Design rationale for circular silos based on finite element analysis

Abstract

The current practice of silo analysis is based on several assumptions and idealisation. Analysis of silo by Finite Element method and comparison of the results with corresponding values obtained from conventional method reveals that the. conventional method cannot predict all the stress resultants (forces and moments) required for silo design. Again the functions predicted by the conventional method deviate largely from the actual values for a region near the ring beam of a silo. Conventional method can analyse a silo for axisymmetric loading only and cannot evaluate various types of moments which may be of considerable magnitude. Since silo is an elevated structure it may be subjected to a considerable amount. of wind load which is non-axisymmetric in nature. Earthquake loading haS significant effect on silo behaviour which is also nonaxisymmetric. Finite Element approach can analyse a silo for axisymmetric as well as non-axisymmetric loadings easily. It was, therefore, felt that the application of Finite Element will lead to a realistic analysis and to a more rational design procedure for silos. With this objective an extensive investigation was carried out on the behaviour of silos of different types under various loading conditions using the Axisymmetric thick shell Finite. Element program by Alunad. A number of parameters influencing silo behaviour were selected and a detail parametric study has been carried out to reveal the sensitivity of stress resultants with respect to a particular parameter. From this study it became obvious that the effect of restraint provided by the ring beam at the bottom of vertical wall can not be ignored. Actually, the moments developed in a silo are due to the restraint provided by the thickened ring beam at the bottom of vertical wall (top of conical hopper). The behaviour of silos under non-axisymmetric loading, such as wind load, has been studied elaborately. It was observed that wind load produces considerable meridional force and circumferential moment in the vertical wall which must be considered in the design of a silo. A detail study has also been carried out to know the effect of temperature difference between inside and outside of a silo. The conventional equations for the computation of meridional moment and circumferential moment have been modified to take into account the effect of restraint provided by the ring beam. Finally, on the basis of the study a design rationale has been presented. Using this rationale the stress resultants required for silo design can easily be computed.