Alternate tank staging configurations with reduced torsional vulnerability

The companion paper [Dutta SC, Jain SK, Murty CVR. Assessing the seismic to rsional vulnerability of elevated tanks with RC frame-type staging. Soil Dy namics and Earthquake Engineering 2000;19(3):183-97] shows that many of the currently designed reinforced concrete elevated water tanks supported on f rame-type stagings have the ratio of torsional and lateral natural periods, tau, in the critical range of 0.7-1.25. This may amplify the effect of sma ll accidental eccentricity and cause large torsional vibration during trans lational ground shaking in earthquakes [Dutta SC. Torsional behaviour of el evated water tanks with reinforced concrete frame-type stagings during eart hquakes. PhD thesis. Department of Civil Engineering, Indian Institute of T echnology Kanpur, Kanpur 208 016, India. 1995 and Dutta SC, Murty CVR, Jain SK. Torsional failure of elevated water tanks: the problems and some solut ions. Paper No. 287. Eleventh World Conference on Earthquake Engineering. A capulco, Mexico, 1996 June 23-28: Elsevier, Amsterdam, 1996]. It was seen t hat the period ratio (tau) of the frame-type stagings cannot be changed app reciably by adjusting properties of the basic configuration, e.g. number of panels N-p, number of columns N-c, and the parameter K-r related to the re lative stiffness of columns and beams. Hence, alternate structural configur ations are needed for such stagings so that tau lies outside the critical r egion. In this paper, approximate closed-form expressions are derived for l ateral and torsional stiffnesses for a few alternate configurations. These expressions are used for studying the natural period ratio (tau) for those alternate configurations. The alternate configurations are made by adding t o the basic configuration (a) radial beams, (b) radial beams and a central column, (c) another concentric row of columns connected through radial and circumferential beams, and (d) diagonal braces. First three of these config urations have higher value of tau while the fourth configuration has a lowe r tau when compared with that of the basic configuration. The paper present s a limited parametric study to evaluate the effectiveness of each of these configurations in changing tau. A systematic stepwise approach is proposed for checking torsional vulnerability of the tanks and choosing a suitable configuration. Since the magnitude and direction of eccentricity for elevat ed water tanks is often not known, configuration-based solution may be pref erable to the conventional increased strength design. (C) 2000 Elsevier Sci ence Ltd. All rights reserved.