Last reviewed 30 Sept 2026 · 6 min read
Why the method matters
The way a bridge is built decides its stresses during construction, its cost, its speed and the impact on traffic or water below. The method is chosen at the design stage and the structure is designed for it: a girder designed for a launching sequence needs different reinforcement or prestress than the same girder cast on falsework.
Cast-in-situ on falsework
The girder or slab is cast in place on staging (falsework) supported from the ground.
- Advantages: no heavy plant, flexible geometry, monolithic behaviour.
- Limits: needs firm ground below; blocks traffic or water; slow; the formwork must be adequate against settlement of the staging.
- Care: design of the staging for the wet-concrete load with safety factors, camber for deflection, sequence of pour to control cracking, and striking only when concrete reaches the required strength.
- Suits short viaducts, flyovers over low ground, and skew or curved decks.
Precast girder launching
Girders are made in a casting yard and brought to the site.
| Method | Description | Suitable for |
|---|---|---|
| Crane erection | Mobile or crawler cranes lift each girder onto the piers | Short girders up to about 40 m; accessible ground |
| Twin cranes / gantry | Two cranes lift together | Heavier girders |
| Launching truss / gantry | A truss spans between piers and moves forward on rails; trolleys lower the girder | Viaducts; long girders; rivers or heavy traffic below |
| Beam launcher | Lighter launching frame moving over piers | Highway and metro girders |
| Barge / floating crane | Girders carried by barge and lifted | Wide rivers and sea crossings |
Before launching the girder is checked for lifting stresses (pick-up points), stability against lateral tilting during lifting and on the launcher (the top flange in compression can buckle laterally), and bearing seating. Lateral stability during transport and placement is often the governing check for slender I-girders.
Balanced cantilever construction
The bridge is built outward from the pier in both directions at the same time, so the moments on either side balance.
- Cast-in-situ: a form traveller (moving formwork) casts segments 3–5 m long; each segment is prestressed to the previous one after the concrete has gained strength.
- Precast segments: segments are cast in a yard (match-cast against each other so they fit) and lifted into position by a lifting frame, crane or launching gantry; they are glued with epoxy and prestressed.
- Stability: the cantilever is stabilised by a temporary pier table fixed to the pier or by props; unbalanced load (one segment more than the other, or a crane) is checked.
- Closure: the two cantilevers meet at mid-span; a closing segment is cast with the cantilevers locked together (with temporary struts) to avoid movement while concrete hardens. Continuity tendons are then stressed.
- Use: spans 60–250 m for PSC box girders; also cable-stayed and extradosed bridges.