How bridges get built · Precast concrete decks · Balanced cantilever with precast segments
Also called: precast segmental cantilever, segmental balanced cantilever, lifting frame erection
How it is done
- Cast the segments short-line or long-line with match-cast faces and shear keys; the pier segment, the heaviest, is often cast separately or in parts.
- Set the pier segment on the pier with temporary fixity (vertical bars and blocks, or stability towers) designed for the unbalanced erection moment.
- Lift segments in pairs with deck-mounted lifting frames, a launching gantry or cranes, delivered by barge below or along the deck behind.
- For each segment, dry-fit it, apply epoxy, close the joint with temporary bars and stress the permanent cantilever tendons for the pair within the epoxy's open time.
- Survey tip level, line and twist after each pair; match-cast joints cannot be adjusted, so corrections are made only at wet joints or at the closure.
- Close at midspan with a cast-in-place closure, using a closure frame or jacks to align the tips, and stress the continuity tendons.
- Release the temporary fixity, transfer the deck to its permanent bearings, grout the tendons and seal the anchorages.
Key numbers
- Span60–150 m
- Segment length 2.5–4 m; weight typically 50–150 t
- Erection rate2–4 segments per day per pier; a 100 m span pair in 3–5 weeks
- Depthabout L/16–L/20 at the pier and L/40–L/50 at midspan
- Temporary joint compression during epoxy cure: about 0.3 MPa average
- Costdeck USD 1,600–2,600 per m2
Where it fits
- Spans 60-150 m over water and roads with many similar spans; marine viaducts where barges deliver segments; sites needing speed.
Where it does not
- Few spans (the yard costs), spans over 150 m (segment weight and cantilever), sites where segment delivery is impossible.
Choosing it
- Choose precast over cast-in-place cantilevers when there are many spans and segments can be delivered by barge or along the deck: 2–4 segments a day against one pair a week.
- Prefer cast in place when there are few spans, segments would be too heavy to lift, or the geometry varies too much for one casting set-up.
- Prefer span-by-span erection for spans below about 50 m.
- Choose a launching gantry over lifting frames on long viaducts over land with no delivery from below; choose lifting frames with barge delivery over water.
Plant, pace and money
PlantCasting yard, lifting frames or gantry, barges or transporters, PT plant.
ProductivityTwo to four segments per day per pier; a 100 m span pair in 3-5 weeks.
CostDeck USD 1,600-2,600 per m2.
Risks and controls
What goes wrong
- Unbalanced moment during erection, epoxy joint quality in heat or rain, segment geometry error, lifting frame anchorage.
Quality assurance
Geometry control from the yard through erection, epoxy tests, stressing records.
Origins
Jean Muller and Campenon Bernard built the first match-cast precast segmental cantilever with epoxy joints at Choisy-le-Roi over the Seine in the early 1960s; the Oléron Viaduct (1966) then showed that segments erected by a launching gantry could build kilometres of 79 m spans.
Examples
Choisy-le-Roi and Oléron (France), JFK Memorial Causeway (Texas) and many marine and urban viaducts; the Confederation Bridge (Canada) is the extreme case, with whole precast cantilevers placed by a heavy-lift vessel.
Case studies
Choisy-le-Roi Bridge, SeineFrance · early 1960sThe first precast segmental bridge with match-cast, epoxy-glued joints, by Campenon Bernard with Jean Muller.
Oléron ViaductFrance · 1966About 2.9 km of 79 m precast segmental cantilever spans erected with a launching gantry.
JFK Memorial Causeway, Corpus Christi, TexasUSA · 1973The first precast segmental bridge in the United States.
Confederation BridgeCanada · 1997The extreme case: 250 m spans built from precast cantilever girders of up to about 190 m and 7,500 t, each placed whole by a heavy-lift vessel rather than segment by segment.
In the bridge game
What Bridge Master applies when you pick it
Deck types: Precast segmental box, Extradosed, Cable-stayed, PSC box, balanced cantilever. Spans 60-130 m, up to 150 m.
- +30: Several medium-long spans: precast cantilever.
- +15: Segments arrive by barge.
- -30: Too few spans for a casting yard.
- -40: Beyond precast cantilever.
- -25: Span-by-span is faster at short spans.
- +10: Precast segments: short site cycles and little disruption in the city.
- not allowed: Precast cantilever beyond feasibility.
Related methods
Balanced cantilever, cast in place (form travellers)Span-by-span precast segmental with gantrySegment casting: short-line and long-line match castingFloating crane erectionProgressive placement of precast segments
Further reading
ASBI Handbook · Podolny and Muller