Analysis and Design of Trestle Type Water Tower with Diagonal Bracing using FEM and Draft Code Provisions

Abstract

Water Tower is a special liquid retaining structure which is designed as crack free structure using Working Stress Method. There are basic discrepancies observed where the design of container is made with Working Stress Method and the supporting structure is designed by Limit Stress Method. As all engineering structures undergo Earthquakes with their distinct behaviour, Water tower also behaves quite differently than ordinary building structures. In earlier codes provisions were given considering Water tank as Single Degree of Freedom system but in Draft Code impulsive and convective water masses of Container water are considered. The Bhuj Earthquake occurred on 26th January 2001, most of the Water tanks built in Gujarat state which were designed by IS 1893 (1984) Code have survived the earthquake. The functioning of Water tanks during recent earthquake is at variance with conservative design approach adopted by provisions of Proposed Draft Code. There are varying opinions regarding provisions for elevated tanks seismic analysis in Proposed Draft Code ‘IS: 1893 (2002) (part II)’. Comments regarding Proposed Draft are also mentioned here. In this study comparison between design forces for Water tanks obtained by the provisions of Old Code and those of the Proposed Drat Code is carried out. The dissertation also deals with the concept where we can achieve economical water tank staging by introducing diagonal bracings thus reducing the design column and horizontal bracing moments considerably and providing lighter sections. Considerable increase in cost results due to adoption Proposed Code, for Water tanks resting on column supports staging or shaft supports. Generally in concrete design, Diagonal bracings are not preferred due to congestion of reinforcement at joints and difficulties in detailing and construction. If these problems are not addressed adequately, there is a potential danger which may lead to the failure of water tank supporting staging. In present dissertation steel diagonal Bracings are considered and the forces in columns and bracings due to conventional peripheral bracing system with and without Diagonal Bracings are compared. The lateral load carrying capacity of structure is based on its stiffness and its load distribution mechanism. The trestle is found to be flexible compared to the shaft and thus subjected with lower lateral forces. For the stiffness calculation STAAD Pro software has been used which gives more accurate stiffness as it incorporates 3D behaviour of frame staging. The provisions regarding Response Reduction Factors adopted for Shaft supports in Proposed Draft Code appears to be based on the thought that shafts are more vulnerable than the Trestle support. Although in Gujarat most of the tanks are Shaft supported and they have performed well during recent Earthquake. The seismic analysis is carried out manually using old code provisions and draft code provisions. The Dynamic analysis of Case study problem is done using STAAD Pro. First chapter includes the introduction part. Second chapter includes the literature survey where the literature available for Water tanks regarding Modeling, design, Draft Code Provisions, Various Bracing System is abstracted. Scope of work is also included in the same chapter. Third chapter includes comparison of Proposed Draft code with IS 1893- 1984 and the discussion for provisions of draft code are reviewed. Fourth chapter consists of Analysis and design of Water tank supported on trestle. Fifth chapter contains the dynamic analysis of water tank carried out using STAAD Pro software which also consists of the time period variation of water tank for various modes of vibrations for various decreasing order capacities. Sixth chapter includes estimation and cost analysis for trestles with and without Diagonal bracings additional to the Peripheral bracings for improving effectiveness of supporting structure with graphical representations. Seventh chapter concludes with graphical representations and discusses future scope of the project.