Movable scaffolding system (MSS)

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

How it is done

Choose the type: underslung (main girders below the deck on pier brackets, deck top clear) or overhead (girder above, hangers carry the forms, suits limited clearance below), and check it for span, curvature, gradient and pier-head shape.
Assemble the MSS at the abutment or on the first span and commission it; load-test it or monitor the first pour closely, since the first span always takes much longer than the routine cycle.
Support the MSS on brackets fixed to the piers (underslung) or on the deck and pier (overhead), and set the forms to casting levels that include the MSS deflection under wet concrete.
Fix reinforcement (prefabricated cages where possible), ducts and anchorages; close the inner forms and pour the span plus a cantilever of about one fifth of the next span, where moments are small.
Stress the tendons, coupling them at the construction joint, then lower and open the forms; underslung forms open transversely to pass the pier.
Launch the MSS to the next span on its own hydraulics within the wind limit, after a launch checklist (hold point), and move the brackets ahead.
Monitor deflection during each pour and bracket bolt tension every cycle; at the end, dismantle the MSS for reuse.

Key numbers

Span30–60 m typical, up to about 70 m with heavy machines (Mersey Gateway approaches about 70 m)
MSS steel weightabout 400–1,500 t (Mersey Gateway machine 1,500 t and 157 m long)
Cycle7–14 days per span once the crew has learnt it
Construction jointabout 0.15–0.25 of the span into the next span
Break-evenroughly 8–10 similar spans per machine
Costmachine USD 3–8 million; deck USD 1,200–1,800 per m2

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.

Choosing it

Choose an MSS over falsework when the ground below is unavailable (water, traffic, soft ground, height) and there are about ten or more similar spans.
Choose an MSS over precast span-by-span when the viaduct is too short or too remote to pay for a casting yard; precast is faster per span on long viaducts.
Prefer balanced cantilever for spans above about 60–70 m.
Choose an overhead MSS when clearance below the deck is limited or pier heads cannot take brackets; choose underslung when the deck top must stay clear for reinforcement and concreting.

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.

Origins

Movable scaffolding systems were developed by German contractors from the late 1950s for long motorway viaducts; self-launching hydraulic machines, some with actively controlled ('organic') prestressing of the main girder, spread worldwide from the 1980s and 2000s.

Examples

Mersey Gateway approach viaducts (UK), D1 motorway viaducts in Slovakia, and many Portuguese, Spanish, Middle Eastern and Asian motorway and rail viaducts with 40–70 m spans.

Case studies

Mersey Gateway approach viaducts, Runcorn-WidnesUK · 2017One MSS, 157 m long and 1,500 t, cast 19 approach spans of about 70 m on both sides of the estuary; its assembly alone took three to four months.

D1 motorway viaducts, Jablonov-JánovceSlovakia · 2014An overhead MSS with organic prestressing cast two box-girder viaducts (412 m and 365 m) with 65 m spans on 14-day cycles, and was moved between them along the new road without dismantling.

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 30-60 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).
+10: Tall piers: the MSS needs no ground support, falsework would.
not allowed: MSS far beyond feasible span.

Try it in the game →