DYNAMIC ANALYSIS OF REINFORCED CONCRETE CHIMNEYS

Abstract

Structural Problem in designing tall reinforced concrete chimneys lies in the determination of its capacity to resist the dynamic forces induced by wind and earthquake. This disser-tation presents lihe elastic dynamic responses of six chimneys of heights ranging between 30 metres to 335 metres using modal method considering first six modes and direct time integration method. Squat ion of motion in the later method is solved by Newmark's constant average acceleration method. Each of the six chimneys is idealized as a vertical cantilever beam with varying stiffness and mass. The bending, shear and rotary inertia effects are considered. Chimneys are assumed to be fixed at the base and soil—structure interaction effect is not considered. Detailed results are presented in the form of periods, bending moments, shear forces and dis-placements. The participation of modal mass in different number of modes is studied. A method of working out elasto-plastic seismic response of reinforced concrete chimneys is presented. It is concluded that the fundamental time periods of chimneys calculated by IS Code are slightly larger than the computed periods by stiffness metr.c approach. Higher modes of vibrations of chimneys signif pantly influence the bending moment and shear force responses. In view of atleast 90% participation of modal mass, the use of first five-m.odes for chimneys of heights 75 metres and below and six to eight modes ( Vi ) for tallert chimneys is suggested in modal .method, Modal method gives response fairly close to the response obtained by direct integration method. For the same values of forces at the base the normalized bending moment and shear force distiibu Lions of ISo1893-1975 are conservative as compared to the distributions obtained by dynamic analysis. There is a need to study the inelastic behaviour of reinforced concrete tall chimneys.