Analysis and Design of Shaft Type Water Towers Using Finite Element Method and Draft Code Provisions

Abstract

Overhead water tanks or elevated service reservoirs are one of the most important components of any efficient water distribution system. The basic purpose of elevated water tanks is to secure constant water supply. There post earthquake functionality makes these structures even more important. There vulnerability to severe earthquakes due to their configuration, involving large mass concentrated at the top of relatively slender staging, is a major concern for all the designers.

In the past earthquakes some of the shaft type of water tanks has shown some distress and a very few has collapsed. The poor performance of such structures should be studied considering various aspects like design considerations, construction quality, quality of post construction maintenance etc. In the proposed draft code the level of forces has been increased to bring it to the level of other international codes and to compensate for the poor performance of shafts. Analysis and design of the supporting structure and foundation of a case study has been carried out by IS: 1893-1984 and Proposed draft for IS: 1893 (Part II) and comparison is made for the design forces and quantities of physical quantities.

When thickness of a shaft is very small compared to its diameter, the shaft behaves as a membrane structure. For access, for maintenance, air ventilation and for the inlet and outlet pipes of water, openings are frequently required in shafts. To minimize the effects of such openings and to have smooth flow of stresses around the openings the opening sizes should be as small as possible. This helps in maintaining constant shell thickness of shaft over the height of openings. Even after all such measures, stress concentration on sides of the openings and stress release above and below the openings are bound to occur. Here an effort has been made to study and understand the effects of stress concentration around such openings on shafts. Further the effects of stress release on foundation of shaft especially due to the inlet and outlet openings located very near to the foundation has been studied.

The most important aspect in the construction of a shaft is to maintain verticality, circularity and uniformity of thickness. Any variation in geometry would result into additional stresses. Generally the workmanship is of good quality, many times (which may not be always the case), some construction error is bound to occur. As a good design engineer it is a good practice to consider effect of such errors by way of minimum tolerances and include them in the design itself. But the Indian Standard code has been silent about these tolerances in the shafts of elevated water tanks. An effort has been made to study the effects of tolerances in the shafts, especially the tolerances in plumb ness of the shafts. Further the addition stresses due to such tolerances has been worked out and probable boundary for such tolerances have been suggested to restrict the additional stresses on the shafts due to poor workmanship.