基于分层壳单元模型超大型冷却塔风致倒塌机制与失效准则

历史上超大型冷却塔风毁事件多次发生,国内外现行规范中缺乏超大型冷却塔风致倒塌机制与失效判别准则。以我国西北地区某在建高228 m的冷却塔为背景,建立分层壳单元模型,考虑塔筒的多尺度壁厚及配筋率变化,通过刚体测压风洞试验获取了冷却塔表面风荷载,结合增量动力分析法分析了典型风速下塔筒位移和内力响应,确定临界倒塌风速为83 m/s;基于节点失效前后von Mises应力变化规律提炼了冷却塔倒塌过程中3种内力重分布机制,结合塔筒喉部位移构建了结构变形失效准则。研究表明：采用分层壳单元模型可以有效模拟超大型冷却塔倒塌全过程,中心破坏区域扩散形成裂隙网,直至完全倒塌破坏;塔筒首个单元失效首先引发滑动面机制,随后以转动铰机制和滑移面机制为主进行内力重分布;当结构变形失效指标(喉部迎风面与背风面的相对水平位移与喉部直径之比)不小于1.5%时,超大型冷却塔失效倒塌。     

尿素造粒塔在风荷载作用下的结构反应分析

采用有限元软件SAP2000研究尿素造粒塔在风荷载作用下塔底内力变化情况;分析风荷载作用下结构物的变形情况;分析风荷载下结构物的响应情况及不同基本风压下外壁的主应力分布情况。得出下述结论:在风荷载作用下,造粒塔塔底内力、位移以及应力变化都与基本风压成线性关系。     

Failure analysis of a transmission tower subjected to combined wind and rainfall excitations

Many transmission towers have collapsed during severe gales and thunderstorms, and these failures have traditionally been attributed simply to wind loading. This study attempts to reveal the full failure mechanism of tower structures under strong wind excitation considering the rainfall effect. First, the calculation of rain load for a tower‐line system is provided. Then, an uncertainty analytical method for estimating the strength capacity and predicting the failure pattern of transmission towers induced by wind and rain loads is presented. Next, a real collapsed transmission line is considered to establish the finite element model, followed by the determination of the most vulnerable tower, which is then used to perform the uncertainty analysis. The results illustrate that the collapse basic wind speed considering the rainfall effect is smaller than the pure wind condition. In addition, the failure probability of the tower body is the largest, which is consistent with the deterministic method, whereas the most vulnerable tower began to fail from the tower leg in reality, indicating that the initial broken position of the transmission tower may not occur in the location with the largest probability and that the deterministic method is invalid in some cases. Finally, the influence of the wind attack angle and bundle number of transmission conductor is investigated. The most unfavorable wind attack angle is 90°, and the rainfall effect becomes increasingly significant with increases in the bundle number; this relationship should be given particular attention.     

考虑百叶窗透风率超大型冷却塔内吸力风振系数研究

为研究不同百叶窗透风率下超大型冷却塔内吸力风振系数的分布特性和取值,以国内某在建210m高超大型冷却塔为工程背景,首先通过同步刚体测压风洞试验,得到了不同透风率（0%、15%、30%及100%）下冷却塔结构内表面平均与脉动风荷载。在此基础上,对比分析不同透风率平均和脉动风压分布特性,并采用有限元方法建立了塔筒-支柱-环基一体化仿真分析模型,对该超大型冷却塔进行四种内吸力作用工况下完全瞬态时域动力计算。对比了四种透风率下内吸力风振系数的一维、二维和三维分布特征,分析了以塔筒径向位移、子午向轴力、von Mises应力和环向弯矩四种典型目标响应下的风振系数取值标准,分别给出了此类超大型冷却塔不同透风率下内吸力风振系数的取值建议,即0%、15%、30%及100%透风率下内吸力风振系数分别取为1.69、1.79、1.69和1.57。     

基于配筋量准则的冷却塔结构抗风设计

通过风洞试验、结构有限元分析和结构配筋设计计算,对某八塔组合冷却塔工程进行系统分析,通过内力加权的方法整合荷载模式多样性的影响,提出以内力加权为准则的冷却塔结构安全评定准则。获得了冷却塔结构空间和时间尺度动态配筋及其包络配筋结果,并与现行规范基于单塔配筋结果进行对比。研究表明,风的脉动效应不可忽略,在配筋层面可以看出其影响不只存在于迎风面,对侧风面与背风面也有显著影响。群塔干扰对塔下部的影响大于对上部的影响,在冷却塔的设计中全塔选择统一的群塔干扰系数与风振系数是不合理的。     

