脉动风作用下结构基本模态的定义和识别

顾名思义,基本模态应该是结构所有模态中最重要的一阶模态,在动力响应分析中具有不可或缺的作用.传统习惯将第一阶模态作为基本模态,这对于风振响应分析是不恰当的.本文通过对模态应变能公式进行定性分析,归纳出脉动风作用下模态应变能最大的那阶模态的振型所具有的基本特征,并将这阶模态定义为结构在脉动风作用下的基本模态.按照这个定义,脉动风作用下高层、高耸结构的基本模态位于第一阶,这与传统定义相同;而对于平均风荷载全为吸力的小矢跨比穹顶结构,脉动风作用下基本模态位于高阶.脉动风空间相关性的强弱对这两种类型结构的影响正好相反.算例分析表明,模态贡献系数可以准确地识别出基本模态在模态序列中的阶次.

Definition and identification of the fundamental mode of structures under fluctuating wind

General speaking,the fundamental mode of a structure should be the most important mode of all,and must be included in the selected modes for dynamic analysis.Traditionally the first mode is defined as the fundamental mode,however,this is not correct for structures under fluctuating wind.Through qualitative analysis of the mode strain energy formulation,the characteristic of the mode shape with the largest mode strain energy is concluded,and this mode is defined as the fundamental mode of the structure under fluctuating wind.According to the new definition,the fundamental mode of the high rise building under fluctuating wind is the first mode,which is consistent with the traditional definition.As for low rise dome-like roofs,on which the mean wind load is suction,the fundamental mode locates on the high order.The influence of the spatial correlation of the fluctuating wind on the two different structures is just opposite.The numerical examples indicate that the mode contribution factor can be applied to identify the fundamental mode from all the modes of the structure.