## 4 Analysis of the Entire Collapse Process
### Analysis of the Collapse Mechanism  
#### 4.2.1 Weakness Analysis  
The basic design wind speed of Shanxi Lu'an Tower is 28.3 m/s. Finite element analysis of the cooling tower under two wind load distribution modes (type A wind field and typhoon wind field) was performed to extract the structural weak points during the collapse of the cooling tower.  
Figure 16 shows the comparative distribution of the stress ratios (a dimensionless ratio of the first principal stress to the strength of concrete) of the large cooling tower under the action of the two wind fields. It is found from the figure that when the cooling tower is in a stable state, the three-dimensional meridional stress is symmetrical around the meridian axis of \(0^{\circ}\). The meridional stress increases from the top of the tower to the throat and the meridional stress from the throat to the bottom of the tower is similar. When the cooling tower is stable under wind load, the hoop stress in the throat is symmetrical around the meridian at \(0^{\circ}\) and the maximum first principal stress occurs not in the middle but at \(\pm 62^{\circ}\) in the meridional direction, thereby forming a folding zone around the throat. Compared with the normal wind, the overall stress ratio of the typhoon is similar, but the distribution range is significantly expanded and derived to the leeward area. In summary, in the analysis of the whole process of wind-induced collapse of the cooling tower, attention should be paid to the section of the concrete tower that is located at the intersection of height 40-120 m and meridians \(\pm 62^{\circ}\). Additionally, the fold area between the high- and low-stress ratios, where the concrete tower is prone to shear cracking, must be considered comprehensively.  
Figure 15: Comparison diagram of the wind-induced collapse of cooling towers. (**a**) Test view. (**b**) Test overall \(30^{\circ}\). (**c**) Test overall \(60^{\circ}\). (**d**) Simulation overhead. (**e**) Simulation overall \(30^{\circ}\). (**f**) Simulation overall \(60^{\circ}\).  
Figure 17 shows the comparative distribution of the horizontal displacement of large cooling towers under the action of the two wind fields. It can be found from the Figure 17 that when the cooling tower is in a stable state, the horizontal displacement is symmetrical around the 0\({}^{\circ}\) meridian axis and the maximum horizontal displacement appears 10-20 m below the throat. The horizontal displacement keeps increasing from the top of the tower to the throat. At the meridians of \(\pm 62^{\circ}\) on both sides of the throat, the tower of the cooling tower protrudes, thereby forming a weak fold and tear zone. The horizontal displacement under the effect of the typhoon is similar to that under the effect of normal wind, but the distribution range is expanded and an overall horizontal displacement occurs. In summary, the position 15 m below the throat is marked as the weak position of the entire collapse process in this paper.  
Figure 16: Comparison of the stress response and stress ratio of the tower between normal wind and typhoon. (**a**) 3D meridional stress in Type A. (**b**) 3D meridional stress in typhoon. (**c**) 2D meridional stress ratio of the tower in Type A. (**d**) 2D meridional stress ratio of the tower in typhoon.