Categories: post-tensioning
      Date: nov 13, 2005
     Title: High speed train line Milan – Bologna / Piacenza 1 and Piacenza 2  viaducts
Piacenza viaduct is one of the structures along the new high-speed network in Italy

Piacenza viaduct is one of the structures along the new high-speed network in Italy, whose aesthetics, environmental impact, care for durability and construction needs have deeply led the process of design, in order to have an efficient solution to high standard specifications for railway bridges coming from the Italian Railways (Ferrovie dello Stato - FS).

The length of Piacenza viaduct is 5070 m with a minimum radius of curvature of 5500 m and a maximum rail slope of 1.2%. The viaduct consists of no. 150 prestressed concrete (PC) simply supported spans, each spanning 33.10 m, and three spans consisting of composite steel and concrete structures, with longer lengths of about 40 - 50 m, for road and railway crossings.

Aesthetical prerequisites connected with the limited height of the line track with respect to ground level, suggested the choice of an architectural contour and curvilinear concrete panels specifically designed to fit harmoniously into the structure.

The structural solution is the viaduct with simply supported spans, which follows the traditional choice of Italian Railways (Ferrovie dello Stato - FS) for ordinary viaducts. This structural solution fits the use of long welded rail as it avoids rail expansion devices, and simplifies the maintenance operations so minimising maintenance costs.

The viaduct has deep foundations with cast-in-place concrete plinths connecting a cluster of ten to twelve 1.2 m diameter bored piles, whose lengths vary from 25 to 35 m. The piers are twin walls structures, completely joined at the bottom and separated at the top, apart from two concrete beams of small dimensions, which bear the so called "stroke end devices", required in order to avoid deck slipping out of pier cap and falling.

The deck is a single full span precast girder, longitudinally prestressed by post-tensioning tendons: it is a twin box-girder, having maximum height of 2.80, maximum width on upper face of 14.02 m. The precast element weight is approximately 10.000 kN.

The absence of head diaphragms on one hand allowed the use of easily removable internal formwork, which can be drawn out from one end of the precast element, but on the other hand implied a considerable increase in the conventional reinforcement to withstand the local head stresses; in order to investigate the behaviour of such congested span heads, several test elements were built.

The monoliths are pre-stressed by means of no. 24 Tensacciai MTAID postensioned cables, of which no.15 made up of 19-0.6" strands each, arranged in paraboloid fashion in the three webs, and no.9 made up of 12-0.6” strands each, arranged in straight lines in the counter-slab.

The prestressing system meet three main requisites: it had to ensure the protection of the cables, preventing corrosion, to enable non-destructive testing of the protection to verify the leak tightness, and to ensure distribution of the stresses in the concrete in the heads areas.
The need of meeting the first two requisites led to the design of electrically isolation of the Tensacciai prestressing system.
In a system of this kind, the whole bundle of strands and its locking arrangement, as well as the injected grout, are contained in a completely sealed plastic envelope that is electrically isolated from the surrounding environment.
It allows the check of grade of insulation through electric impedance measurements, between tendons and reinforcement.

During production, different percentages of success (i.e. measured electrical impedance higher than threshold limit given by Swiss guidelines) have been registered: after the first 5 - 10 decks, the percentage increased to the average of 85% that has been maintained on the current production.
The values registered in the greatest portion of production complied with the limits and the average percentage was considered satisfactory.

The large quantity of experimental data resulting from the experience of the Piacenza Viaduct, which as far as we know is the largest scale application of dielectric cables in the world, is currently being studied in order to improve the scientific knowledge on this innovative system.
The method of construction is full span precasting, with following transport and launching.

It shall be remarked that the full span precasting procedure of construction has the relevant advantage in increasing the speed of construction: the precasting cycle has been designed for the production of two decks per week.

The Piacenza viaduct can be considered an efficient answer to the increasing and demanding prescriptions and specifications coming from the Italian Railways, in the frame of simply supported prestressed concrete deck for railway bridges.

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Project : High speed train line Milan – Bologna / Piacenza 1 and Piacenza 2 viaducts
Location : Po river, Piacenza
Type : P.c. continuous box girder cable stayed deck
Total length of bridge :
400 m.
Stayed span lengths :
main span 192 m. and lateral span 104 m each all staye
Deck width : 15 m
Height of towers from deck level : 60 m
Stay cable strand : 530 tons
No. of stays : 72
Longest stay cable : 105 m
Max no. of strands per anchorage : 91 x 15.7 mm. dia
Main client : TAV SpA
Contractor : ASG scarl
Designer : MPA – Mario Petrangeli e Associati
Period : 2003 - 2006