A Study of Vortex Shedding Acts on Steel Vent Stack Based on EU Standards

Abstract This paper calculates and analysis the crosswind resonance and load acts on the steel vent stack of a UK nuclear power plant which caused by the vortex shedding. It is based on BS EN standards. Both downwind loads and cross wind load which perpendicular to the wind velocity are generated when the wind acts on the structure with a circular section. That may cause a huge displacement or damage of the structure. So some measure to reduce the impact is necessary. This paper discuss impaction on self supports stack which cause by structure free length and the supports form and the fatigue. And compares the difference between the Chinese codes and EU codes. The cross wind load is caused by the vortex shedding effect. When the wind load acts on the structure, it will produce alternating vortices behind both sides of the chimney, and alternately fall off from one side to the other side, forming the vortex shedding effect. When a vortex is created on one side of the structure, the wind speed on the other side increases, causing the air pressure to drop. As a result, the structure is subjected to lateral forces on one side of the vortex. When the vortex alternates on both sides of the structure, a harmonic lateral load with the same frequency as the vortex is formed. Therefore, the fatigue effect caused by fluctuating wind load should be considered in the design of the structure. As a tall structure, the steel stack must control and reduce wind vibration, especially cross wind vibration, to achieve safety and economic indicators. If a horizontal support is anchored on middle of the stack, it can not only share the horizontal load at the bottom of the structure, but also reduce the calculation length of the result and increase the natural vibration period of the structure. It can be seen from the calculation in this paper that the structure without horizontal support needs to increase its damping to avoid dam4age by adding frequency modulated mass dampers or wind rings. The cross-wind load caused by vortex shedding is mainly related to the natural vibration frequency of the structure and the logarithmic damping of the structure in BS EN standards. Therefore, the cross-wind effect can be reduced by increasing the logarithmic damping of the structure, optimizing the structure design, and changing the natural vibration frequency of the structure.