Tornado-induced collapse analysis of a super-large reinforced concrete cooling tower

Abstract

Wind loads are a predominant design factor for the gravity, thermal effects, and seismic actions of thin-walled super-large cooling towers. The current load code provisions relating to cooling towers in various countries are only suitable for straight-line boundary-layer wind fields under normal climates, rather than non-synoptic winds such as tornadoes, which have quite different wind field characteristics and are more vulnerable to structural safety. To further understand the tornado-resistant performances of super-large reinforced concrete cooling towers (RCCTs), wind tunnel tests with respect to a reduced-scale model of a prototype structure with a height of 215 m were conducted using a tornado vortex simulator. Wind loads acting on the tower shell due to tornadoes with different swirl ratios were studied, and then applied to a refined full-scale finite element model. The entire process of tornado-induced RCCT collapse was simulated and the critical failure wind velocities considering nonlinear collapse effects were compared with the results from the buckling stress state (BSS) approach, which is recommended in national codes and linear bifurcation buckling analysis. The results revealed that structural failure is more likely to occur when the RCCT is located at the tornado core radius because the structure experiences the most unfavorable tornadic wind loads within this region. The loss of material strength rather than the loss of stability is the key factor influencing tornado-induced RCCT failure. The structural collapse is initiated by the appearance and development of circumferential cracks in the windward region of the tangential wind flow owing to the tensile failure of the meridian reinforcement inside the tower shell. The cracks around the tower shell lead to the degradation of the natural frequencies of the structure and the change in vibration modes, and finally affect the tornado-resistant performance of the RCCT.