Before Millau, incremental launching was reserved for viaducts of moderate span and predictable geometry — efficient, but never extraordinary. Then France chose a challenge: the Tarn Valley.
Piers reaching approximately 245 m, topped by masts to a structural height of 343 m. A deck of 2,460 m, curved in plan and exposed to valley winds. For the deck erection, tall cranes or continuous false-work across the valley were far less practical than a launch-based solution. The answer was bold: launch the deck from above.
Millau remains a defining case where method drove design — deck launching, temporary works, and cable-stay technology conceived as one integrated system.
On the river spans, the front pylons and part of their permanent stay cables were erected on the deck before and during launching, providing control at the leading cantilevers. Once the two deck halves met and were locked at mid-span, those pylons were completed, the remaining pylons erected on the other piers, the full stay system installed and tensioned, and the temporary steel towers removed. The sequence was not an adaptation of a standard method to an exceptional site. It was a method conceived specifically for this site, at this scale, under these constraints.
For further reading on launch-based bridge erection, see the EE&HL case studies on the Oléron Viaduct — an earlier French precedent for deck launching — and the Woronora River Bridge, where incremental launching was applied to a curved prestressed concrete deck.
Frequently Asked Questions
The valley depth — up to 270 m above the Tarn — made conventional false-work impractical at any reasonable cost. Cantilevering from the pylons would have required the pylons to be erected and their stay systems to be fully operational before any deck could advance, creating a sequencing problem at the leading spans. Incremental launching allowed the deck to be assembled on flat ground at both abutment plateaux and pushed progressively across the valley using temporary intermediate supports, deferring the permanent cable-stay system to a later construction phase.
The six permanent spans each measure 342 m — far beyond what an unlaunched orthotropic steel deck could sustain without excessive negative bending at the leading cantilever. Temporary steel towers at intermediate positions within each span reduced the effective span during each launch increment to approximately 171 m. A steel launching nose at the deck's leading edge further reduced the negative bending moment as the deck crossed from one support to the next. The hydraulic wedge system on each support allowed continuous adjustment of vertical reactions throughout.
For the two central river spans — where piers P2 and P3 are tallest — the steel pylons were pre-assembled on the ground, placed horizontally on the deck, and installed with a partial set of permanent stay cables before launching began. As the deck advanced, these cables provided vertical support at the leading cantilever. They were installed at very low initial tension (reportedly around 5% of final design tension) to accommodate the changing deck geometry through the launch, then re-tensioned to full design load after deck closure.
The northern and southern deck sections met above the Tarn in May 2004. Satellite-guided positioning and hydraulic control of the launching equipment allowed the leading edges to align within approximately one centimetre, enabling the welded closure joint to proceed under controlled conditions 270 m above the valley floor. The operation was timed to coincide with favourable wind conditions.
Incremental launching (also called the push-launch or ILM method) is an erection technique in which the bridge deck is assembled in sections at one or both abutments, then hydraulically pushed forward in increments across the piers until it reaches its final position. A steel launching nose reduces bending during cantilever phases. The method avoids false-work in the valley below, making it suited to sites where access is difficult or spans are long. At Millau, the scale and pier height pushed the technique beyond any previous application of the principle.