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!