Twin precast, pretensioned concrete bridges were recently built over Tlalpan Freeway - in one of Mexico City's most densely populated areas. Designed for a clear span of 170 ft (52 m) over a major freeway, they bring a new dimension to bridges for heavy traffic urban areas in seismic zones. Each of the twin bridges consists of three longitudinal girders with a total length of 1539 ft (469 m). Each girder comprises 13 pretensioned elements of hollow box cross section with two upper flanges. The substructure consists of cast-in-place columns supported on a foundation of cast-in-place caissons and precast, reinforced concrete piles. Seismic forces are resisted by longitudinal frames integrating columns with main support elements, and by transverse frames consisting of columns and transverse reinforced concrete beams.
