For the construction of the Allonne bridge over the A16 motorway to the south of Beauvais in the Oise region of France, Freyssinet’s teams implemented prestressing and guying systems for the structure and demonstrated their mastery of an unusual and highly technical process: the construction of a structure by rotation.

  • Owner
    French ministry of Ecological Transition, transport manager
  • Client
    Oise department
  • Delivery date
  • Partners
    Consortium with Chantiers Modernes BTP (Vinci Construction France, Lead contractor)

A design and construction method guided by technical constraints

The Allonne bridge construction project had to meet severe construction constraints: create a long-span structure (90 m) without an intermediate pier (passage of a fibre-optic network under the lane), and without disrupting traffic on the A16 while guaranteeing unfailing safety for the workers carrying out the work.

To meet this technical challenge, the project designers opted for a 125-m long cable-stayed solution, prefabricated parallel to the motorway. Comprising an 87-m central span and a 22-m rear counterweight span on either side of a 47-m high central tower, the bridge was then installed by rotation over the traffic lanes.

“This is a very complete project which is a showcase of our know-how with a compendium of specialised Freyssinet techniques for the construction of structures. The customer was impressed by the ability of our teams to manage difficulties and all types of unforeseen events during sensitive operations.”

Martin, Fomer Project Engineer – Centralised Cables and Handling Department

  • 7,500t
    Weight of the structure (tonnes)
  • 125m
    Length of the structure (meters)
  • 6h
    Duration of the sliding operation by rotation (hours)
Allone bridge rotation

The rotation of a 7,500 t bridge in a few hours

The pivoting manoeuvre of this 7,500 t mammoth defying the laws of gravity to closely follow the abutment built on the opposite bank took place in just six hours. Placed on a pot bearing specially designed by our teams, the south mast was the vertical axis of rotation. A first curved beam, located under the support of the North mast, supported the slide chair, equipped with stainless steel sheets and neoprene pads allowing to limit friction to a minimum. The counterweight abutment rested on a 2nd curved beam.

The sliding was carried out with the help of a 140-t jack. By progressively shifting it on a series of anchor tubes on this second curved beam, it enabled the counterweight abutment to be brought to its final position, by pushing it by 50 cm, thus making the entire structure pivot. A final jacking was carried out to transfer the bridge on the final supports under the Eastern abutment and in line with the Northern mast, prior to the installation of the superstructure and the expansion joints completing the work.

Pre-installation of 30 stay cables, including 2 temporary ones of 100 metres

But before achieving this spectacular result, several months of preparation were necessary. The deck was first formed and poured on temporary ring beams with longitudinal prestressing (units 19 and 25 C15). Freyssinet then proceeded to install the twenty-eight 30 to 80 metre long stay cables, which we tensioned to 70% before decentring. Two additional 100 metre long stay cables had to be temporarily placed to stiffen the end of the central span during the cantilever rotation. Due to the curvature and asymmetry of this complex structure, each stay cable had to be specifically sized. Before it was commissioned, a final tensioning operation with very precise adjustment was carried out following the positioning of the structure on its final supports, and the pavement has been laid.

Allone bridge stay cables

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