Effect of Dynamic Loads on Tall RCC Chimneys of Different Heights with Elliptical and Circular Cross sections

Abstract

Tall RC chimneys form an important component of major industries and power plants. Since pollution control standards are getting stringent every passing day, the mandatory height requirements of chimneys are increasing. Such increased heights pose structural challenges for designing against dynamic loads such as wind gust and seismic forces. Arriving at an economic and efficient structural profile is one of the important aspects of the analysis and design of tall chimneys. Most of the chimneys constructed in industry are of circular cross section. With increase in heights, chimneys become more vulnerable to dynamic loads. In order to combat these loads and design an economical and efficient structural solution needs to be evolved. Elliptical cross-section provides one such possible alternate solution. Therefore evaluating the behavior of chimneys of varying heights with elliptical cross-section and comparing the same with that of circular cross-section assume significance. Chimneys with circular and elliptical cross sections of five different heights viz. 275m, 300m, 325m, 350m and 400m with two different profiles in elevation - one tapering from bottom to top and the other tapering from bottom to 2/3 H and straight afterwards - have been analyzed for seismic and vortex shedding effects caused by wind forces; further, analyses have been carried out for three R/t ratios - 15, 20 and 25. In total 120 models have been analyzed 60 for seismic forces and 60 for vortex shedding effects. Analytical results in terms of stresses induced on the structure, earthquake base shear, joint acceleration, joint displacements and vortex shedding base shear are evaluated. Maximum values of seismic forces induced base shear is only 1% of the self weight of the structure for all heights. It is interesting to note that chimneys with elliptical cross section experience less accelerations compared to that with circular cross section. Circular chimneys undergo higher displacement than the elliptical chimneys which is attributed to the higher accelerations they are subjected to by seismic forces. As the height of the chimneys is increased vortex shedding phenomenon induces increased forces compared to that due to earthquakes.