Modern methods for constructing prestressed concrete bridge decks and viaducts

The construction of prestressed reinforced concrete bridge decks and viaducts is currently dominated by a set of well-established methods and specialist equipment. Broadly, these solutions can be divided into in situ concrete bridges and prefabricated bridges, each with its own structural and construction logic.

Among in situ methods, three types of equipment are particularly widespread today:

• Ground-supported falsework
• Pier-supported falsework
• Incremental Launching equipment

   

Pier-supported falsework: Formtravellers (FT) and Movable Scaffolding Systems (MSS)

Pier-supported falsework is especially relevant when the site conditions make ground-supported systems impractical.

Two types of equipment are currently dominant:

• Formtravellers (FT)
• Movable Scaffolding Systems (MSS)

   

They serve different structural solutions:

• Formtravellers are used for decks constructed by the cantilever method, typically segment by segment, balanced around piers.
• Movable Scaffolding Systems are used for decks designed as continuous concrete beams supported on the columns, where each span is concreted in its final position.

   

In both cases, the deck is concreted in sections:

• With Formtravellers, typical segment lengths are up to 5 m, and in special cases can reach 10 m.
• With Movable Scaffolding Systems, the deck is cast span by span. Construction joints are usually placed at about 1/5 of the span length, where bending moments are close to zero.

   

Incremental Launching Method (ILM)

The Incremental Launching Method is used for the construction of continuous decks in concrete, steel or composite (steel–concrete) solutions. The deck is prefabricated in sections behind an abutment and then pushed longitudinally over the piers using hydraulic cylinders.

Structurally, ILM is also suited to beam-type decks, which naturally raises the comparison with MSS.

MSS vs Incremental Launching: key differences

Both MSS and ILM are used to build continuous beam decks, but the way the deck is formed and loaded during construction is very different:

• In Incremental Launching:
  • The deck is prefabricated by sections behind the abutment.
  • The entire deck is pushed over the top of the columns with hydraulic cylinders.
  • During construction, any given section of the deck may be subjected to very high stresses, often close to or higher than those in service.

     

• In Movable Scaffolding Systems:
  • The MSS incorporates the full formwork and support structure.
  • Each deck span is concreted directly in its final position, supported by the system and the piers.
  • The forces in the deck and columns during construction are of the same order of magnitude as in the final service stage.

     

Material efficiency and structural design implications

From a structural design perspective, Movable Scaffolding Systems typically allow the minimum consumption of concrete, reinforcement and prestressing steel. The reason is simple:

• With MSS, construction loads resemble service loads, so the deck does not need to be over-dimensioned for extreme temporary load cases.
• With ILM, the launching process causes unfavourable and often critical stress states along the deck during construction, which must be considered in design.

There are also geometric and project-level advantages that favour MSS in many cases:

• MSS allows decks with variable radii both in plan and elevation, adapting well to complex motorway or railway alignments.
• MSS solutions are independent of ground bearing capacity and relief, since they are supported on the piers rather than the ground.

   

For designers and contractors, this often translates into more flexible geometry, leaner structures and fewer constraints from site ground conditions when MSS is used.

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