Formwork, Mechanisation and Site Operations in Movable Scaffolding Systems

Beyond the main steel structure, the formwork system and site operations are critical to the performance and cost-effectiveness of a Movable Scaffolding System.

Internal and external formwork: configuration and mechanisation

In box-girder decks, both internal and external formwork can be integrated into Overhead or Underslung MSS. The internal formwork configuration is governed by the internal geometry of the box, not by the MSS type, so the solutions are broadly similar.

Typical characteristics:

• Normal panel length (both internal and external): 5–6 m.
  
• Internal formwork systems are most efficient when they are hydraulically operated:
  
  • The system often consists of formwork panels 5–6 m long,
    
  • A rail system, and
    
  • A transport trolley that moves the folded internal formwork from span to span, powered hydraulically.
    

To ensure efficient operations, external and internal formwork panels are usually connected by threaded ties (e.g. Dywidag bars) passing through the deck webs.

Geometric conditions for using mechanised internal formwork

To make full use of mechanised internal formwork, some design decisions must be taken early in the bridge conception:

• The internal diaphragm typically located above the pier axis should be designed with a central opening of adequate size.
  
• This opening must allow the passage of the folded internal formwork on its transport trolley.
  
• Traditional diaphragms with narrow man-access openings are not compatible with fully mechanised formwork.
  

In theory, diaphragms can be concreted in two stages to create a larger passage opening, but:

• This usually requires extensive use of reinforcement couplers.
  
• The resulting cost is high and usually not competitive.
  

A better solution is to adjust the slab and web thicknesses near the diaphragm, allowing the diaphragm to be sized with a sufficiently large opening for the internal formwork.

Crossfall, rotation of the section and pour sequence

Deck geometry in cross-section is also important for MSS optimisation:

• Ideally, the deck cross-section should be geometrically constant, and variations in transverse slope should be obtained by rotating the whole cross-section.
  
• If the top slab rotates with the change in crossfall while the bottom slab remains horizontal, the web height becomes variable.
  
• This often forces the deck to be concreted in two stages, complicating both the formwork and the construction cycle.
  

Designing with MSS in mind means favouring simple, repeatable cross-sections and rotations.

Formwork surfaces: plywood vs steel

The most common formwork surface in MSS is:

• Phenolic plywood, typically 21 mm thick.
  

Steel formwork skins are technically possible but usually less attractive in practice:

• Rebuilding or adapting steel formwork for future projects is expensive.
  
• With plywood:
  
  • The underlying steel structure is easily reused and modified for new projects.
    
  • Plywood sheets are screwed to timber sections bolted to the steel ribs and can be replaced if the number of spans is large or the surface wears out.
    

For typical deck lengths and project sizes, phenolic plywood offers the best balance of cost, flexibility and finish quality.

Handling of reinforcement with Overhead MSS

Overhead Movable Scaffolding Systems are often equipped with electric winches that allow the transport of pre-assembled reinforcement cages or large reinforcement modules.

Some winch systems use toothed racks and toothed wheels, which:

• Ensure safe handling of loads on decks with longitudinal slopes.
  
• Reduce manual handling and increase productivity on site.
  
  

MSS as machinery under the Machinery Directive

Due to the presence of movement (launching, formwork opening/closing, internal trolley, etc.), Movable Scaffolding Systems and Formtravellers fall under the definition of machines in the Machinery Directive.

This classification implies:

• Detailed risk analyses must be carried out.
  
• The design must consider the risk hierarchy defined in the Directive.
  
• All user safety aspects (access, fall protection, emergency stops, guards, etc.) must be addressed systematically.
  

The result is equipment that is not only structurally safe, but also safe to operate.

Construction cycle, crew and launching speed

The typical operations when using an MSS include:

• Opening the formwork
  
• Launching the MSS to the next span
  
• Closing and adjusting the formwork (including cambering)
  
• Preparing for reinforcement
  
• Concreting and prestressing
  

The crew size required depends on span length, deck width and cycle time, but typically:

• An MSS requires a team of around 12–14 people to handle operations efficiently.
  

Regarding launching:

• A typical launching speed is about 10 m per hour.
  
• Higher launching speeds are technically possible, but:
  
  • The kinetic energy of the moving MSS increases with speed.
    
  • Any accidental contact with supports or obstacles leads to much higher impact forces.
    
