Launching of the San Cristobal Bridge

The San Cristobal Bridge in Chiapas, Mexico, is a three-span (71.5, 180, and 71.5 m) curved steel composite and orthotropic box girder erected by incremental launching. Lessons learned from the collapse during launching of the San Cristobal Bridge as well as the redesign and relaunching of the new bridge are discussed. After the collapse of the San Cristobal Bridge, T. Y. Lin International was hired by the new contractor, Ingenieros Civiles Asociados, to investigate the cause of the collapse of the original bridge, check the redesign of the bridge, and perform the erection engineering of the bridge. From the site investigation, it was concluded that the collapse of the structure was caused primarily by the failure of the shear studs and the consequent loss of composite action of the girder cross section over Pier 2. To ensure the safe erection of the structure, the redesign of the new bridge included the following changes: addition of shear studs, increase of deck posttensioning during launching, increase of the concrete slab strength, and addition of stiffeners to the bottom flange and webs. To match the predicted and measured deflections during launching, a calibration of the effective deck inertias (consistent with a 12 × t_slab and contrary to the AASHTO specifications) was necessary to account for the effects of shear lag and the actual effective slab width.