Analysis of Conical Water Tank With Wind Load

Elevated water tanks are essential components of urban and rural water supply systems, since they provide the pressure needed to distribute water by gravity to consumers. Among the different shapes used for elevated containers, the conical or Intze-type container with a conical bottom is widely adopted because the inclined wall transfers a large part of the water load efficiently to the supporting ring beam. This study presents the analysis of an elevated reinforced concrete conical water tank subjected to wind load, with the aim of understanding how wind forces combine with hydrostatic and gravity loads in the container and the supporting staging. The tank considered has a capacity of 500 cubic metres and is supported on a frame staging of eight columns with bracing at regular intervals, giving a total height of 18 m from the foundation to the bottom of the container. The container consists of a top dome, top ring beam, cylindrical wall, bottom ring beam, conical wall and bottom dome, proportioned according to common practice for Intze tanks. The tank is assumed to be located at Visakhapatnam, a coastal city with a basic wind speed of 50 m/s, where wind is expected to be a significant design action. The structure is modelled in STAAD.Pro using plate elements for the shells and domes and beam elements for the ring beams, columns and braces. Dead load, hydrostatic pressure for the tank-full condition, and wind load according to IS 875 (Part 3):2015 are applied. Wind pressure on the container is calculated using force coefficients for circular sections and is distributed around the circumference, while the wind on the staging is applied to the columns and braces. Both the tank-full and tank-empty conditions are considered, since the empty tank is more vulnerable to overturning and the full tank produces the largest axial forces in the columns. The analysis provides the hoop tension and meridional forces in the conical and cylindrical walls, the moments in the ring beams, the axial forces and moments in the staging columns, the forces in the braces, and the lateral displacement at the top of the tank. The results are compared with the forces obtained by the conventional membrane analysis of the container and by the approximate portal analysis of the staging. The design of the container walls is checked for the limit state of cracking in accordance with IS 3370 (Parts 1 and 2):2009 so that the structure remains watertight. The study shows that wind load has only a small effect on the stresses in the container walls but produces significant bending moments in the staging columns and braces, particularly at the lowest panel. The maximum lateral displacement at the top of the container is found to be within acceptable limits. The combination of dead load, tankempty condition and wind load governs the check against uplift at the foundation. The work demonstrates the value of finite element modelling for elevated tanks and can be extended to seismic analysis using the impulsive and convective mass model of IS 1893 (Part 2).