Incremental launching with temporary piers

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

Design the temporary piers (steel lattice towers or braced tubular columns) for the launching reactions, friction and guide forces, wind and nose landing, on piles or spread footings checked for scour.
Build them before the launch reaches them, check their settlement, and fit sliding bearings and lateral guides at the same profile as the permanent piers; jacks on top allow levels to be adjusted.
Launch over the doubled number of supports, monitoring temporary pier deflection, reactions and settlement at every stroke.
After launching, stress the additional tendons the long final spans need while the temporary piers still support the deck.
Lower the temporary supports with their jacks in a controlled sequence, transferring the load to the permanent structure while surveying the deck.
Remove the temporary piers and their foundations and restore the river bed or valley floor.

Key numbers

Final span60–110 m with one temporary pier per span, halving the launching span
Deck depthset by the launching span (about 1/12–1/16 of the shortened span), not by the final span
Costabout 10–20 % above plain launching for the temporary piers
Programmea few weeks before and after launching for building, releasing and removing the temporary piers

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.

Choosing it

Choose it over balanced cantilever for 60–110 m spans when cheap temporary piers can be founded (shallow water, firm ground) and a constant-depth, constant-curvature deck is acceptable.
Prefer temporary stays from a mast on the deck over temporary piers in deep water, tall valleys or navigable channels.
Prefer balanced cantilever when temporary pier positions are in deep or fast water, or would cost more than a pair of form travellers.

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.

Origins

Temporary intermediate supports were part of incremental launching from its first concrete use on the Caroní River bridge (1964), whose 96 m spans were launched over temporary intermediate supports; the same principle later carried the steel deck of the Millau Viaduct over 342 m spans.

Examples

German and Swiss valley viaducts; Korean river bridges over shallow beds.

Case studies

Caroní River BridgeVenezuela · 1964The first concrete incremental launch; its 96 m spans, far beyond an unsupported launch, were launched over temporary intermediate supports.

Millau ViaductFrance · 2004Steel deck rather than concrete, but the same principle: seven temporary towers halved the 342 m spans for launching.

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.

Try it in the game →