  • The small time savings from faster launching rarely justify the increased risk.
    

In practice, controlled, moderate speed is the standard for safe MSS operations.

Pre-assembly, transport and bolted connections

During original manufacture:

• MSS steel structures are usually partially pre-assembled in the workshop.
  
• All components are marked with references to simplify correct on-site assembly.
  

For transport:

• Parts are typically designed to fit into 40’ containers or onto standard TIR truck platforms.
  

On site, proper planning of assembly and dismantling is essential:

• Correct definition of lifting points and sequences.
  
• Control of the centre of gravity in each assembly phase.
  
• Safe access for cranes and auxiliary equipment.
  

Bolted connections in MSS can be:

• Friction (slip-critical) connections with pre-stressed bolts, or
  
• Shear connections with non-pre-stressed bolts.
  

Each option has implications:

• Friction connections require:
  
  • Calibrated torque wrenches,
    
  • Strict tightening procedures,
    
  • Non-reuse of pre-stressed bolts.
    
• Shear connections are usually simpler and cheaper, and are commonly adopted when design allows it.
  
  

Design Criteria Document: the key to choosing the right MSS

To evaluate whether a given MSS is suitable for a specific deck, a Design Criteria Document is essential. This document must clearly define:

• Loads from the fresh concrete
  
• Safety factors
  
• Wind speeds:
  
  • During launching
    
  • During concreting
    
  • Under storm conditions
    
• Materials and steel grades
  
• Maximum span and deck weight
  
• Live loads and construction loads, among other parameters
  

The cost and site performance of an MSS depend heavily on these technical definitions. A well-prepared Design Criteria Document is the foundation for choosing or designing the right system for each project.

STRUKTURAS
WE MAKE IT SIMPLE!

Movable Scaffolding Systems (MSS): Types, Design Rules and Advantages

Movable Scaffolding Systems (MSS) have become one of the most efficient solutions for constructing prestressed concrete bridge and viaduct decks on piers, span by span. They combine structural efficiency with repeatable construction cycles and long equipment life.

Types of Movable Scaffolding Systems

There are two main types of MSS:

1. Overhead Movable Scaffolding Systems
   
   • The main steel structure is located above the deck.
     
   • The formwork is suspended from this upper structure.
     
     
2. Underslung Movable Scaffolding Systems
   
   • The main structure is located under the deck.
     
   • The formwork rests on the lower structure.
     
     

From the point of view of weight and cost, both types are generally equivalent when designed for the same maximum span and deck weight per meter.

Site assembly: practical differences between Overhead and Underslung MSS

The main difference between the two systems appears during on-site assembly and dismantling:

• Overhead MSS
  
  • Can typically be assembled and dismantled behind the abutments.
    
  • This can simplify logistics, especially where access under the bridge is difficult (rivers, railways, deep valleys, congested traffic).
    
    
• Underslung MSS
  
  • Is usually assembled between the abutment and the first (or last) pier.
    
  • Dismantling is typically done between the abutment and the first/last pier as well, unless a phased construction of the abutment allows part of the MSS to pass through it.
    

These aspects are crucial when planning crane operations, temporary works, and site sequencing.

Structural performance and deformability limits

In the structural design of MSS, deformations are carefully controlled:

• For the overall MSS span, a maximum total deformation of L/400 is usually adopted, where L is the span of the MSS (axis-to-axis distance of its supports).
  
• For local elements, such as individual formwork profiles, a typical limit is L/250 for the relevant element span.
  

This control ensures that the final deck geometry (including camber and alignment) stays within strict tolerances and that formwork reactions are close to those assumed in design.

Design codes and fabrication standards

The steel structure of a Movable Scaffolding System is usually designed and manufactured under well-defined standards:

• Eurocode is used for the assessment of structural safety.
  
• EN 1090 governs the quality control of steel fabrication.
  

In practice, MSS structures are also:

• Modelled in 3D or represented in detailed 2D drawings.
  
• Supplied with precise weights for all components, essential for planning lifting and assembly operations safely.
  
  

General advantages of MSS solutions

Key advantages of Movable Scaffolding Systems include:

• Optimized material consumption in the bridge deck (concrete, reinforcement and prestressing) due to favorable construction load conditions.
  
• Environmental benefits: MSS equipment typically has a useful life exceeding 50 years, making it reusable on many projects.
  
