Construction methods · Cast-in-place concrete decks

Incremental launching (ILM)

The deck is cast in 15-30 m segments on a fixed casting bed behind the abutment and pushed out over the piers with hydraulic jacks, a steel launching nose ahead of it reducing the cantilever moment. All the work happens in one place at ground level; the deck arrives at the far abutment span by span.

How bridges get built · Cast-in-place concrete decks · Incremental launching (ILM)

Also called: incremental launching method, push-launch, ILM

abutment +casting bedlaunching nosepush 5-10 m/hsliding bearingssegment cast behind the abutment, launched, repeatIncremental launching (ILM)
Concept diagram (TheBridgeEng)

How it is done

  1. Build the casting bed and the launching abutment; install sliding bearings with PTFE pads on every pier.
  2. Cast a segment on the bed (one to two weeks), stress the centric launching tendons, launch by jacks (lift-and-push or friction launchers) at 5-10 m per hour.
  3. Repeat until the deck is in place; then install permanent bearings, add the continuity tendons, remove the nose.

Where it fits

  • Straight or constant-curvature alignments, constant depth box girders, spans 30-60 m (up to 100 m with temporary piers), sites with no access under the deck: valleys, rivers, railways, environmentally protected ground.

Where it does not

  • Variable alignments and spans, very long spans without temporary piers, decks that cannot be constant depth.

Plant, pace and money

PlantCasting bed with formwork, launching jacks or friction launchers, launching nose (0.6 span long), temporary sliding bearings, lateral guides.
ProductivityOne segment per 7-14 days; 20-30 m of bridge per cycle.
CostDeck USD 1,300-2,000 per m2; the bed and nose USD 1.5-4 million.

Risks and controls

What goes wrong

  • Nose landing on the pier misaligned, friction and pier deflection during launching, overstress of the deck as it passes the pier (every section becomes a support section), unbalanced friction pulling the deck sideways.

Quality assurance

Launch procedure with pier deflection monitoring, bearing friction checks, survey of the nose landing, tendon stressing records.

Examples

Rio-Caroni (the first), Millau Viaduct (steel), hundreds of European and Korean valley and railway crossings.

In the bridge game

What Bridge Master applies when you pick it

Deck types: PSC box (cast in place), RC box, PSC box, balanced cantilever. Spans 30-50 m, up to 65 m.

  • +30: Straight, repetitive, no ground access: ILM.
  • +20: Launching over rail needs no possession except for the nose passage.
  • +15: Nothing in the river during construction.
  • -45: Variable curvature cannot be launched.
  • -35: Launching moments too high; temporary piers or another method.
  • -30: Too short to set up a casting bed.
  • +5: Tall piers: launching avoids tall falsework, but pier top forces need checking.
  • not allowed: ILM on a variable-radius alignment.

Try it in the game →

Related methods

Incremental launching with temporary piersLaunching of steel girders and boxesCast-in-place post-tensioned box and slab decks

Further reading

Rosignoli, Bridge Launching · SETRA guide on incremental launching

← Balanced cantilever, cast in place (form travellers)Incremental launching with temporary piers →