External Post-tensioning on Portugal's Second Longest Bridge
The second longest bridge in Portugal, is located just 70 km north of Lisbon. With a total length of 5 km, 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. The approach bridges consist of 105 spans (42 m long) cast in three stages. 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 than 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. Info No. 6 describes this application in detail. The 6 m 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.. With 2 DSI project managers on the job, 8 installation workers could be trained within a short period of time. The newly trained team gained the confidence of the contractor as well as the site supervision.