子午线型对冷却塔结构特性和抗风性能的影响机理

Meridian curves have great influences on the structure behaviors and wind-resistant performance of hyperboliodal cooling towers. The defective meridian curve was also regarded as one of the reasons resulting in Ferrybridge accident. There have been some fragmentary findings about these influences but not comprehensive, and the influence mechanism is quite far beyond our perception. This obstructs the pursuing for optimized meridian curves and is not favorable for the flourishing construction of cooling towers. In this project, numerical methods will be employed to give comprehensive comparative study on the structure behaviors firstly, including the mechanical behavior, dynamic properties, buckling behavior, global and local stiffness, from which the critical parameters of meridian curves would be picked up and the influence patterns would be found out. Then, the influence mechanism of meridian curves on the structure behaviors of hyperboloidal cooling towers would be recognized, according to those previous findings and theoretical analyses based on structure mechanics of plate and shell. Based on the comprehensive understanding of the structure behaviors, rigid and aeroelastic model tests followed by dynamic response analysis would be conducted to peruse the influence of meridian curves on wind induced vibration responses, in which meridian curve and dynamic property parameters would be included. On the other hand, the ultimate wind loads and failure mode of hyperboloidal cooling towers would be obtained from numerical simulation, and the influence from meridian curves would also be studied. Then based on the wind induced vibration and ultimate wind loads analysis, wind induced vibration study on imperfect structures would be conducted. Finally, the optimizing strategy would be proposed based on the above fundamental study and the practical design process.

双曲冷却塔的子午线型对结构特性和抗风性能存在显著影响，不合理的子午线型也被认为是渡桥电厂风毁事故的原因之一。线型对结构特性和抗风性能的影响形式已有零星发现，但并不系统全面，对影响机理还没有明确认识，阻碍了冷却塔线型设计和优化工作的开展，亦不利于我国当前高涨的建设实践。本项目拟通过数值计算，对比分析子午线型参数对结构特性的具体影响形式和关键影响因素，包括其力学行为模式、动力特性、稳定性以及整体和局部刚度特征等；借助板壳结构力学理论分析，明确线型参数对结构特性的影响机理。以此为基础上升到冷却塔抗风性能的两个关键问题，风振响应和极限承载能力：经刚体和气弹模型风洞试验和风致动力响应分析，考虑线型和动力参数，揭示子午线型对风振响应的影响规律；通过数值仿真模拟，分析子午线型对风荷载极限承载能力和破坏模式的影响；并将两者结合展开针对在役非完善结构的风振响应研究。最终结合实际设计过程探究线型优化策略。

在前期工作的基础上，首先以一座代表性冷却塔为例，通过线型方程的参数调整获得五组线型，独立分析塔筒上下缘直线段、喉部以上线型、下缘倾角、喉部高度和整体半径对结构在多种荷载下受力性能的影响，并通过结构设计评价各参数对塔筒材料用量的影响。另外，上述参数对结构基频和稳定性的影响也进行了分析，并结合结构振型解释了线型对基频的影响机理。通过上述研究揭示了子午线型尤其是线型斜率/曲率对结构特性的影响机理，提出了以塔筒配筋量为指标的子午线型优化方法并对子午线型的构造方法提出了一定要求。同时，为便于将配筋量作为优化目标，还根据冷却塔的受力特性提出了一种简化且可靠的配筋计算方法，从而可以在结构配筋计算时避免繁复的多风向组合。在动力范畴，在风洞试验的基础上对冷却塔的风振效应也进行了系统研究，详细分析了各响应的环向和子午向分布特征以及各响应背景和共振分量在总脉动响应中的权重，并综合各响应的时程特征、塔筒结构特性及配筋设计原则，将塔筒各响应的阵风响应因子从环向和子午向的二维分布简化为分别针对环向和子午向配筋设计的两个独立数值并作为结构设计的风振系数。在此过程中，还提出了在响应时程中准确分离背景和共振分量的方法并进行了验证。另外，对结构风振响应的各个影响因素如结构基频、阻尼、风速、子午向相关性等进行了全面分析，明确了各自对对共振效应的影响规律并给出了拟合表达式。结果表明，在基频大于0.7Hz时，结构的共振效应并不明显，而在整体双曲线型控制下，子午线型变化对结构基频的影响较为有限，故而线型对结构风振效应的影响亦非常有限。在极限承载力方面，已经建立了考虑塔筒钢筋混凝土材料非线性的有限元计算模型并得到了初步的结果，塔筒在破坏前的非线性效应极弱，而一旦局部混凝土开裂后，结构刚度急剧下降且位移急剧增加，从而造成计算终止，但此时钢筋仍未屈服。受时间所限，后续仍将继续展开极限承载力方面的研究。