Estimation of damping ratio of TV towers based on ambient vibration monitoring

Damping ratio is an important component in the dynamic analysis and plays a key role in design of supertall buildings. It is necessary to have reliable field measurement records for estimation of structural dynamic characteristics. The main objective of this paper is to estimate eigen mode damping ratio of TV towers as a kind of supertall buildings with the height of more than 250 m. In this regard, field measurements were performed on Milad TV tower, located in Tehran, which has a height of 435 m by ambient vibration monitoring. Damping ratio of this tower is determined using random decrement technique and compared with available data of a few other TV towers. For proposing eigen mode damping ratio of TV towers, it is shown that Rayleigh damping coefficients could not be calibrated properly for tall slender TV towers. A damping estimation chart of TV tower that is a function of natural frequency and structural aspect ratio is proposed, and the chart validity has been checked with field measurement results of other TV towers. The chart shows that structural damping for wind analysis proposed in design codes and standards is overestimated for tall slender TV towers. Copyright © 2011 John Wiley & Sons, Ltd.     

覆冰导线三自由度耦合舞动稳定性判定及气动阻尼研究

利用风洞内支架式三自由度频率可调的弹簧悬挂装置对D形覆冰六分裂导线节段气弹模型进行舞动试验,在不同气动力特性的风攻角下调整竖/扭自振频率比及风速,激发并记录纯竖向、纯扭转、竖扭耦合和扭转水平耦合等失稳舞动。分别基于实测响应和三自由度舞动响应非线性数值模拟结果识别导线的竖向和扭转气动阻尼并与Den Hartog或Nigol舞动理论计算值进行对比,研究导线气动阻尼与气动特性、自振特性及风速之间的关系。试验结果发现：由于Den Hartog或Nigol系数正负不同以及竖/扭频率比不同,导线气动负阻尼绝对值会随风速提高以指数形式增大或者是随风速提高先增大后减小至小于结构阻尼;用单自由度舞动理论来预测三自由度舞动存在较大缺陷且偏于不安全。基于特征值摄动法提出覆冰分裂导线竖向-水平-扭转三自由度耦合的舞动稳定性判断条件式,解释了无法用Den Hartog或Nigol单自由度稳定性判断条件式说明的舞动现象,并得到试验结果的验证。     

抑制斜拉桥拉索风雨激振的气动措施研究

在驰振理论的基础上,运用带有运动水线的二维拉索模型对风雨激振进行理论分析,得到了拉索发生风雨激振的充分条件、必要条件及临界发散风速。并将此分析方法运用到设有气动措施的拉索上,引入等效阻尼比指标对气动措施减振效果进行定量分析。结合静气动三分力试验和分析计算,对螺旋线、纵向肋条和椭圆环三种气动措施的减振效果进行评价分析,螺旋线、纵向肋条的减振效果好于椭圆环。通过人工降雨试验,对气动措施减振效果的分析方法进行检验。     

正六边形超高层建筑横风向气弹效应研究

为研究正六边形超高层建筑横风向风致响应和气动阻尼比,开展了一系列多自由度气弹模型风洞试验。测量模型顶部风致位移和加速度响应,基于随机减量法识别了横风向气动阻尼比。结果表明,在顶角迎风时,正六边形超高层建筑易于出现大幅涡振现象,在立面迎风时没有出现涡振现象。顶角迎风时,横风向气动阻尼比随折算风速增大呈现出“先增大到最大正值、再迅速转为最小负值,再平稳回升到零值附近”完整过程。而立面迎风时,横风向气动阻尼比与折算风速近似呈线性关系。最后,建立横风向气动阻尼比的经验评估公式。相关研究可为正六边形超高层建筑的抗风设计和规范完善提供参考。     

大跨度空间膜结构随环境风速模态变异性分析

由于膜结构具有较强的几何非线性特性,同时受流固耦合效应的影响使其动力参数识别难度加大,基于此点,本文利用小波包、随机减量技术和Laplace小波联合方法对大跨度空间膜结构的模态参数进行了基于实测数据的识别,并进一步分析了大跨度空间膜结构模态参数随风速的变化规律及其相关性,分析结果显示,该方法可有效识别膜结构的动力参数,且具有较好的抗噪能力,对于骨架支撑式膜结构而言,由于存在气动阻尼作用,其结构自身总阻尼较常规结构为大,这有利于结构气动稳定性,但该种结构并不属于风致敏感性结构。     

Identification of damping ratio and its influences on wind‐ and earthquake‐induced effects for large cooling towers

