Two case studies on structural analysis of transmission towers under downburst

Downbursts are of great harm to transmission lines and many towers can even be destroyed. The downburst wind field model by Chen and Letchford was applied, and the wind loads of two typical transmission towers in inland areas and littoral areas were calculated separately. Spatial finite element models of the transmission towers were established by elastic beam and link elements. The wind loads as well as the dead loads of conductors and nsulators were simplified and applied on the suspension points by concentrated form. Structural analysis on two typical transmission towers under normal wind and downburst was completed. The bearing characteristics and the failure modes of the transmission towers under downburst were determined. The failure state of tower members can be judged by the calculated stress ratios. It shows that stress states of the tower members were mainly controlled by 45 degree wind load. For the inland areas with low deign wind velocity, though the structural height is not in the highest wind velocity zone of downburst, the wind load under downburst is much higher than that under normal wind. The main members above the transverse separator of the legs will be firstly destroyed. For the littoral areas with high deign wind velocity, the wind load under downburst is lower than under normal wind. Transmission towers are not controlled by the wind loads from downbursts in design process.

下击暴流对输电线路危害极大，甚至能摧毁许多铁塔。应用了陈和莱奇福德提出的下击暴流风场模型，分别计算了内陆地区和沿海地区两座典型输电铁塔的风荷载。利用弹性梁单元和杆单元建立了输电铁塔的空间有限元模型。将风荷载以及导线和绝缘子的恒荷载简化并以集中力形式施加在悬挂点上。完成了两座典型输电铁塔在正常风和下击暴流作用下的结构分析。确定了下击暴流作用下输电铁塔的承载特性和破坏模式。通过计算应力比可以判断铁塔杆件的破坏状态。结果表明，铁塔杆件的应力状态主要受45度风荷载控制。对于设计风速较低的内陆地区，尽管结构高度不在下击暴流的最大风速区域内，但下击暴流作用下的风荷载远高于正常风荷载。塔腿横向分隔器上方的主要杆件将首先遭到破坏。对于设计风速较高的沿海地区，下击暴流作用下的风荷载低于正常风荷载。在设计过程中，输电铁塔不受下击暴流风荷载控制。