竖向地震作用对超大型冷却塔结构的影响分析

采用ABAQUS有限元分析软件,分别对不考虑和考虑土-结构相互作用的超大型冷却塔结构进行了弹性和弹塑性时程分析,研究了竖向地震作用对超大型冷却塔结构的影响以及考虑土-结构相互作用后竖向地震作用影响的差异。结果表明:竖向地震作用对冷却塔塔壳的竖向加速度和竖向位移、塔壳喉部主应力影响明显,对支柱轴力有一定影响,对结构水平向的地震响应影响相对较小;考虑土-结构相互作用后,竖向地震作用对支柱的竖向加速度、支柱内力以及塔壳喉部的主应力影响略有增大。     

风电中空夹层钢管混凝土结构塔筒在往复扭矩作用下的破坏机理研究

本文基于ABAQUS有限元软件,建立了风电中空夹层钢管混凝土结构塔筒在纯扭往复荷载作用下的“壳-实体”精细有限元模型,并进行了数值模拟计算。有限元模型计算结果与试验结果吻合较好。在此基础上,对风电中空夹层钢管混凝土结构塔筒在纯扭往复荷载作用下的变形过程、破坏形态和机理进行了分析。研究表明,风电中空夹层钢管混凝土结构塔筒在达到极限承载力之前,具有较好的组合工作性能,风电中空夹层钢管混凝土结构塔筒在往复扭矩作用下的破坏过程为钢管的鼓曲变形进一步发展为鼓曲区域钢材低周疲劳开裂破坏,同时提出了风电中空夹层钢管混凝土结构塔筒受扭承载力简化计算方法,计算结果与试验结果吻合较好,可用于工程设计参考。     

中空夹层钢管混凝土风力机塔架风振性能研究

传统风力发电机组塔架大多为锥形单管钢薄壁细长结构,此类结构在叶片转动及风荷载作用下易发生大的变形和振动。为克服传统风电塔自身发展的局限性并发挥中空夹层钢管混凝土(CFDST)结构优良的力学性能,基于某锥形钢塔筒,通过承载力等效提出CFDST塔筒结构形式,利用ABAQUS软件建立其风振性能有限元模型,对比了两种塔筒的振动模态。从时域和频域对二者在不同荷载工况下的动力响应特征进行对比分析,并对塔筒与叶片是否共振及瞬态冲击荷载下的振动进行了研究。结果表明:CFDST塔筒在保证原有钢塔筒抗弯承载力和刚度的同时,底部截面尺寸减小了25.6%,且不会与叶轮转动产生的谐波激励发生共振; 阻尼对风机塔筒位移、速度、加速度及应力响应幅值影响显著; 与钢塔筒相比,CFDST塔筒在正常运行荷载工况下峰值位移、加速度幅值和最大等效应力分别降低21.1%、30.2%和41.6%,而在暴风荷载工况下,分别减小14.4%、32.2%和36.3%; 研究成果可为相关塔筒的设计优化提供参考。     

钢塔架结构静力计算的分层空间桁架法研究

通过理论推导,得出在满足分层空间桁架法基本假定条件下的计算公式,用空间桁架法设计塔架,对比分析刚性、柔性斜杆塔架的区别。同时,推导了三角形截面塔架的内力及位移计算公式,用分层空间桁架法及逐次渐进法计算四种截面形状的刚性结构及柔性结构的内力,用规范方法进行构件验算,得到应力比处于0.75~0.85之间时的结构用钢量;并得出如下结论:刚性斜杆塔架及柔性斜杆塔架用钢量基本一致,两种塔架应力比处于同一水平时刚性斜杆塔架刚度较大;由承载力控制时,按照两种形式斜杆设计的塔架用钢量基本相同,由刚度控制时,采用刚性斜杆有助于降低结构用钢量。     

Full-scale and model tests on wind-induced,static and dynamic stresses in cooling tower shells

The response of the shell of concrete cooling towers to turbulent wind is investigated by experimental methods. Calculations based on membrane theory are used to verify the reliability of the test results with respect to the membrane forces. The tests include measurements of wind load and shell response in model and full-scale conditions. For the model tests, dynamic similarity of the model was observed, and the atmospheric boundary layer was simulated. The Schmehausen cooling tower was equipped with special strain and pressure pick-ups for the full-scale observations. The results suggest that there is considerable bending in circumferential stresses, and that dynamic stresses are not always correctly calculated by assuming a static design wind load, affecting in particular the buckling safety of the shell.     