Standard 5% damping ratio for high‐rise concrete structures is generally used for dynamic analysis under the action of wind and earthquakes in the existing cooling tower regulations and researches. But considering the unique configuration and material attributes of large cooling towers, the actual damping ratio must be far smaller than the recommended. However, only a few field measurements of damping ratio for large cooling towers have been conducted; neither are there thorough investigation into the qualitative and quantification of wind and seismic effects under different damping ratio. To fill this gap, field measurements of a large cooling tower standing 179 m in northwestern China was performed and acceleration vibration signals at representative positions of the tower under ambient excitation were obtained. The vibration signals were preprocessed combining random decrement technique and natural excitation technique. Three pattern recognition methods (auto‐regressive and moving mean model, Ibrahim time domain, and spare time domain (STD)) were applied to analyze the frequencies, damping ratios, and modes of vibration for the first 10 order modes. Following the line of thought of modal combination, the equivalent synthetic damping ratio was derived. Under 5 damping ratios (0.5%, 1%, 2%, 3%, and 5%), a comparative analysis on the dynamic responses of the cooling tower to wind and single seismic loading by using full transient method was performed. On this basis, the patterns of influence of damping ratio on wind‐induced vibration, wind vibration coefficient, and time history and extrema of seismic responses were extracted. Finally, different combinations of dead weight, wind, temperature in winter, sunshine duration, and seismic intensity and those of accidental seismic effects (8 working conditions) were considered, using equivalent synthetic damping ratio and standard damping ratio. Thus, the most unfavorable working conditions were identified under actual and standard damping ratios for the large cooling tower. 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.     

深圳京基金融中心横风向气动阻尼试验研究

针对439 m的深圳京基金融中心（KFT）工程,采用气动弹性模型技术研究其横风向气动阻尼特性。根据工程地址的100 a重现期风速、缩尺比例以及前期刚体模型同步测压所得到结构风敏感风向（东西向）的广义力功率谱密度特征确定气弹模型试验的风速范围,通过风洞试验获取不同风速下气动弹性模型敏感风向的顶部加速度响应数据,应用随机减量技术计算分析横风向气动阻尼随折算风速的变化规律,试验考虑结构模态阻尼对气动阻尼的影响。结果表明：在折算风速为4~14的范围内,结构气动阻尼值均大于0且随风速的增大而增大,上游地王大厦(DWT)的干扰效应对气动阻尼未产生不利影响,KFT的基本频率和DWT脱落在尾流中的漩涡频率一致的100 a重现期风速时的气动阻尼比为1.25%,表明前期采用刚性模型试验对KFT进行抗风设计时,不考虑气动阻尼影响计算得到的结构风致荷载和风致响应偏于保守和安全。     

大跨越输电塔线体系风振控制研究

本文通过模型风洞试验和对江阴大跨越输电塔线体系风振控制设计计算,研究了大跨越输电塔线体系动力特性和风振控制。通过悬挂水箱、粘弹性阻尼摆和悬挂水箱加粘弹性阻尼摆组合控制三种制振方案的气弹模型风洞试验研究,验证了设置振动控制装置可有效地减小大跨越输电塔的风振响应,为江阴大跨越工程风振控制设计及应用提供了依据。最后,对正在建设中高346.5m的江阴大跨越输电塔线体系进行风振控制设计,提出用调频质量阻尼器(TMD)和粘弹性阻尼器(VED)的控制方案,理论计算表明具有较好控制效果。     

大跨越输电塔线体系动力特性及风振响应

以正在建设中的世界第一高 3 46 5m的输电塔———江阴大跨越为工程背景 ,建立了考虑输电线、绝缘子和塔架耦合的两塔三段线分析模型 ,用非线性有限元分析方法对大跨越塔线体系进行了风振响应分析 ,并进行了气动弹性模型风洞试验。通过理论计算和试验研究 ,指出输电线对塔架的自振频率影响较小 ,而结构体系风振响应分析则应考虑塔线耦合影响 ,提出的分析方法和得到的结论可供实际工程抗风设计参考     

江阴高压输电线路脉动实测及分析

采用环境随机激振方法对江阴500kV输电线路的一座拉线塔和自立塔分别进行了实测。得到了拉线塔横向、纵向及扭转向的一阶频率、振形和阻尼比。并对自立塔和拉线塔的实测结果进行了比较。     

