External Post-tensioning used on Portugal's Second Longest Bridge

Context

The bridge spans the Tejo river delta south of Santarem to allow travelers to cross the delta despite yearly recurring flooding in the area.

As early as 1997, the Portuguese Department of Transportation awarded the contract to the joint venture CONDURIL/MSF for the construction of a new bridge designed by J.L. Cancio Martins.

The river crossing includes a cable-stayed bridge with a main span of 246 m. The approaches on both sides of the main span are to be constructed with a total of 3 advanced shoring systems and consist of 105 spans (42 m long) cast in three stages.

Solution

The first stage is an open single cell box which allows for easy removal of the inner formwork and reduces the loading on the shoring system. The shoring system was designed for the lighter open section and not for the full box girder. Steel pipes are placed in the first stage, which later will allow the DYWIDAG post-tensioning bars to be installed.

Once the post-tensioning bars are installed, and the remaining reinforcement is placed, the second stage deck slab is poured and stressed. A trailing gantry lifts then the precast cantilever elements in position. The preinstalled post-tensioning bars in the cantilever segments are coupled with the bars in the inner cell.

Once the longitudinal DYWIDAG bars 32mm dia. are placed, the outer deck slab is cast in a third stage.

The completed cross-section will then be reinforced with the proven DYWIDAG external strand system 19-0.62". The external system has already been utilized successfully in the viaduct de Scardon in France. The 6m long segments of the main span are incrementally cast and tensioned with up to 28 pieces DYWIDAG single bar tendons 36 mm dia. to the preceding segment. Transverse post-tensioning of the main bridge consists of 8 pieces DYWIDAG bars 32 mm dia..