programs, the reliability analysis and the simulation of MATLAB software in order to find one for the practical calculations.The time history analysis of calculating the base isolation structures is commonly with fortran programs, being applicable for both the normal and the severe earthquakes. It is, however, a difficult problem to choose seismic records reasonably, as it is pointed out in many references that the results of calculations vary widely with the different seismic records applied to the same structure, even though all of them can satisfy the corresponding regulations in the seismic code. How to choose the seismic records is introduced in the explanation of the regulations about time history analysis, which means that it may be reasonable to modify the time step of seismic records. If the time step is modified, however, the seismic records are also changed in the characteristic of frequency and therefore the calculations in this paper are carried out according to the invariable time step.It is reasonable to calculate the seismic structures with the method of reliability analysis because the seis- mic action is the statistical results of all the seismic records which happened in history, and thus it is unneces- sary to choose the seismic records. As for this method, its formulas are not the same for the linear systems as those for the nonlinear ones, and both of them are complex terribly. Furthermore, to deduce these formulas needs more profound mathematical knowledge concerning the base isolation systems. These causes restrict the application of this method and it is more important that the number of historical seismic records is not enough to analyze the earthquake statistically at all, especially the number of historical seismic records is much less in our county. Therefore, the method of reliability analysis is not consummate highly.The simulation of MATLAB software is another form of the time history analysis in essence and sim- ilarly. The difficult problem to adopt this method is how to choose the seismic records reasonably, too. Tocompare the common method with the fortran programs, however, it is clear that its apparent benefits are per- spicuous and simple. Meanwhile, the simulation models are not understood in the mechanical aspect, and consequently few simple algorithms can be accomplished unless the mathematical knowledge is sufficient concerning the base isolation. Just for this reason, the few simple algorithms are completed in this paper.It is evidently that the method of reliability analysis is a developing tendency. However, it is advisable to adopt the practical calculating method of the simulation of MATLAB software if the some mathematical knowledge is studied seriously.