Progressive placement of precast segments

How bridges get built · Precast concrete decks · Progressive placement of precast segments

Also called: progressive placing, progressive cantilever erection, one-direction segmental erection

How it is done

Cast the segments short-line to the curved geometry and plan the erection from the deck: a swing crane, derrick, stiffleg crane or gantry at the tip, and transporters bringing segments over the completed deck.
Start at the abutment with the first span on falsework or temporary support, and give the first pier the temporary fixity the erection analysis requires.
Place each segment at the tip: dry-fit, epoxy the match-cast face, apply temporary bars, and stress the permanent top-slab cantilever tendons as the cantilever grows.
Once the cantilever passes the length it can carry alone, support it with temporary stays from a mast over the last completed pier, or with a temporary bent near midspan.
Land the cantilever on the next pier (pier segment) by jacking to level, and stress the bottom-slab continuity tendons for the completed span.
Transfer the load from the stays or bent to the permanent structure in the designed sequence, then move the mast or bent forward and repeat span by span.
Close at the far abutment, stress the final continuity tendons, grout and seal.

Key numbers

Spanabout 30–60 m (Linn Cove maximum 55 m)
Segment length 2.5–3.5 m; weight typically 30–80 t (Linn Cove 2.6 m and about 45 t)
Temporary support of the advancing spanstays from a mast over the last completed pier, or a temporary bent near midspan
Erection ratelimited by moving the lifting equipment; about four segments a week on Linn Cove
Jointsepoxied match-cast faces under about 0.3 MPa temporary compression, as in other precast segmental work

Where it fits

Spans of about 30–60 m on tightly curved or reverse-curved alignments where erection must come from the deck already built
Environmentally sensitive or steep sites where only the permanent piers may touch the ground
Viaducts that can be started from one abutment, with segments delivered over the completed deck

Where it does not

Spans over about 60–70 m, where balanced cantilever from the piers handles the moments more economically
Open sites where cranes from the ground or a span-by-span gantry are cheaper and faster
Teams without stage analysis, stay-force control and segmental geometry control experience

Choosing it

Choose it over balanced cantilever when the deck must advance from one end only (no access to the piers from below, no ground under the spans) and spans are moderate.
Prefer balanced cantilever when spans exceed about 60–70 m, several erection fronts are needed, or the piers can be reached: the cantilever is then balanced and needs no stays.
Prefer a span-by-span gantry on straight or gently curved viaducts with spans under about 50 m: faster, with no cantilever stage.
Choose temporary stays from a deck mast to keep the ground clear; choose temporary bents where a midspan foundation is acceptable or tight curvature makes the stayed cantilever too torsionally demanding.

Plant, pace and money

PlantShort-line casting yard, deck-mounted swing crane, derrick or stiffleg crane (or an overhead gantry), temporary mast with stay cables and jacks or temporary bents, segment transporters on the deck, stressing and grouting equipment.

ProductivitySet by moving the lifting equipment and by the stay or bent transfer at each pier; the Linn Cove Viaduct averaged about four segments a week with a stiffleg crane, and a swing crane would have been faster.

CostIndicative: deck USD 2,000–3,500 per m2, roughly 10–30 % above precast balanced cantilever on an accessible site; the premium buys erection without ground access.

Risks and controls

What goes wrong

Large cantilever moments and tip deflection before the next pier is reached
Stay or temporary bent force errors distorting the profile
Geometry errors accumulating in one direction with no midspan closure to absorb them
Torsion on tightly curved cantilevers, the reason Linn Cove used temporary bents instead of stays
Stability of the first span and pier during the longest cantilever stage

Quality assurance

Independent check of the erection stage analysis, stay forces measured with load cells or lift-off tests, tip survey at a fixed time of day after each segment, casting curve updated from the erection survey, and pier landing tolerances and the release sequence of temporary supports recorded.

Origins

Progressive placement, normally with temporary stays from a mast on the deck, was part of the precast segmental toolkit developed by Jean Muller and others in the 1970s; the Linn Cove Viaduct (completed 1983) was the first segmental bridge in the US built this way, using temporary bents at midspan instead of stays because of its tight curves.

Examples

Linn Cove Viaduct, Blue Ridge Parkway, North Carolina (completed 1983): 379 m long, 153 precast segments of which only one is straight, curves down to about 76 m radius, erected from the top in one direction with temporary bents at midspan; even the piers were precast and placed from the deck.

Case studies

Linn Cove Viaduct, Blue Ridge ParkwayUSA · 1983379 m of 153 precast segments on curves down to about 76 m radius, erected from the top in one direction by a stiffleg crane with temporary bents at midspan; the first US segmental bridge built by progressive placement.