环境激励下房屋建筑的阻尼比识别

阻尼比的准确取值一直是结构动力分析的难点,由于阻尼机理非常复杂,对阻尼比的取值一般通过实测得到。传统的模态参数识别方法需要同时利用激励和响应信号,但对实际建筑结构,激励施加存在困难。探讨了基于环境激励下的模态识别方法,利用环境激励(如施工机械振动、风、车辆、行人、地脉动等)下的结构振动响应数据,采用随机子空间(Stochastic Subspace Identification)法,有效地识别实际建筑结构的模态参数的方法。工程实例的模态参数识别结果表明,该方法不仅能准确识别出结构的模态频率,还能相对准确地得到结构的模态阻尼比。     

输电塔结构动力特性研究

本文根据多条输电线路工程中不同类型、不同高度输电塔为原型 ,建立空间有限元模型 ,利用模态分析研究输电塔结构的动力特性 ,计算出结构第 1自振周期及其振型 ,以实测所得的周期估算公式为基础 ,经过修正得出较为精确的输电塔结构第 1自振周期近似计算方法和细化后的结构第 1振型系数 ,可用于输电塔设计中风振系数计算 ,最后用实际算例验证了本文研究成果对工程设计有参考意义。     

Experimental study of across‐wind aerodynamic damping of super high‐rise buildings with aerodynamically modified square cross‐sections

Across‐wind aerodynamic damping ratios are determined from the wind‐induced acceleration responses of 10 aeroelastic models of square super high‐rise buildings in an urban flow condition (exposure category C in the Chinese code) using the random decrement technique. Moreover, the influences of amplitude‐dependent structural damping ratio on the estimation of aerodynamic damping ratio are discussed. The validity of estimated damping is examined through a comparison with previous research achievements. On the basis of the estimated results, the characteristics of the across‐wind aerodynamic damping ratios of modified square high‐rise buildings are studied. The effects of aerodynamically modified cross‐sections, such as chamfered, slotted and tapered cross‐section, on the across‐wind aerodynamic damping ratio are investigated. The results indicate that modifications of cross‐sections are not always effective in suppressing the aeroelastic effects of super high‐rise buildings. Low corner‐cut ratios (chamfer ratios from 5% to 20% and slot ratios from 5% to 10%) and low taper ratio (1%) significantly decrease the magnitudes of absolute aerodynamic damping ratios. However, large modifications of cross‐sections (slot ratio of 20% and taper ratios from 3% to 5%) increase wind‐induced responses by changing the aerodynamic damping ratios. According to the database, empirical aerodynamic damping function parameters are fitted for high‐rise buildings with aerodynamically modified square cross‐sections. Copyright © 2013 John Wiley & Sons, Ltd.     

超高层建筑气动弹性效应双向受迫振动风洞试验研究

通过双向受迫振动风洞试验对高347m的长沙世茂广场模型的气动弹性效应进行研究,模拟结构平面两个轴向的一阶振动,同步测量了振动模型上的表面风压和模型顶部位移。在对振动模型横风向和顺风向的气动弹性力分析基础上,识别了该模型气动阻尼比和气动刚度比,计算并分析了气动弹性效应对结构风致响应和等效风荷载的影响。分析结果表明,在100年重现期风速作用下,该模型气动阻尼比为正值,气动刚度为负,气动刚度相对于结构刚度较小,对结构自振频率影响不大。考虑气动弹性参数后,顶部最大位移响应可减小5%,最高居住层最大加速度响应可减小10%,由等效风荷载计算得到的基底总剪力和基底总弯矩减小1.1%左右。分析表明,双向受迫振动风洞试验是一种有效且有实用前景的超高建筑气动弹性参数识别方法。     

封闭式单向张拉膜结构气弹失稳机理研究

在均匀流场中进行了封闭式单向张拉膜结构气弹模型风洞试验,考察了膜预张力和来流风速对结构振幅、振动模态和总阻尼比的影响,以及膜面位移与膜上方流场的频谱相关性,分析了封闭式膜结构的气弹失稳机理,给出了气弹失稳无量纲临界风速。研究结果表明：低风速下(无量纲风速小于1.2时),流场中的旋涡主频远低于结构基频,结构以受迫振动为主;随着风速增大,流场中出现与结构2阶反对称模态频率接近的旋涡,导致结构发生以2阶模态大幅振动为特征的涡激共振,且在无量纲风速不小于1.2 的风速范围内出现振动锁定现象,结构总阻尼比迅速衰减接近0。由此可认为,封闭式膜结构的气弹失稳由涡激共振引起,结构振幅的突然增大、主导振动模态的突然改变以及总阻尼比的突然减小为其主要特征;气弹失稳无量纲临界风速约为1.2。     

Evaluation of wind loads on super‐tall buildings from field‐measured wind‐induced acceleration response

