环境激励下冷却塔结构模态测试与阻尼比特性研究

现行冷却塔结构阻尼比均借鉴荷载规范中钢筋混凝土的5%取值,考虑到冷却塔结构自身构型和材料属性,在理论上其真实阻尼比应小于规范值;阻尼比作为风与地震动力分析的重要输入参数,其取值大小将直接影响冷却塔抗风及抗震安全性,然而,现阶段国内外均缺乏大型冷却塔的现场实测和阻尼比取值研究。选取国内8座典型塔高和塔型的冷却塔进行现场测试,获取了环境激励下塔筒典型部位的加速度响应振动信号。首先采用随机减量法和自然激励技术对实测信号进行预处理,然后结合ARMA,ITD和STD三种模态识别方法获得冷却塔前10阶结构自振频率和阻尼比,并将实测值与有限元结果进行对比和误差分析,再借鉴振型组合的思路推荐了8座测试塔的等效综合阻尼比,最后给出了以基频为目标函数的前10阶模态阻尼比和等效综合阻尼比的估算公式。研究表明,8座冷却塔实测频率与有限元分析结果较为一致,基频最大相差为4.4%;阻尼比识别结果分布较为离散,前10阶模态阻尼比最大为2.86%;8座测试塔的综合等效阻尼比范围均在1.13%-2.16%,误差分析结果表明本文提出的阻尼比拟合公式精度高、稳定性好。

Research of damping ratio characteristics and of full-scale measurements for large cooling towers under the ambient excitation

Standard 5%damping ratio for reinforced concrete is generally used for the current damping ratio value of cooling towers,but considering the unique configuration and material attributes of cooling towers,the actual damping ratio must be smaller than the recommended.Damping ratio is the important input parameters of wind and earthquake dynamic analysis,and the wind resistance and earthquake safety of cooling towers are affected by its value.However,very few research of field measurements and damping ratios for large cooling towers has been conducted.Eight cooling towers with typical height and type in the domestic are selected for the field measurements,and acceleration response at representative positions of the tower are obtained under ambient excitation.Firstly,the vibration signals are preprocessed combining random decrement technique and natural excitation technique.Then three modal identification methods(ARMA,ITD and STD)are applied to analyze the frequencies and damping ratios for the first 10 order modes,and the cooperation and error analysis between measured values and results of the finite element are carried out.Following the thought of modal combination,equivalent synthetic damping ratios of eight cooling towers are derived.Finally,the formulas of the first 10 order equivalent modal damping ratios and comprehensive equivalent damping ratio are given.The results show that the measured frequencies of eight cooling towers are consistent with the results of finite element analysis,and maximum difference of fundamental frequency is 4.4%.Distribution of identified damping ratios is discrete,and the maximum value of the first 10 order damping ratios is 2.86%.The range of the comprehensive equivalent damping ratios of eight cooling towers are 1.13%-2.16%,and error analysis results show that high precision and good stability of the damping formulas in this paper.Our research findings provide reference for determining the value of damping ratio in large cooling towers and deepening the understanding on the influence mechanism of damping ratio.