Incremental launching (ILM)

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

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

How it is done

Build the casting bed behind the abutment (one or two segment lengths, with fixed steel formwork and shelter), the launching jacks and the steel launching nose of about 0.6–0.7 of the span.
Install temporary sliding bearings (stainless steel plates with PTFE pads inserted as the deck passes) and lateral guides on every pier, and check the piers for launching friction and gradient forces.
Cast each segment against the previous one, cure it, and stress the centric launching tendons across the joint so every section can carry the moments that travel with it.
Launch with lift-and-push jacks or strand pulling jacks, monitoring pier-top deflection, jack forces and guide loads; on arrival, jack the nose tip up onto the next pier to recover its deflection.
Repeat about weekly; the bed geometry fixes the alignment, so only straight or constant-curvature alignments can be launched.
After the last launch, remove the nose, jack the deck onto its permanent bearings and stress the continuity tendons needed for the final span moments.
Grout the tendons, seal the anchorages and remove the bed.

Key numbers

Span30–60 m (up to about 100 m with temporary piers or stays)
Depthabout L/12–L/16, deeper than other methods because every section passes over a support
Segment length15–30 m; cycle 7–10 days
Launching noseabout 0.6–0.7 of the span, steel
Launching frictionabout 2–5 % on PTFE sliding bearings, plus the gradient component
Alignmentstraight or constant radius in plan and profile

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.

Choosing it

Choose it over balanced cantilever for 30–60 m spans on straight or constant-curvature alignments where nothing can be placed below and there is room for a casting bed behind an abutment.
Choose it over an MSS when access along the alignment is poor, since all the heavy work stays at the bed; prefer the MSS when curvature varies or the deck cannot be of constant depth.
Prefer launching with temporary piers, or balanced cantilever, for spans above about 60–70 m.
Prefer precast girders or segments when the alignment or depth varies, or when the deck is too short to pay for the bed and nose.

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.

Origins

First used for a concrete deck by Fritz Leonhardt and Willi Baur on the Caroní River bridge in Venezuela (completed 1964), then on the Inn bridge at Kufstein, Austria (1965); it became standard for European valley viaducts from the 1970s.

Examples

Caroní River bridge (Venezuela, the first concrete ILM, 1964), Inn bridge at Kufstein (1965), the steel-deck Millau Viaduct (2004) and hundreds of European and Korean valley and railway crossings.

Case studies

Caroní River BridgeVenezuela · 1964The first incrementally launched concrete bridge, by Leonhardt and Baur, with 96 m spans.

Inn Bridge, KufsteinAustria · 1965The second concrete deck launched by Leonhardt and Baur, which brought the method to Europe.

Injaka Bridge, MpumalangaSouth Africa · 1998The deck collapsed during launching and 14 people died; studied since as a failure of launching design checks and temporary works control.

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 35-60 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.
-10: On open, low ground falsework or precast girders are cheaper than a casting bed and launching nose.
not allowed: ILM on a variable-radius alignment.

Try it in the game →