Structural Design, Geometry and Safety of Formtraveller 

Underslung Form Traveller Ariari

Structural Design, Geometry and Safety of Formtraveller 

Formtraveller are complex steel structures that support fresh concrete, reinforcement, construction live loads and self-weight during both casting and launching stages. Their structural design must comply with strict criteria for deformation, safety and geometric compatibility with the bridge deck.

 

Deformation limits and design codes

During the design stage, maximum deformability is carefully controlled:

  • For the main structure of the Formtraveller, a maximum global deformation of L/400 is normally considered, where L is the span of the main structure.
  • For local elements, such as individual formwork profiles, the usual limit is L/250 for the span of the profile in question.

These limits ensure proper deck geometry, correct camber and acceptable stress distributions in both the temporary equipment and the permanent structure.

Regarding design standards:

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

Connections, anchorage and checks on the permanent structure

Formtravellers are connected to the concrete deck using anchors made of threaded bars, which pass through blockouts in the bottom and top slabs. These anchors transmit:

  • Vertical loads
  • Stability-related forces during both casting and launching

Before casting a segment, it is essential that the reactions from the Formtraveller (in both casting and launching configurations) are submitted to the bridge designer. The designer must:

  • Verify the columns and
  • Check the deck section
     for all relevant construction-stage load combinations.

These checks are critical, since cantilever construction can produce more severe stresses in some sections during construction than in the final service condition.

Geometric conditions: diaphragms, web spacing and internal formwork

The geometric conception of the deck has a strong influence on the practicality of using Formtraveller, particularly when internal formwork is present.

Key aspects include:

  • Position of diaphragms
    • To allow mechanised internal formwork to be launched from segment to segment, the internal diaphragm should be placed at the rear of the segment. Same is applied to the external diaphragms and external formwork.
    • If diaphragms are located near the front of the segment, the internal formwork behind them cannot be launched forward and must be dismantled or lowered, which is inefficient and time-consuming.
    • Concreting diaphragms in two stages to create a passage for the internal formwork may be possible in some cases, but this is a decision for the bridge designer and may complicate reinforcement arrangement.

  • Distance between deck webs
    • For optimal use of Formtraveller formwork, it is very important that the distance between the deck webs remains constant along the deck.
    • Constant web spacing facilitates repetitive, standardised internal and external formwork and simplifies tie arrangements.

Longitudinal slope and launching safety

From a theoretical point of view, there is no absolute limit on longitudinal slope for decks built with Formtraveller Systems. However:

  • When the deck longitudinal slope is large, some modifications are needed:
    • The Formtraveller must be able to stay horizontal during casting, to ensure correct concrete distribution and work conditions.
    • During launching, the system must be able to follow the deck slope.
  • A braking device must be included, ensuring that the Formtraveller remains safely in position on top of the rails during launching, especially on significant slopes.

The hydraulic cylinders used for longitudinal launching typically include a safety valve:

  • In the event of a hydraulic pipe rupture, this valve blocks the oil inside the cylinder.
  • This instantly stops the movement and prevents uncontrolled displacement of the Formtraveller.

Classification under the Machinery Directive

Because Formtraveller involve movement (launching, formwork opening/closing, internal formwork repositioning), they fall under the definition of machinery in the Machinery Directive.

This classification requires:

  • Detailed risk analyses, covering all operating modes.
  • Application of the risk reduction hierarchy defined in the Directive.
  • Design solutions that address user safety in a systematic way (access, fall protection, guarding, emergency stopping, etc.).

In practice, a compliant Formtraveller is not only structurally sound, but also engineered as a machine with safety-integrated design.

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If you have any questions or would like to discuss the possibilities for your bridge project, please get in touch with your local agent or our head office in Norway: CONTACTS  

Formwork, Operations and Logistics in Formtraveller

Formwork, Operations and Logistics in Formtraveller 

The efficiency of a Formtraveller depends heavily on how the formwork is designed and handled, and how site operations and logistics are planned. This includes panel dimensions, formwork surfaces, internal formwork launching, crew size, launching speed and workshop pre-assembly.

 

Internal and external formwork: configuration and handling

In typical applications, the external and internal formwork panels have a length of about 5.8 m, assuming a maximum segment length of 5 m in the concrete deck.

