New Stress Ribbon Bridge for Historical Spa in Tuscany

The town of Bagni di Lucca (literally: baths of Lucca) is an old thermal spa in Tuscany. A new pedestrian bridge over the Lima River is currently being built as part of the town’s development plan. The bridge has been designed as a stress ribbon structure and fits perfectly into the historical townscape. In addition, this special construction method has many technical and economic advantages.

The 3.20m wide prestressed concrete stress-ribbon structure features an 87m long main span. This pedestrian bridge has a maximum rise of 1.70m under permanent loads and has been designed with a maximum slope of 5% to guarantee the complete accessibility of the link. The cross section consists of a compact 15cm thick concrete ribbon and two 40cm high beams running parallel over the entire bridge length.

The longitudinal bar system consists of two resistant systems: the first one stabilizes the structure during construction, and the second one is used to prestress the completed structure. The suspension cables are made up of two groups of 45x0.6" St 1670/1860 strands that are arranged in a rectangular shape in five layers of nine strands each. The two groups are located in a special channel and positioned at the centre of gravity of the cross section.

DYWIT, Italy was chosen to supply the DYWIDAG Strand Post-Tensioning System for this project. A total of 15t of DYWIDAG Strand Tendons were supplied for the bridge and installed with the assistance of DYWIT employees. The prestressing system is formed by two kinds of tendons: the first consists of 16 four strand tendons that are located at the centre of gravity height of the 15cm thick slab, and the second consists of two tendons with 12 strands each.

The shape of the section and the positioning of the tendons had been studied in detail in order to keep the position of the centre of gravity constant in all construction phases and under service loads.

Work on the structure began with the construction of the two foundation blocks using micro piles. Afterwards, the suspension cables were positioned along with the safety ropes for the workers. Once the precast elements for the bridge deck had been installed, the tendons were prestressed.