With the nonstationary wind‐induced acceleration data from full‐scale measurements, an approach for estimation of the wind‐induced overturning bending moments for super‐tall buildings was proposed in this paper. The empirical mode decomposition was employed to decompose the measured acceleration data into a set of intrinsic mode functions and a residual component. To remove the baseline offset, the residual component and the intrinsic mode function components with long‐period were eliminated before their integrations into velocity and displacement components. Then, the intrinsic mode function components, which have the same dominant periods as the natural periods of the studied tall buildings, were extracted from the original signals, and the natural frequency and damping ratio for the first vibration mode of the building were identified. Finally, the wind‐induced overturning bending moments of the building were obtained from the generalized wind loads for the first vibration mode, which could be obtained from the time history analysis of dynamic equation. The Hilbert spectrum of wind‐induced overturning bending moments was utilized to observe its characteristics in both time and frequency domains, and the Strouhal number was thus identified. The proposed scheme and some selected results may be helpful for further understanding of wind effects on super‐tall buildings. Copyright © 2012 John Wiley & Sons, Ltd.     

斜拉桥拉索风雨激振的两质量三自由度理论模型

斜拉索的风雨激振严重地影响到斜拉索以至于斜拉桥的安全,其机理至今仍无定论。同时考虑拉索面内和面外两个自由度,并假设拉索和水线之间的作用力为库仑阻尼力和线性阻尼力,建立斜拉桥拉索风雨激振的两质量三自由度理论模型。采用Runge-Kutta法对拉索和水线的耦合方程进行数值求解,得到拉索和水线的运动规律。与仅考虑拉索面内自由度的两质量两自由度理论模型相比,考虑拉索面内面外自由度的两质量三自由度理论模型的起振风速区更多,这主要是因为拉索的水平气动力系数也存在突降,且拉索面内面外运动是相互耦合的。水线上的气动力和重力为水线提供总体的平衡;阻尼力则消耗水线的能量;至少一个方向的拉索运动引起的附加惯性力(竖向或水平)一直对水线做正功。     

Structural behaviour of lattice transmission towers subjected to wind load

Hurricane Wilma hit Yucatan Peninsula in 2005, causing substantial damage to local electrical transmission towers. Based on the failure characteristics observed on such towers, an analytical study is performed to reproduce such failures and assess their vulnerability. Two latticed transmission towers are analysed under the action of 14 different wind velocity patterns corresponding to several national and international wind design codes. Displacement-controlled pushover analyses are performed to reproduce impending failure mechanism for considered wind patterns, and associated gradient wind speed is computed. Results illustrate that consistent cyclonic wind speed patterns lead to better estimates of failure mechanism and gradient wind values than traditional non-cyclonic patterns.     

桥梁箱型吊杆涡振与驰振耦合振动的数值模拟

为研究桥梁箱型吊杆涡振与驰振耦合状态下的风致振动机理,基于某大跨度钢桁架拱桥大长细比箱型吊杆的节段模型风洞试验参数与试验结果,采用计算流体动力学（CFD）软件FLUENT对模型的耦合振动进行了数值模拟,通过FLUENT的后处理功能进一步研究分析了不同振动状态下的气动力和尾流旋涡特征信息。研究结果表明:数值计算得到的风振曲线与试验的实测结果吻合良好,在涡振与驰振的耦合振动过程中,随着振动幅值的不断增大,尾流旋涡脱落由小振幅阶段规则的卡门旋涡形态转化为大振幅阶段的复合模态特征,气动升力也由小振幅阶段的单频特征逐步转化为大振幅阶段的多频特征。     

The influence of the design methodology in the response of transmission towers to wind loading

From a theoretical approach, the design procedure for the establishment of wind loading on transmission towers was reviewed and current procedures, such as Davenport's gust response factor (GRF), were compared with the statistical method using influence lines (SIL), which is considered more realistic. This latter approach can account for unbalanced loading effects, shear and axial loads and the effects of higher modes of vibration in the calculation of the response factors. Several responses due to certain assumed transverse wind characteristics were calculated for some typical transmission towers. The main findings were: (a) Peak loads calculated using SIL were larger than peak loads given by the GRF. (b) The dynamic response of transmission structures is strongly dependent on the turbulence intensity level and its spectrum. (c) For members in which there is reversal in the forces on the load position, the resonant response in the second mode of vibration was bigger, even by four to five times, than the corresponding one in the first mode. Although this effect is not as severe in terms of resulting stresses when all the components are computed in the peak responses, it can lead to fatigue problems. From the current results it can be concluded that the incorporation of the dynamic properties of transmission structures in the design methodologies is needed and that the statistical method using influence lines is a more correct approach since it allows for the inclusion of a larger number of factors in the design methodology.