双曲冷却塔塔筒水平地震响应分析

为明确冷却塔在水平地震下的内力环向分布特征及内在原因,同时探究不同地震波时程与规范反应谱所得内力差异的原因,以某大型双曲冷却塔为例,在动力特性分析的基础上,通过反应谱方法和时程方法的水平地震响应计算及对比分析,对上述两个问题进行了研究。研究表明:由于仅侧弯振型对水平地震有贡献,而塔筒的侧弯振型和实际响应均表现为整体侧倾并伴随微弱的截面“流动”变形,这也使塔筒各内力的环向分布分别呈现正弦、余弦分布特征;其整体侧倾可类比于悬臂杆结构,塔筒子午向轴力FY、子午向弯矩MY、剪力FXY和扭矩MXY的环向分布可借助悬臂杆侧倾时截面正应力和剪应力分布来解释;而截面“流动”变形则决定了环向轴力FX和环向弯矩MX的环向分布;整体侧移显著而截面变形极小也使FY和FXY的幅值在塔筒中下部明显大于FX;由于冷却塔第1阶侧弯振型在水平地震响应中往往起绝对主导作用,因此可先对所选地震波计算得到其反应谱,对比第1阶侧弯振型周期对应的水平地震影响系数α值,即可初步推断不同时程及规范反应谱方法所得结果的大小关系。

Horizontal seismic response features of hyperbolic cooling tower shells

In order to illustrate latitude distributions of internal forces of hyperbolic cooling tower(HCT)shells in horizontal earthquakes and the reasons,as well as reasons for the differences between results got from response spectrum analysis of code and response history analysis of different ground accelerations,dynamic calculations by the two methods and comparisons are conducted for a certain HCT based on dynamic properties analyses.It shows that the contributions to horizontal seismic responses all come from the lateral bending modes,and the bending modes and seismic deformation display the same feature which is global lateral bending accompanied by minute circular flow displacement of section.This feature determines the latitude distributions of internal forces as sine or cosine.The global lateral bending of HCT is like the bending of cantilever columns,and latitude distributions of meridian axial force FY,meridian moment MY,shear force FXY and torsional moment MXY of HCT can be explained by the sectional normal stress and shear stress.Latitude distributions of latitude axial force FX and latitude moment MX are determined by the minute circular flow displacement of section.On the other hand,the amplitudes of FY and FXY are much larger than FX,just as the displacement amplitudes of lateral bending and the sectional circular flow.The first lateral bending mode is always prime dominant for horizontal seismic responses,so the following method is proposed for approximate estimation of internal force amplitudes without time-consuming response history analysis:getting the response spectrums of the selected ground accelerations and then comparing values of response spectrums at the natural period of first lateral bending mode.