Balanced cantilever, cast in place (form travellers)

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

How it is done

Build the pier and cast the pier table (typically 8–14 m long) on brackets or falsework; where the deck is not monolithic with the pier, install temporary fixity (vertical bars through temporary blocks, or props) designed for the unbalanced moment.
Erect the two form travellers back to back on the pier table and load-test them, or monitor the first segments closely.
Cycle each segment pair: advance the travellers, set the forms to the casting curve, fix reinforcement and ducts, pour both sides within the allowed imbalance, cure to stressing strength and stress the cantilever tendons.
Control geometry by surveying the tips at a fixed time of day after each stage and updating the predicted deflections for creep, shrinkage, traveller deformation and temperature.
Complete the side spans: cast the end segments on falsework at the abutment, or use a counterweight or temporary support for the last cantilever segments.
At midspan, lock the two tips together with a steel closure frame at a defined temperature, cast the closure segment (1.5–3 m) and stress the continuity tendons.
Release the temporary fixity, transfer the deck to its permanent bearings (jacking them if reactions need correcting), and remove the travellers backwards or by crane.

Key numbers

Span60–250 m, economic about 80–200 m; records Stolma 301 m and Raftsundet 298 m (1998)
Depthabout L/16–L/20 at the pier and L/40–L/50 at midspan
Segment length3–5 m; pier table 8–14 m
Cycle5–9 days per segment pair, about one week typical
Closure segment1.5–3 m
Costdeck USD 1,800–3,000 per m2; travellers USD 1–2 million per pair

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).

Choosing it

Choose it over an MSS or launching when spans exceed about 70–80 m, spans vary, the alignment is curved, or nothing can stand below the deck.
Choose cast in place over precast cantilevers when there are too few spans for a casting yard or segments cannot be delivered; precast erects 2–4 segments a day against one pair a week.
Prefer extradosed or cable-stayed above about 200–250 m, where girder depth and long-term creep deflection become hard to control.

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.

Origins

Emilio Baumgart built the first reinforced concrete balanced cantilever over the Rio do Peixe at Herval, Brazil, in 1930 (68 m); Ulrich Finsterwalder of Dyckerhoff & Widmann turned it into the prestressed method with form travellers and threaded bars on the Lahn bridge at Balduinstein (1950–51) and the Nibelungen Bridge at Worms (1953).

Examples

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

Case studies

Nibelungen Bridge, WormsGermany · 1953Main span of about 114 m, one of the first large prestressed concrete cantilevers built with form travellers by Finsterwalder's method.

Gateway Bridge (now Sir Leo Hielscher Bridges), BrisbaneAustralia · 1986260 m main span cast in place by balanced cantilever, among the longest concrete girder spans of its time; a duplicate was added in 2010.

Stolma BridgeNorway · 1998301 m main span, the longest all-concrete girder span; lightweight concrete in the central part reduced the cantilever weight.

Koror-Babeldaob BridgePalau · 1977241 m span, a record when built; creep deflection of more than a metre at midspan led to a retrofit, and the bridge collapsed in 1996 shortly afterwards. The standard lesson on long-term deflection.

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.
+10: Nothing below the deck: the channel stays open throughout.
-20: Short for travellers; precast cantilever or a launched box is quicker at this span.
not allowed: CIP cantilever beyond any built span.

Try it in the game →