The construction of a beltway allowing traffic decongestion in the city of La Paz, Bolivia, was completed in 2010. The new elevated road crosses three parallel valleys with signature bridges, providing a direct connection between the two opposite parts of the city. These three consecutive bridges have similar features and as a result are called “Trillizos” (Triplets) in Spanish.
The Triplets have three spans each and total lengths of 233.5 m, 191.5 m and 218.8 m (from west to east). The first bridge has the longest span at 131.5 m. They consist of an extradosed prestressed concrete deck. Extradosed bridges are a cable supported bridge type in which the cables can endure higher stress levels at the expense of the deck carrying most of the live load, which results in a more favorable and economical design when spans are not too long. The deck is 14.8 m wide and accommodates four road lanes and sidewalks. It is made of a box girder of 2.8 m in depth which grows to 3.5 m over the pylons, where it is monolithically connected. The stays, 6 to 7 for every tower, consist of single cables that run through the pylon to be anchored on the deck.
The rugged topography of the site means the decks reach heights between 40 m and 60 m above the bottom of the valley, which made a regular cable-stayed bridge option, with pylons higher than 25 m above the deck, inappropriate for the site. Therefore, the extradosed bridge typology was proposed, which reduces the height of the pylons. The design includes a single plane of stays for a more transparent view. At the same time, it implies the introduction of a new and modern typology in Latin America. This project was awarded the Eugene C. Figg Medal at the IBC 2012, Pittsburgh, PA.
Design was carried out meeting the AASHTO LRFD requirements. A global analysis model in advanced software RMBridge v8i was used for the structural analysis of the bridges. This model included sub- and superstructure elements as well as the uplift resisting system of the abutments. The structures were built using balanced cantilever segmental construction with sliding formwork.
Sobrino, J.A.; Bridges “The Triplets” in La Paz, Bolivia. (in Spanish, Puentes Trillizos en La Paz, Bolivia), Cauce 2000: Revista de la ingeniería civil, (ISSN-0212-761X), pp 56-63, September 2010.
Sobrino J.A.; Trillizos Bridges (The Triplets) in La Paz, Bolivia. Proceedings of the Large Structures and Infrastructures for Environmentally Constrained and Urbanized Areas IABSE Symposium. Venice, September, 2010.