Construction methods · Cast-in-place concrete decks

Movable scaffolding system (MSS)

A self-launching steel truss or girder that carries the complete formwork for one span, casts it, lowers the forms and rolls to the next pier. Overhead and underslung types exist; both remove the ground from the equation, so long viaducts over water, roads and soft ground can be cast in place at one span every 7-14 days.

How bridges get built · Cast-in-place concrete decks · Movable scaffolding system (MSS)

Also called: MSS, span-by-span cast-in-place, travelling formwork, form traveller for spans

self-launching truss carries the whole span formpier bracketscast, stress, lower, launch: 7-14 days per spanMovable scaffolding system (MSS)
Concept diagram (TheBridgeEng)

How it is done

  1. Assemble the MSS (100-200 m long, 500-1,500 t) at the abutment; support it on pier brackets.
  2. Close the forms, fix reinforcement and tendons, pour the span with a construction joint at the quarter point of the next span.
  3. Stress, lower the forms, launch the MSS to the next pier on its own hydraulics; cycle 7-14 days.

Where it fits

  • Viaducts with 30-60 m spans and more than about 10 spans, constant section, over water, roads, rail or soft ground; light rail and highway viaducts.

Where it does not

  • Fewer than 8-10 spans (mobilisation is not recovered), spans over 60-70 m, sharply curved or widely varying spans.

Plant, pace and money

PlantThe MSS, pier brackets, concrete pump, tower or mobile crane for reinforcement.
Productivity7-14 days per span.
CostMSS purchase or hire USD 3-8 million; deck USD 1,200-1,800 per m2.

Risks and controls

What goes wrong

  • MSS collapse during launching or pouring (several fatal accidents), bracket failure, wind during launch.

Quality assurance

Launching procedure with hold points, bracket bolt tension checks, deflection monitoring during the pour.

Examples

Sheikh Zayed Bridge approaches, Taiwan and Korean high-speed rail viaducts, Portuguese and Spanish motorway viaducts.

In the bridge game

What Bridge Master applies when you pick it

Deck types: PSC box (cast in place), Voided slab, RC box, RC T-beam, RC slab. Spans 30-55 m, up to 70 m.

  • +30: Long viaduct of repetitive spans: MSS.
  • +15: Builds over water and traffic.
  • -35: Too few spans to amortise the MSS.
  • -40: Beyond MSS span.
  • -25: Curved MSS possible but slow; consider curved steel or CIP on falsework.
  • +10: Underslung MSS passes over track without ground contact (check clearance to the girder).
  • not allowed: MSS far beyond feasible span.

Try it in the game →

Related methods

Full staging method (ground falsework)Span-by-span precast segmental with gantryCast-in-place post-tensioned box and slab decks

Further reading

Rosignoli, Bridge Construction Equipment · fib Bulletin on construction methods

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