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The “Tre ponti” full story with photo and drone video

In Lunigiana, Tuscany, three new suspension bridges, which substitute those destroyed during the 2011 flooding, are now completed and open.

The bridges overpass the Magra, Mangiola and Teglia rivers, without piers in water, with main spans of 140, 100 and 50 mt and have a common feature: they are suspension bridges, and are the first road bridges of this type in Italy.

Anyway the structural systems, even within the homogeneity of the same structural figure of suspension bridge, are very different: two are earth-anchored, one is self-anchored, all show different and asymmetric layout.

THE STORY
In November 2011 a severe flood occurred in Lunigiana, the western part of Tuscany, adjacent to Liguria.
that caused the collapse of three bridges that crossed rivers Magra, Mangiola and Teglia.
The Tuscany Region launched an international design competition for the choice of the best technical and formal solution for the new bridges construction, and DMA was declared winner in July 2012.

These projects were then developed at a definitive and executive level in 2013, and took place between 2015 and 2017.
The three bridges have a common feature: they are suspended bridges: with catenary suspension system, and are therefore the first modern suspended road bridges in Italy.
In 1850 some small chain supported bridges for pedestrians and horse carriages were erected,just in Tuscany and Campania. Since then, only arch and girder bridges, and later cable-stayed bridges were built.
The homogeneity in the construction typology was a choice oriented to the characterization and recognition of the intervention as uniform and consistent, as expressly required in the tender.

 

TECHNICAL CHOICES

The static schemes, despite the bridges are all “suspended”:
– two bridges are anchored to the ground, and one is self-anchored;
– Stadano bridge is provided with seismic isolation, being the longest one.
– all three exhibit, but in very varied forms, significant asymmetries.

The choice of suspended bridge comes from different reflections and needs.
With the suspended bridge a structure with the following advantages was obtained:

  • a very thin deck;
  • of low-height masts, smaller than in case of cable-stayed bridge;
  • the absence of piles in the riverbed during construction for the two major ones which has allowed to avoid the risks associated with the violent and sudden floods that characterize these streams.

The level of the new deck was positioned 1,5 mt above the water level that is predicted to occurr during a 200 year period flood.

It is interesting to note at this point that the suspension bridge, typically destined for large spans or light and pedestrian structures, has demonstrated to be able to build, under certain conditions and overcoming some uncomplicated constructive problems, an alternative to the cable-stayed bridge, in technical, formal and economic terms.

 

STADANO BRIDGE

The bridge has a total lenght of 295 mt, of which crosses the river Magra with a suspended main span of 139 mt, with 40 and 20 mt spars.
The total length of the bridge between the two outer joints is 300 mt.
The main deck structure is metallic with rope suspension system.
The viaduct in golena is made of reinforced concrete with a traditional construction system: span by span.

The south side tower, on the left bank, is a single pylon with r.c. structure and rectangular section, with special surface finish, located behind the abutments, leaving the necessary space for the roadway curve.
The north side tower, of the tween type, consists of a pair of reinforced concrete shafts with prismatic profile and almost rectangular section.
The suspension system therefore has a pair of cables that start joining the south side antenna and diverge toward the top of the north side pylons.
The deck has a width of 12 mt and includes two lanes, and two pedestrian paths.

 

Fig. 1 and 2. View of the Stadano bridge from the road, towards the south tower, and view of the inferior structure from the belvedere obtained in the first pier.

Fig. 1 and 2. View of the Stadano bridge from the road, towards the south tower, and view of the inferior structure from the belvedere obtained in the first pier.

The assembly phase, which required the use of some work equipment in the riverbed, ie the one for which there was a real risk of sudden floods, lasted only one month.

Constructive technologies
The configuration of deck has enabled a strong prefabrication of the structural elements and hence at a high construction speed.
The modularity of the construction elements has made it possible to standardize production, manufacture and assembly activities and, above all, reduce learning times in assembly operations for safety and operating speed.

The Project has been implemented without any work variant.

 

 

THE BRIDGE OF MULAZZO

The Mulazzo bridge crosses the Mangiola river with a 100 mt span.
A 24 mt side span is the junction with the left bank.

Figures 3 and 4. View from the bridge of Mulazzo, downstream at the end of the construction. Detail of antennas on the left bank.

Figures 3 and 4. View from the bridge of Mulazzo, downstream at the end of the construction. Detail of towers on the left bank.

The Construction
The Mulazzo structure was erected with the same system as Stadano, but by installing 10 mt long, macro-segments, after having assembled them at job-site by welding box segments and transverse girders.
This process involved the provision of an assembly base outside the flooding area, the workload transfer of part of the welds to be carried out, and required the use of larger cranes and a more difficult control of the hanger forces in the various phases.
It has, however, allowed a very fast installation of segments, made within just 10 working days, for about 124 mt of deck.
The construction of the Mulazzo bridge has developed over the course of 13 months, from June 2016 to July 2017.

 

THE CASTAGNETOLI BRIDGE

The bridge on the Teglia torrent, in particular, has an original static scheme:

  • it is a suspension bridge, that is, with the deck girder supported by a system of parabolic cables;
  • is a self-anchored suspended bridge, ie with the closure and the balance of the horizontal components of the cable forces inside the deck girder and not on the ground;
  • is an asymmetric bridge in the longitudinal profile, as it has a single tower located on the left bank;
  • is an asymmetric bridge in the transversal plane, as the main structure – consisting of a tower, and a suspension system
  • is located in the valley section, leaving the upstream area completely free from engineering structures.

In this way, it was possible to allow upstream access to the temporary road access, and some essentiality was attributed to the structural system.

Figures 5 and 6. Castagnetoli bridge, from the valley during the load tests, and detail of the scaffold structures.

Figures 5 and 6. Castagnetoli bridge, from the valley during the load tests, and detail of the scaffold structures.

 

The Construction
The first stages of construction were the execution of substructures, foundations and abutments and the mooring block.
The construction was then continued through the assembly of structures on provisional trestles, with the simultaneous realization of the tower.
Subsequently, the concrete slab was built and the construction was completed with the installation and tensioning of the suspension system.
The tensioning of cables took place through the tension of the hangers, followed by the disarming of the deck and the subsequent removal of the temporary trestles.
The construction of the bridge, contracted by the Tuscany Region, which has been using DMA’s design as well as technical assistance for its guidance and control, began in November 2015 and ended in May 2017, with a duration of 18 months.

For all three bridges the project has been implemented without any variation in the course of the work and within the expected costs.

 

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