The Petronas Towers in Kuala Lumpur, Malaysia are twin skyscrapers designed by architect Cesar Pelli and structural engineers Thornton-Tomasetti and Ranhill Bersekutu. Each tower has a core structure composed of 16 reinforced concrete cylindrical columns varying in diameter from 2.4 to 1.2 meters. The towers are built upon an underground forest of concrete and steel columns supporting a concrete slab. A concrete core and column structure was used instead of metal to reduce vibrations high in the towers and because Malaysian builders preferred working with concrete rather than steel.