How bridges get built · Cast-in-place concrete decks · Incremental launching with temporary piers
Also called: ILM with temporary bents, launching over temporary supports
How it is done
- Install temporary steel piers on spread footings or piles at mid-span of the long spans, braced against wind and launching friction.
- Launch the deck over the doubled number of supports; after launching, remove the temporary piers and stress the final tendons for the long spans.
- Sometimes the temporary piers are used only for the launching phase and the deck is post-tensioned for the long span afterwards.
Where it fits
- Spans 60-110 m, rivers and valleys with shallow water or firm ground at mid-span, sites where launching keeps traffic below untouched.
Where it does not
- Deep water at the temporary pier position, navigable routes, sites where the temporary piers cost more than a cantilever traveller.
Plant, pace and money
PlantAs for ILM plus temporary piers (often modular steel trusses) with foundations.
ProductivityAs for ILM; the temporary piers add a few weeks before and after.
Cost10-20 % above plain launching for the temporary piers.
Risks and controls
What goes wrong
- Temporary pier settlement during launching, buckling of tall temporary piers, removal in flood season.
Quality assurance
Temporary pier design check, settlement monitoring, launch load records.
Examples
German and Swiss valley viaducts; Korean river bridges over shallow beds.
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, Steel box girder, Steel plate girder. Spans 60-100 m, up to 120 m.
- +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.
- -30: Too short to set up a casting bed.
- +5: Tall piers: launching avoids tall falsework, but pier top forces need checking.
- +25: Temporary piers make the launching span manageable.
- -25: Temporary piers in deep water become a marine job of their own.
- -20: Temporary piers in a navigation route need permits and protection.
- not allowed: ILM on a variable-radius alignment.
Related methods
Incremental launching (ILM)Launching of steel girders and boxesBalanced cantilever, cast in place (form travellers)
Further reading
Rosignoli, Bridge Launching