Internal and external panels are connected using threaded ties, such as Dywidag bars, passing through the deck webs. This creates a closed, stable formwork box that can carry fresh concrete and construction loads.

The internal formwork in Formtravellers is usually launched manually from one segment to the next:

  • Chain tackles are commonly used to move the internal formwork along rails.
  • Hydraulic cylinders can also be adopted, but the standard solution in many projects remains manual.
  • The rails on which the internal formwork rests are launched together with the main structure of the Formtraveller.

As mentioned earlier, the position of internal diaphragms must allow the internal formwork to pass. If diaphragms are too close to the front of the segment, the internal formwork may need to be dismantled behind the diaphragm.

 

Formwork surfaces and materials

The most common formwork surface used in Formtraveller Systems is:

  • Phenolic plywood, usually 21 mm thick.

Steel formwork surfaces are technically possible but rarely preferred:

  • Rebuilding steel formwork for future projects is costly.
  • For typical segment lengths and project sizes, plywood offers a better economic balance.

The usual arrangement is:

  • Plywood sheets are screwed to timber sections.
  • Timber sections are bolted to the steel ribs of the formwork structure.
  • Parts of the internal and external web formwork are often made using wooden beams.

If the number of segments is very high, the phenolic plywood can be replaced during the life of the Formtraveller, while the underlying steel structure remains reusable.

 

Handling reinforcement: limits and special solutions

As a rule:

  • Overhead Formtraveller do not normally allow the transport of pre-assembled reinforcement cages on the system itself.

However, in some specific projects where:

  • The deck webs are vertical or nearly vertical, and
  • Pre-assembled web rebar panels are desired,

it may be possible to develop special transport rails integrated into the Formtraveller. In such cases:

  • A crane installs the pre-assembled web reinforcement panel.
  • The load is then transferred from the crane to the special rails of the Formtraveller.

These are project-specific solutions, not the standard configuration.

 

Crew size, launching speed and construction cycle

To operate a Formtraveller– including:

  • Opening the formwork
  • Launching from one segment to the next
  • Closing the formwork
  • Introducing camber
  • Preparing the formwork for reinforcement
  • Concreting the segment

– a dedicated crew is required. 

The exact crew size depends on the segment weight, deck width and the target construction cycle, but in general:

  • A team of 8 to 10 people per pair of Formtravellers is typical.

Regarding relocation speed:

  • The normal launching speed of a Formtraveller is around 10 m per hour.
  • Increasing the launching speed does not make practical sense, since the maximum segment length is only about 5 m. The marginal time savings are minimal, and the risks associated with higher movement speeds increase.

The typical construction cycle for a pair of segments using Formtravellers is approximately:

  • 1 week per pair of segments,
     assuming standard times for reinforcement assembly, prestressing operations, concrete curing, and formwork operations.

Workshop pre-assembly, transport and on-site planning

During the original manufacture of a Formtraveller System:

  • The steel structure is usually partially pre-assembled in the workshop.
  • All components are marked with references, indicating their position in the final assembly.

For transport:

  • Parts are designed to fit in 40-foot containers or on TIR truck platforms.

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

  • Correct definition of lifting points for sub-assemblies.
  • Ensuring that the centre of gravity of each assembly stage remains within safe limits.
  • Guaranteeing suitable access for cranes and transport equipment.

Accurate weights of all parts must be known:

  • Typically derived from 3D models or detailed 2D drawings.
  • If there is any uncertainty, scales can be used to confirm the component weights.

Design Criteria Document: selecting the right Formtraveller 

To evaluate whether a given Formtraveller is suitable for a specific bridge deck, it is essential to prepare a Design Criteria Document. This document must clearly define at least:

  • Loads from the concrete segments and launching situation
  • Safety factors
  • Wind speeds:
    • During launching
    • During concreting
    • Under storm conditions
  • Materials and steel grades
  • Maximum span
  • Live loads and other construction loads

The cost and site performance of a Formtraveller depend strongly on these definitions. A well-prepared Design Criteria Document is therefore the key technical basis for:

  • Deciding whether an existing system can be reused or adapted, or
  • Designing a new system optimised for the specific project.

In most successful cantilever projects, the Formtraveller is not an afterthought but an integral part of the initial design strategy for the bridge.

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