How bridges get built · Heavy lift, moving and accelerated construction · Top-down construction over wetlands
Also called: top-down viaduct, end-on construction, leapfrog construction
How it is done
- Build the first spans from the bank conventionally; from then on, work only from the deck.
- Drive piles with a crane-hung hammer at the end of the deck, cast or set precast caps, deliver girders along the deck, place with a launcher or an end-crane.
- Cast or set the deck slab as the launcher moves on; keep the completed deck load-rated for construction traffic.
Where it fits
- Wetlands, mangroves, protected marshes, mudflats, sites where trestles are prohibited or too expensive; spans 25-45 m.
Where it does not
- Spans that a launching girder cannot handle, foundations that need bored piles with slurry plants.
Plant, pace and money
PlantLaunching girder or end-crane, crane with hammer, deck transporters.
ProductivityOne span per two to five days.
Cost5-15 % above trestle-based construction per metre; cheaper where the trestle would be long.
Risks and controls
What goes wrong
- Overloading the fresh deck, pile positioning accuracy from the deck, crane stability at the deck end.
Quality assurance
Deck load ratings posted, pile position survey, crane outrigger positions on the girders.
Examples
Louisiana and Florida wetland viaducts (Lake Pontchartrain, Everglades), Hong Kong and Singapore mangrove crossings; the game's wetland sites.
In the bridge game
What Bridge Master applies when you pick it
Deck types: PSC I-girder, PSC U-girder, Precast segmental box, Voided slab, Spliced PSC girder, Steel plate girder, Steel box girder, RC slab, RC T-beam. Spans 20-45 m, up to 55 m.
- +35: Wetland rule: build from the deck.
- +25: No temporary works allowed below: top-down.
- -20: Unnecessary where access is free.
Related methods
Launching girder (beam launcher) for precast girdersPHC spun concrete pilesTemporary trestles and access bridges
Further reading
FHWA ABC Manual · Louisiana DOTD top-down construction guidance