大型冷却塔表面脉动风压原型实测与分布准则

我国冷却塔规范风荷载条款仍源自20世纪80年代原型冷却塔（约90m高）实测资料,且仅规定了塔筒表面静态风压分布。事实上,超大型冷却塔（高度≥165m）风振问题与风荷载脉动作用关系更加密切,由此导致了冷却塔数学和物理试验模型雷诺数效应模拟准则的不完整性,难于准确再现冷却塔表面动态风荷载与来流条件、塔群组合状况等参数间的合理关系,已成为制约大型冷却塔抗风性能研究和结构设计的瓶颈。为此,采用全天候动态风压采集设备,对某电厂冷却塔（约166m高）进行通风筒表面动、静态风压长期现场观测,量化表面脉动压力与来流紊流度之间的影响关系,提出具有原创性的冷却塔超高雷诺数条件（Re≥6E7）脉动风压雷诺数效应模拟准则。     

大跨度斜拉桥倒Y形混凝土桥塔的横向拟静力试验

为研究强震下大跨度斜拉桥倒Y形混凝土桥塔的横向破坏机理及屈服后性能,文中以苏通大桥为背景,基于OpenSees平台建立了有限元分析模型,根据动力特性等效和恒载轴力等效原则将全桥模型简化为单塔模型,并确定了两点位移加载模式,采用1:35的几何缩尺比设计了试验模型并进行拟静力推倒试验,最后结合数值模拟分析,得到地震下桥塔的屈服顺序和破坏位置,并分析屈服后性能。研究结果表明：提出的简化单塔模型可替代全桥模型进行横向地震破坏机理研究;在横向强震下,横梁上侧塔柱截面最先屈服,塔底截面其次屈服,横梁下侧塔柱截面最后屈服,最后破坏位置为横梁上侧塔柱或塔底截面;在塔柱截面屈服后,桥塔整体强度增幅明显,破坏位移可达屈服位移的4~6倍,具有延性破坏特性;塔柱截面配箍率的增加会显著增大桥塔的整体强度和位移能力。     

小型双曲线冷却塔风荷载有限元分析

自然通风冷却塔是火力发电厂的重要组成部分,通常由旋转壳,人字柱,环梁及基础等部分组成。在大型电厂中冷却塔一般有140m,对于这些冷却塔国内外研究得比较多,需要做风洞试验并采用专门的设计程序如LBSD,RBSA等,而在小型火力发电厂中采用的双曲线冷却塔一般塔高60m．很少有资料涉及这部分。文章以无锡某电厂的冷却塔为例,采用通用有限元程序ANSYS分析了冷却塔在自重,风荷栽作用下的内力和变形,及其线性稳定性分析。今后工程中如有类似冷却塔。本文结果可以作为参考。     

考虑脉动风影响的风电塔风致动力响应数值分析与现场监测比较

本文基于"风轮-机舱-塔体"三维仿真模型,进行风电塔在暴风荷载作用下的风致动力响应分析,并基于规范中的风荷载计算方法与动力分析方法对风电塔的应力、位移和内力进行计算分析,同时研究风电塔风轮受不利风向影响时的力学特性变化规律,并对风电塔在常规风荷载作用下的应力和位移进行现场监测比较研究。结果表明,基于规范计算的风电塔的应力、位移及内力要比动力分析的计算值偏小;脉动风时程激励下,风电塔的最大应力值低于结构的最小许用应力值,但结构顶部的水平位移超过结构的位移限值,在风电塔设计中应予以注意。风电塔受不利方向的暴风影响会产生很大的内力,塔底最大剪力和弯矩增大了91%和106%,在风电塔设计过程中应该重点考虑;基于常规风荷载作用下的应力和位移的数值仿真结果与监测结果吻合较好。     

Collapse mechanisms of power towers under wind loading

In this paper, nonlinear static pushover analyses were carried out to assess the capacity and collapse mechanism of two existing transmission towers under wind loading. Different load distribution patterns were adopted from the application of the codified design wind load of the following countries: Mexico, United States, India, Japan and the New Zealand – Australian Code. Three-dimensional inelastic response analyses were performed using open access software. Analyses were employed to define the capacity curves, stress-strain curves for structural elements, the yielding mapping sequences and the collapse mechanisms, and to evaluate the influence of the tower body deformation and to assess the theoretical overstrength and ductility capacities. Results show a damage concentration, which leads to a fragile collapse mechanism with important strength reserves and a non-uniform distribution of yielding within the tower height. Since the collapse mechanism is not compatible with the desired performance inherent to the design philosophy, recommendations for the design stage are proposed, which pretend to ensure that the inelastic behaviour be consistent with the goals implicit in a code-based design to prevent tower collapses.     

