## 4 Analysis of the Entire Collapse Process
### Analysis of the Collapse Mechanism
#### 4.2.2 Analysis of Collapsed Form  
Based on the incremental dynamic analysis (IDA) method, the structure is analyzed non-linearly with the basic wind speed starting at 25 m/s for stepwise loading. The typical weakness (Section 4.2.1) is used as the collapse control member. The loading wind speed step is 5 m/s. When the wind speed increases to the tensile failure of the concrete (tensile strength \(\geq\) 1.71 MPa), the concrete cracks in the local area and the rebar are stretched. As the wind speed increases further, the compression zone of the tower is close to the limit stress state (compressive strength \(\geq\) 19.1 MPa) and the wind-induced response of the cooling tower increases significantly. Therefore, the rapid increase in the maximum displacement of the cooling tower is used as the basis for judging its collapse. Figure 18 shows the maximum displacement of the cooling tower and throat under the typhoon wind field. It can be found from this figure that there is an obvious critical wind speed (80 m/s) in the displacement wind speed curve. Before reaching this critical wind speed, the displacement and wind speed are linear (0-60 m/s) and some nonlinear characteristics begin to appear (60-80 m/s). The cooling tower undergoes a small deformation and is in the approximate elastic stage. After this critical wind speed, the displacement suddenly increases, and the cooling tower undergoes a large deformation and enters the stage of plastic continuous collapse. Research shows that the critical wind pressure of the numerical simulation result is 18.62 kPa. The critical wind pressure of the design code (GBT 50102-2014) when considering the overall stability is 17.67 kPa. The difference between the two is 5.10% and the code is relatively conservative compared with the numerical simulation results. In summary, under the coupling of wind load and self-weight load, the critical wind speed for the collapse of this large cooling tower is 80 m/s. Figure 17: Schematic diagram of the 3D horizontal displacement of towers in two wind fields (units: m). (**a**) Type A wind field. (**b**) Typhoon wind field. Figure 18: Schematic diagram of the maximum displacement of cooling tower with wind speed. Figure 19 shows the displacement schematic diagram and the first principal stress schematic cloud diagram of the numerical simulation of the collapse of the large cooling tower at the critical wind speed of 80 m/s. From the analysis of this figure, it can be seen that the whole process of wind-induced cooling tower collapse can be roughly divided into four stages. First, the throat is depressed and becomes ovoid when viewed from the top. At this time, the first principal stress in the cooling tower extends from the throat to the bottom on the windward side. Then, the depression continues to develop in the throat and the rigid ring on the top is depressed more slowly. The throat is dragged and two folds are formed obliquely upward from the windward point of the cooling tower throat. At the same time, the first principal stress also develops upward with the pleats transversely. Then, as the rigid ring on the top of the cooling tower continues to sag, the strain reaches the failure point. During the collapse, the rigid ring collapses inward from the windward point and, at the same time, the shear tear at the crease of the throat breaks down. The first main stress develops from the throat to the rigid ring in the meridional direction. Finally, the top rigid ring fragment impacts the back of the cooling tower. Under the combined action of gravity and wind loading, the shell fails and continuous collapse occurs. Compared with the effect of the normal wind field ([PERSON], 2020), it is found that there are some differences in the weak position and the degree of continuous collapse, but there is a similar failure trend under the effect of typhoon and the conventional wind field. Generally speaking, there is a similar wind-induced failure mechanism.