The Chicago Planning Commission greenlighted the fourth proposed version of Santiago Calatrava's 2,000-foot, 7-sided glass and steel spire. The Spanish architect has compared his twisting design to trees and smoke, but the building has a practical side, too — its shape will actually help dissipate the Windy City's powerful downdrafts. The spire — which will be the tallest building in North America — is being built on a 2.2-acre lakefront site and features a light emanating directly from the top of its structure. In a related story, Calatrava's Bridge of Strings at Jerusalem's entrance is starting to be assembled with the hope of bearing light-rail travelers by 2009.