Concrete Shear Strength of Elevated Water Storage Tank Pedestals

Concrete pedestals of elevated water storage tanks support a steel vessel, schematically shown in Figure 1, or a concrete vessel, schematically shown in Figure 2. ACI Report 371R-08 [1] recommends that elevated tank pedestals be designed in accordance with ACI 318 (Code) [2]. The Code permits the shear strength of members to be calculated as the sum of the strength provided by concrete alone and the strength provided by shear reinforcement. For the concrete contribution to shear strength, Chapter 11 of the Code allows the designer of tank pedestals the choice of considering the pedestal as a compression member or as a shear wall. The Code provisions for compression members are based on tests of beams and beam columns of rectangular and solid circular cross sections. The Code provisions for shear walls are based on tests of walls of rectangular sections. The pedestals of tanks exemplified by the pedestals shown in Figures 1 and 2 are of thin-walled circular hollow sections. In addition, Chapter 21 of the Code provides requirements for structural walls subjected to seismic forces. This paper discusses the concrete shear strengths of elevated tank pedestals obtained from ACI 318, proposes alternate concrete shear strengths based on theory and test results on plain concrete, and presents concrete shear strength and limit state obtained from a pushover analysis of the pedestal shown in Figure 2.