Construction methods · Cast-in-place concrete decks

Balanced cantilever, cast in place (form travellers)

From each pier, the box girder grows outward in 3-5 m segments cast in a pair of form travellers, one on each side to keep the pier balanced, until the cantilevers from adjacent piers meet at midspan and are joined by a closure pour. The method for 80-250 m concrete spans over rivers, valleys and railways where nothing can stand below.

How bridges get built · Cast-in-place concrete decks · Balanced cantilever, cast in place (form travellers)

Also called: FCM, free cantilever method, balanced cantilever, form traveller construction

travellertravellerpier tabletemporary fixitysegments 3-5 m cast in pairs, balanced; closure at midspanBalanced cantilever, cast in place
Concept diagram (TheBridgeEng)

How it is done

  1. Build the pier table (the first 8-12 m over the pier) on brackets, with a temporary fixity to the pier (bars, blocks or props) if the pier is not monolithic.
  2. Erect the form travellers; cycle: move traveller, set forms, fix reinforcement and ducts, pour, cure, stress the cantilever tendons, repeat every 5-9 days per pair of segments.
  3. Control geometry with a camber curve updated from survey after every segment.
  4. Close at midspan with a closure segment, stress the continuity tendons, remove the temporary fixity, adjust the bearings.

Where it fits

  • Spans 80-250 m, deep valleys, navigable rivers, railway crossings, tall piers, sites where a gantry or falsework cannot be placed.

Where it does not

  • Spans under 60-70 m (precast or MSS is faster), sites with strong wind on the free cantilever without tie-downs, soft piers without temporary fixity (the game's unbalanced_fcm check).

Plant, pace and money

PlantForm travellers (two per pier, 50-100 t each), pier brackets, tower crane or crawler crane, concrete pump, stressing equipment.
Productivity5-9 days per segment pair; a 150 m span from two piers in 5-7 months.
CostDeck USD 1,800-3,000 per m2; travellers USD 1-2 million per pair.

Risks and controls

What goes wrong

  • Unbalanced moment during construction (traveller or segment lost on one side), geometry error at closure, creep and shrinkage deflections misjudged, traveller anchorage failure.

Quality assurance

Survey every segment with temperature correction, tendon stressing records, traveller anchor checks, closure survey.

Examples

Gateway Bridge Brisbane, Stolma Bridge (301 m, lightweight concrete), Korean and Japanese river bridges by the hundred.

In the bridge game

What Bridge Master applies when you pick it

Deck types: PSC box, balanced cantilever, Extradosed, Concrete arch, Rigid frame, PSC box (cast in place), Cable-stayed. Spans 80-200 m, up to 250 m.

  • +30: This is how you build an FCM box.
  • +15: No support below required.
  • -10: Long free cantilevers in strong wind need temporary ties.
  • -35: Too short to justify travellers.
  • -10: Many medium spans: precast segmental cantilever is faster.
  • not allowed: CIP cantilever beyond any built span.

Try it in the game →

Related methods

Balanced cantilever with precast segmentsExtradosed bridge constructionForm travellers and traveller anchorageCable-stayed cantilever erection

Further reading

Podolny and Muller, Construction and Design of Prestressed Concrete Segmental Bridges · fib Bulletin 4

← Movable scaffolding system (MSS)Incremental launching (ILM) →