配有高耸塔结构的大射电望远镜馈源支撑系统的风振分析

通过时域法分析配有高耸塔结构的大射电望远镜馈源支撑系统的风振响应。首先,由非线性静力分析确定结构初始静态参考位形和初应力;其次,模拟作用在悬索、馈源舱和高耸塔上的随机风荷;最后,对比分析考虑索塔和不考虑索塔两种情况下的馈源支撑系统50m模型的风响应。结果验证了高耸塔变形对舱体定位精度有一定的影响。     

基于能量法的下击暴流作用下输电塔线体系失效 倒塌研究

对下击暴流作用下的输电塔线体系进行了基于能量法的失效倒塌分析。采用ABAQUS软件建立了 “一塔两线” 的输电塔体系有限元模型; 基于改进的OBV模型的时变平均风速模拟方法,推导并得到了输电塔及导线各节点的下击暴流风速时程曲线; 采用基于特征能量的动力稳定性判别方法对塔线体系在3个不同风速的下击暴流动力工况时的响应进行分析,确定了下击暴流作用下输电塔失效的时刻,探讨了大跨越输电塔线体系的失效倒塌机制。结果表明:采用能量法对结构体系进行分析时,最后一次特征能量超过输入能量的时刻即为结构的失效时刻; 基于能量法对下击暴流作用下的输电塔线体系进行分析,可以快速准确地判断输电塔线体系整体失稳倒塌的时刻,具有较强的工程实用价值; 在倒塌时刻,主材失稳引发的薄弱区域杆件部分失效会导致输电塔的整体失稳,最终使结构失效倒塌。     

Non‐Gaussian characteristics and extreme distribution of fluctuating wind pressures on large cylindrical–conical steel cooling towers

Large cylindrical–conical steel cooling tower (SCT) represents a new configuration of cooling tower, and its wind load distribution pattern and forming mechanism are very different from those of the traditional hyperbolic cooling towers. Large eddy simulation was used for the numerical simulation on a superlarge cylindrical–conical SCT that exceeds the specification limit, which is also the highest (189 m) SCT under construction in Asia. The surface flow field and time history of 3‐D aerodynamic force were obtained for the cylindrical and conical parts, respectively. Comparison with the measurements of other large cooling towers and the results of wind tunnel test confirmed the validity of the numerical simulation. Then, based on the probability density distribution and spatial correlation of representative measuring points, regions of non‐Gaussian distribution were identified. The forming mechanism of non‐Gaussian wind pressure distribution was revealed from the perspective of the correlation of non‐Gaussian distribution versus flow separation and eddy motion. The criteria for classifying the region of non‐Gaussian distribution for the cylindrical and conical parts were analyzed, respectively. Research shows that the wind pressures in the windward regions of conical and cylindrical parts obey Gaussian distribution; however, the wind pressures from the region of extreme negative pressure to the region of flow separation are largely non‐Gaussian and the wind pressures of the conical part are generally non‐Gaussian in the leeward region. Finally, the three algorithms for calculating the extreme values of wind pressure were used, namely, peak factor method, improved peak factor method, and Sadek–Simiu method. The distribution patterns of peak factors and extreme values of wind pressure in SCT towers were analyzed comparatively. The 2‐D formulae for fitting the extreme values of wind values for the cylindrical and conical parts were derived by nonlinear least square method. Moreover, strategy for value determination was also presented. The present research aimed to strengthen the understanding of the fluctuating wind pressure distribution and its forming mechanism for large cylindrical–conical SCT towers.     

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.     

双曲线冷却塔有限元结构分析

通过建立双曲线冷却塔的ANSYS有限元模型,分析了冷却塔在自重、风荷载作用下的应力及变形分布规律,分析结果对于理解冷却塔在不同工况作用下的力学性能具有重要意义,可为该类型冷却塔的结构设计提供参考。