• Low operating energy consumption compared to many alternative heavy construction methods.
  
• Independence from ground bearing capacity and terrain relief, since the system is supported by the piers.
  

These features explain why MSS is often the preferred solution for repetitive spans on viaducts and long elevated structures.

Longitudinal slopes and safety in MSS launching

In theoretical terms, there is no strict limit on longitudinal slope for decks built with MSS. The longitudinal launching system is:

• Hydraulically driven, and
  
• Equipped with a mechanical brake, typically a pin system, that holds the MSS safely in place when the drive cylinder is retracted.
  

In addition, the hydraulic cylinders used for MSS launching are usually equipped with safety valves:

• In case of a hydraulic pipe rupture, these valves block the oil inside the cylinder.
  
• This stops unintentional movement and prevents uncontrolled displacement of the MSS.
  

These safety and control systems allow MSS to be used safely even on decks with significant longitudinal slopes.

STRUKTURAS
WE MAKE IT SIMPLE!

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.

STRUKTURAS
WE MAKE IT SIMPLE!

Construction Methods for Prestressed Concrete Bridge Decks and the Role of Formtravellers

The construction of prestressed reinforced concrete bridge decks and viaducts can generally be divided into two large families:

• In situ concrete bridges, where the deck is cast on site.
  
• Prefabricated bridges, where large structural elements are manufactured in a factory environment and later assembled on site.
  

Within the in situ family, several types of equipment are commonly used for the construction of prestressed concrete decks. Today, the main ones are:

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

Among these, pier-supported falsework plays a key role when ground access is difficult (rivers, valleys, railways) or when ground-supported systems would be uneconomical or unsafe.

Pier-supported falsework: Formtravellers and Movable Scaffolding Systems

The most widely used pier-supported systems are:

• Formtravellers
  
• Movable Scaffolding Systems
  

These two families serve different structural solutions:

• Formtravellers are used to construct bridge and viaduct decks by the cantilever method. The deck is built in successive segments projecting from a pier, usually in symmetric pairs, one on each side of the pier axis.
  
• Movable Scaffolding Systems are used for bridge and viaduct decks designed as concrete beams supported on columns, where spans are cast in situ with support on the piers.
  

In both cases, the deck is constructed in sections:

• With Formtravellers, the deck is built in segments, typically up to 5 m long. In some special cases, segment length can be increased up to 10 m.
  
• With Formtraveller Systems, the deck is also constructed in sections, but typically one full span at a time. Concreting joints are usually placed at approximately 1/5 of the span length, where the bending moment is close to zero.
  
  

Types of Formtraveller 

Formtraveller have evolved into several specialised configurations to respond to different types of structures:

1. Overhead Formtraveller
   
   • The main structure is located above the deck.
     
   • The formwork is suspended from this upper structure.
     
2. Underslung Formtraveller
   
   • The main structure is located under the deck.
     
   • The formwork rests on the lower structure.
     
     
3. Arch Formtraveller
   
   • A specialised solution developed to allow the casting of concrete arch segments.
     
     
4. Wing Formtraveller
   
   • Typically used to cast deck wings, either for composite decks or for fully concrete decks.
     
     

Advantages of Formtraveller 

Formtraveller offer several important advantages for the construction of bridges and viaducts:

• They allow large spans to be cast in situ by the cantilever method, segment by segment.
  
• Segments are usually cast in symmetric pairs from each column, which balances cantilever moments and is structurally efficient.
  
• They are environmentally friendly, as their useful life can exceed 50 years, allowing reuse on many projects.
  
• Their energy consumption during operation is generally low compared to alternative heavy construction methods.
  
  

Overhead vs Underslung Formtraveller: how to choose?

• For the same segment length and segment weight, the Overhead Formtraveller System is usually lighter and cheaper than the Underslung solution.
  
  

This makes the overhead configuration particularly attractive.

STRUKTURAS
WE MAKE IT SIMPLE!

The Strukturas rental Underslung MSS Randselva was dismantled autumn 2021 in Norway and sent directly to Germany for the demolition of the Krondorf Bridge on the A3 south of Nürnberg.

The northbound deck is being demolished as the traffic is running on the southbound deck. After domolished deck and new construction on the north bond deck, the Strukturas rental Underslung MSS will be used to demolish and build also the south bond deck.