Design of Millau Viaduct (short summary)

The Millau viaduct is a bridge and the longest-stayed bridge on record. The bridge comprises eight steel box deck sections of 32.05 m in width and 346 m the longest, spans 2460 m, and sits on seven RCC piers of height ranging from 78 m is 244.8 m. The need for the bridge arose to shorten the 4 hrs—congestion at Millau for travelers arriving from north of Europe to Spain.

Besides the minimal modern architectural design in harmony with the surrounding green landscape, the structural engineers designed planar centered eight cable-stayed spans instead of having a single main cable-stayed span anchored near an abutment or pier. The pylons and the piers mainly resist the structure longitudinal bending resulting from the loaded adjoining flexible spans. Plus, Minor forces are transmitted between adjacent bridge spans, and their vertical movement is reduced.

The trapezoidal cross-section of the deck is composed of a steel box girder with a max height of 4.2 m with an upper orthotropic decking made up of steel sheets 12-14 mm thick to resist fatigue. This final thickness is further increased around the 88.92 m high pylons to resist bending, torsion, and punching shear. The bottom of the box girder consists of 25-80 mm thick steel sheets, with rigidity provided by stiffeners of 14-16 mm thickness. Two vertical steel webs 4 m apart from thickness 20-40 mm run the entire length of the structure in turn, stiffened on their lower part by two longitudinal trapezoidal stiffeners. Lattice steel diaphragms provide the transverse stiffening of the deck at 4.17 m spacing on the spans.

The 88.92 m high - 7 steel pylons are set into the 7 piers. Longitudinal continuity is assured between the steel sheets of the webs of the central box girder and those of the steel walls of the 38m high steel pylon legs. , Lateral rigidity is provided by a frame covering the bearings found on each pier shaft. The steel grade of materials used ranges from S355 to S460.

The stranded staying cables consist of T15 strands of class 1860 MPA galvanized, sheathed, and waxed. The number of strands varies between 45 near the pylons and 91 in the middle of each span. The cables are fixed to the pylons and adjustable on the deck side.

With stiff piers and pylons, the attached deck is designed with reduced inertia (less thick) to stand the resistance and deformability it is subject to, consequently leading to an advantage of reduced deck surface resistance to wind effects. However, having stiff piers and pylons goes against the flexibility required for the thermal dilatation of the bridge deck.

The solution chosen to assure the deck does not rotate and provide the bridge decks the flexibility to expand longitudinally 0.6 m from each end due to thermal variation was for the bridge end piers shafts (P1 & P7) to be split into two. Every single shaft of the end piers had reduced inertia, consequently reducing the effects produced by the thermal dilation of the deck. Moreover, doubling of bearings in the longitudinal sense ensures that bending of the bridge deck structure is reduced to the minimum. For aesthetic homogeneity, all piers have been designed with two shafts. Similarly, the connected pylons are designed to the piers to assure the stiffness against bending and rotation, and secondly, the flexibility for bridge expansion due to thermal variation.

References:

Bergsjö, Cathrina, and Marcus Pettersson, ‘A Simulation of the Millau Viaduct’, 119

Buonomo, M, Claude Servant, and Michel Virlogeux, ‘The Design and the Construction of the Millau Viaduct’, 2004, 18

Magalhães, Filipe, Elsa Caetano, Álvaro Cunha, Olivier Flamand, and Gérard Grillaud, ‘Ambient and Free Vibration Tests of the Millau Viaduct: Evaluation of Alternative Processing Strategies’, Engineering Structures, 45 (2012), 372–84 <https://doi.org/10.1016/j.engstruct.2012.06.038>

‘Millau Viaduct Geotechnical Studies and Foundations | Elsevier Enhanced Reader’ <https://doi.org/10.1016/j.jrmge.2013.05.002>

‘Millau Viaduct (Millau/Creissels, 2004)’, Structurae <https://structurae.net/en/structures/millau-viaduct> [accessed 19 February 2021