Construction methods · Precast concrete decks

Span-by-span precast segmental with gantry

All the match-cast segments of one span (2.5-3.5 m each) are hung from an overhead or underslung gantry, glued with epoxy, and post-tensioned into a span in two to four days. The fastest viaduct method ever built and the standard for metros, expressways and long approach viaducts.

How bridges get built · Precast concrete decks · Span-by-span precast segmental with gantry

Also called: span-by-span erection, overhead gantry, underslung gantry, segmental viaduct

overhead gantrysegments hung, epoxied, stressed into one span2-4 days per spanSpan-by-span precast segmental
Concept diagram (TheBridgeEng)

How it is done

  1. Cast segments short-line in a yard (one segment per cell per day), store, deliver by truck or along the deck.
  2. Hang all segments of the span from the gantry on temporary bars, align, apply epoxy, close with temporary PT.
  3. Thread and stress the external or internal tendons, set the span on bearings, launch the gantry.

Where it fits

  • Viaducts with 30-50 m spans and more than 15-20 spans, urban and marine settings, metros, tight sites with only overhead access.

Where it does not

  • Few spans, varying spans, spans over 50-55 m (weight), sharp curves for overhead gantries.

Plant, pace and money

PlantGantry 80-120 m, 300-600 t; casting yard with short-line cells; segment transport.
ProductivityOne span per two to four days per gantry.
CostDeck USD 1,300-1,900 per m2 including yard and gantry over a long viaduct.

Risks and controls

What goes wrong

  • Gantry collapse during launching (several accidents), epoxy joint failure, geometry error accumulating across segments, temporary bar failure.

Quality assurance

Match-cast geometry control in the yard, epoxy application in temperature limits, stressing records, gantry launch procedure.

Examples

Dubai Metro, Bangkok, Hong Kong-Zhuhai-Macau approaches, Sheikh Zayed Bridge approaches, many US urban viaducts.

In the bridge game

What Bridge Master applies when you pick it

Deck types: Precast segmental box, PSC box (cast in place), PSC I-girder, PSC U-girder, Voided slab. Spans 30-50 m, up to 60 m.

  • +35: Kilometres of 30-55 m spans: gantry span-by-span.
  • +15: Marine or urban viaduct with no ground access.
  • -40: Gantry and yard cannot be amortised.
  • -40: Beyond span-by-span.
  • -20: Gantry geometry on tight curves is awkward.
  • not allowed: Span-by-span gantry beyond feasibility.

Try it in the game →

Related methods

Balanced cantilever with precast segmentsSegment casting: short-line and long-line match castingMovable scaffolding system (MSS)Launching girder (beam launcher) for precast girders

Further reading

ASBI Construction Practices Handbook · AASHTO Guide Specifications for Segmental Bridges

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