The Research On Principle Of Mode Number Decision In The Seismic Design Of Tall Building Structures

In the construction seismic design, along with the building's height increasing, its basic period will be more longer, the long period mode has a big ratio in the higher modes, which will occur analogy sympathetic vibration easy with the long period component in the seismic wave, therefore the influence of higher modes of vibration is more remarkable .So it is important to considering the influence of higher modes of vibration to reflect the responses under the seismic more exactly.Firstly, in this paper, according to present research situation of the reasonable mode number decision, the existing algorithms insufficiency in the domestic and foreign has been pointed out, some concepts of the mode have been contrasted, and also a forecast of the applicable scope, computational method, simplification calculation model, and the mode selection principle has been made.Secondly, by the investigation of structure dynamics research results of UC Berkeley from1950 to 1990, as well as the procedure development history of the ETABS and SAP, the origin and mechanism of the theory of the effective mass method has been delved .Obtained that the modal participating mass ratio is equivalent to the base shear,It is necessary to increase other coefficient (which based on displacement equivalent).Thirdly, the modal combination principle of the multi- degrees of freedom system is discussed in mode-superposition response spectrum method, It is shown that the SRSS law often overestimated the result compared to CQC. From the contrasting of effective mass with seismic load, the connotative relationship between them has been obtained.Moreover, the dynamic response spectrum analysis of an ideal frame model has been carried on by ETABS, with the contrasting of different seismic fortification intensity, site, mode number and layer analysis, conclusion of mode decision having nothing to do with them could be gained, which could only affect the response value. The story shear and displacements increase along with the seismic fortification intensity and site increasing, the story stiffness has been reduced slightly; Regarding to the complex structure, the contribution to the base shear of each mode is not descending along with mode number increasing, the modal combination number is different with different inspirit direction.Finally, some summaries and the suggestions have been made to the influence factor and the decision principle of mode decision. It is pointed out that the mode participation ratio was decided by the mode and exterior inspirit, the modal participating mass ratio is related with modal participation ratio and mass, but not with seismic fortification intensity and site .The decision principle is: the sum of modal participating mass ratio should exceed 90 percent, when the number of mode has a bigger increase, the base shear and top displacement should not has a big increase, the error should be assure below 5 percent, to the irregular structure, the dynamic load participation ratio should be exceed 95 percent.