Natural draft steel hyperbolic cooling towers: Optimization and performance evaluation Mahdi Izadi * ,† and Khosrow Bargi School of Civil Engineering, University of Tehran, Tehran, Iran SUMMARY Demand for efficient and economical hyperbolic cooling towers has driven engineers toward designing tall and lightweight towers, specially, in regions with high-seismic ground motions. An effective way to achieve this goal is using steel cooling towers. Also, by using tubular diagonal grid (Diagrid) system as the structural system, the weight of cooling tower will reduce, significantly. Therefore, in this investigation, a steel hyperbolic cooling tower is modeled with Diagrid system. A linear analysis, under dead and wind load, is performed on several systems with different angles of Diagrids to reach the optimal angle of grids. Furthermore, the effect of stiffening rings on the cooling tower is examined, and it is observed that the use of stiffening rings makes the tower lighter. And, by performing nonlinear time history and stability analyses, the behavior of the cooling tower is investigated under strong earthquakes, and it is represented that the system shows very good performance during strong earthquakes. Copyright © 2013 John Wiley & Sons, Ltd. Received 11 June 2012; Revised 29 October 2012; Accepted 10 January 2013 KEYWORDS: hyperbolic steel cooling towers; diagonal grid system; linear and nonlinear time history analyses; stability analysis; buckling factor 1. INTRODUCTION 1.1. Necessity of investigation on steel cooling towers Hyperbolic cooling towers are used in a wide range of power plants. These types of cooling towers are mostly made of reinforced concrete (RC) in the form of thin shells. Given the prevalent opinion that RC cooling towers are, as a rule, more economic than steel ones, at first look, it does not seem justified to build cooling towers of steel. Thus, it has to be investigated which factors can render a reticulated steel cooling tower competitive with RC towers. If the tower is to be built in a high-seismic zone, substantial advantages can be earned by reducing the weight of the tower, since seismic forces are proportional to weight. Harsh climatic conditions, hindering the pouring of concrete for most of the years or necessitating costly protection measures, call for some kind of prefabrication. 1.2. Steel cooling towers First, steel cooling towers were in the shape of a pyramid at the bottom part of the cooling tower and the prismatic part mounted on the pyramidal part. The common beam–column frame systems with braces were used as the structural system of the cooling towers, and the elements were made of angle sections that connected to each other as a truss. Afterward, structures with diagonal grid (Diagrid) system were used to decrease the weight of towers that had the same shape and elements. But, this *Correspondence to: M. Izadi, School of Civil Engineering, University of Tehran, Tehran, Iran † E-mail: m.izadin@ut.ac.ir THE STRUCTURAL DESIGN OF TALL AND SPECIAL BUILDINGS Struct. Design Tall Spec. Build. (2013) Published online in Wiley Online Library (wileyonlinelibrary.com/journal/tal). DOI: 10.1002/tal.1081 Copyright © 2013 John Wiley & Sons, Ltd.