Why is the natural vibration period calculated during the design of towers?

Natural vibration characteristics (natural vibration period, mode shape, and damping) are essential for dynamic analysis of tower-type containers. When calculating tower containers under seismic or wind loads, it is necessary to first determine their natural vibration period and mode shapes.

Effect of natural vibration period on calculated load. 1. Impact on wind load calculations: As the natural vibration period increases, the pulsation coefficient also increases, and the horizontal wind force acting on the calculated section grows. Therefore, increasing the wall thickness during calculations can help reduce this horizontal wind force. 2. Impact on the calculation of seismic loads: The effects are relatively complex. Generally speaking, changes in the natural vibration period affect the value of the seismic influence coefficient, which in turn directly influences the magnitude of the horizontal seismic forces. The relationship between the seismic influence coefficient and the natural vibration period can be divided into three stages, namely the rising stage, the plateau stage, and the falling stage. The natural vibration period increases, and its effect on the seismic influence coefficient varies across the three stages. (1) During the rising phase, the seismic impact coefficient increases as the natural vibration period increases. (2) During the falling phase, the situation is exactly the opposite. It can be inferred that an increase in wall thickness can sometimes reduce and sometimes increase horizontal seismic forces. Therefore, when calculating the strength and stability of tower-shaped containers, if it is necessary to adjust the wall thickness to meet the strength or stability requirements, increasing the wall thickness is not the only option; different load conditions require separate consideration. At the same time, for segmented towers, increasing the wall thickness of different tower sections does not have the same effect. Therefore, specific issues need to be analyzed individually during calculation in order to make the design more reasonable. Excerpt from \"Training Manual for Pressure Vessel Design Engineers\", compiled by Li Shiyu, P422