Frequently asked questions
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FAQ
Frequently asked questions
This FAQ provides a comprehensive overview of prestressed concrete bridge deck and viaduct construction, with a strong focus on the equipment, methods, and technical considerations involved—particularly the use of formtravellers and movable scaffolding systems. It covers construction techniques, types of equipment, structural solutions, design standards, operational procedures, and safety requirements. Additionally, it addresses practical aspects such as segment lengths, formwork systems, team requirements, logistics, and planning, making it a valuable resource for understanding both the engineering principles and on-site execution of modern bridge construction.
What are the general methods of construction of prestressed reinforced concrete bridge decks and viaducts that currently exist?
The general methods of construction of prestressed reinforced concrete bridge decks and viaducts that currently exist can be classified into two broad groups, in situ concrete bridges and prefabricated bridges.
What is the most used prestressed reinforced concrete bridge deck construction equipment today?
The most used prestressed reinforced concrete bridge deck construction equipment today is Ground-supported falsework, Pier-supported falsework and Incremental Launching.
What is the most used Pier-supported falsework today?
The main groups of Pier-supported falsework in common use today are the Formtravellers and the Movable Scaffolding Systems.
What type of structural solution for bridges and viaducts are these types of Pier-supported falsework used in?
Formtravellers are used to construct bridge and viaduct decks by the cantilever method, while Movable Scaffolding Systems are used in the construction of bridge and viaduct decks designed as concrete beams supported on the columns.
What is the maximum casting length used with the Formtravellers and the Movable Scaffolding Systems?
When Formtravellers are used, the deck is built by sections usually called segments, whose maximum length is usually 5m, however in some cases it can be higher, sometimes reaching 10m. When the Movable Scaffolding Systems are used, the deck is also built in sections, but in this case with the length of a span, positioning the concreting joints approximately 1/5 of the length of the span, a position that corresponds to approximately zero values of the bending moment.
What types of Formtraveller are there?
There are four types of Formtraveller, the Overhead, the Underslung, the Arch and the Wing. The Overhead Formtraveller include a main structure located above the deck from which a formwork is suspended; while the Lower Formtraveller include a main structure located under the deck, on which the formwork rests. The Arch Formtraveller is a special solution developed to allow to cast concrete arch segments. The Wing Formtraveller is usually used to cast the deck wings either at composite decks or at concrete ones.
What general advantages can we attribute to a Formtraveller?
The main advantages that we can attribute to the Formtraveller are the fact that cast in situ big spans can be built by the cantilever method, that consist of casting a pair of segments from a column usually in a symmetrical way, they allow to minimize the consumption of materials used in the design and construction of the bridges and viaducts build by the cantilever method, they are environmentally friendly because their useful life can exceed 50 years and they have a low energy consumption.
Is there any determining factor that helps to choose between an Overhead and a Underslung Formtraveller?
From the point of view of their weight and price, the Overhead is usually lighter and cheaper for the same segment weight and length.
Are both Formtraveller solutions equipped with formwork adjustment devices, including cambering adjustment?
Yes, in both cases the adjustment of the formwork including the cambering adjustment is possible using the devices included in the Formtraveller.
What is the maximum limit of deformability considered during the structural design stage of a Formtraveller?
Normally, a maximum general deformation of L/400 is considered, where L is the span of the Formtraveller main structure. For local elements such as formwork profiles, this value usually corresponds to the span of the profile in question divided by 250.
What are the main codes used in the design and manufacture of the steel structure of a Formtraveller?
Normally the Eurocode is used in the assessment of structural safety and EN1090 is used in the quality control of the manufacture of the steel structure.
Is the internal formwork of the Formtraveller mechanized, allowing its repositioning from span to span using mechanical means or is the operation fundamentally manual?
The internal formwork of the Formtraveller is launched from one segment to the next, usually by manual means, using chain tackles. Hydraulic cylinders can also be used but the standard solution is manual. The rails on which the internal formwork rests are launched together with the main structure of the Formtraveller.
What is the normal length of the external and internal formwork panels?
Normally the length of these panels is 5,8m assuming that maximum segment length at concrete deck is 5m.
Are the internal and external formwork panels connected to each other in both types of Formtraveller?
Yes, the panels are connected to each other by threaded ties like Dywidag bars passing through the deck webs.
How are the Formtravellers connected to the concrete deck?
Formtravellers are connected to the concrete deck by anchors made by threaded bars, passing through the bottom and top slab blockouts.
Before the segment is casted is it needed to check the bridge columns and deck for the construction stage?
Yes, Formtraveller weight and reactions transmitted to the deck during casting and launching stages must be submitted to the bridge designer that will use this data to make the necessary checks
Is there any special care to be taken regarding the conception and design of the concrete deck to make it possible to use a mechanized internal formwork?
Yes. It is essential that the internal diaphragm in case existing is placed at the rear of the segment allowing the internal formwork to be launch. In case internal diaphragm inside the concrete deck are located close to the front of the segment it will oblige to dismantle the internal formwork placed behind the diaphragm since it can’t be launched
Is it possible to concrete the diaphragm in two stages, allowing the internal formwork to pass through it folded over the transport trolley?
Probably yes but this is a decision to be taken by the bridge designer.
Is there any geometric factor to consider during the conception and design stage of the bridges and viaducts concrete decks so that the Formtraveller formwork can be used optimally?
Yes. The main factor to consider has to do with the distance in between the deck webs. It’s very important that this distance is constant along the deck
Is there any limit in terms of longitudinal slope for the decks to be built with Formtraveller?
In theoretical terms there is no such limit. When slope is big then some Formtraveller modifications must be implemented allowing it to be at horizontal position during casting and follow the deck longitudinal slope during launching. A breaking device must be included to keep Formtraveller safe on top of the rails during launching stage
Is there a safety system in the hydraulic cylinders of the longitudinal launching system that blocks the movement in case of a rupture of a hydraulic pipe?
Yes, this safety system normally exists and is formed by a valve which stops the oil inside the cylinder in case of a hydraulic pipe rupture.
What is the most common type of formwork surface used in the Formtraveller formwork?
The most common type is the one that includes phenolic plywood sheets, usually 21mm thick. Steel formwork solutions are also possible, but usually little used because the cost of their future rebuilt for other projects is very high. The solution that uses the phenolic plywood is more economical for the usual lengths of deck because it includes a steel structure that is easy to rebuild to future projects. Plywood is usually attached to the steel structure of the formwork using wood screws that attach the plywood sheets to the wooden sections bolted to the steel ribs. If the number of segments to be casted is very high, phenolic plywood can always be replaced during the use of the Formtraveller. Part of the internal and external web formwork panels are also usually made by wooden beams.
Do the Overhead Formtraveller allow the transport of the deck pre-assembled reinforcement cages?
Normally not! At some projects when the deck webs are vertical or close to it it’s possible to add to the Formtraveller some special transport rails that will allow to install web rebar preassembled panels using a crane and then transferring this panel load to the Formtraveller specials transport rails.
Can a Formtraveller be classified as a machine under the Machinery Directive?
The definition of machine included in the Machinery Directive requires that this type of equipment for the construction of bridge decks and viaducts be classified as such because its use involves movement. The classification of this equipment as machinery within the scope of this Directive requires detailed risk analyses to be carried out that consider the recommendations of the Directive in terms of risk to users, requiring that in the design of the machine the solutions found consider the hierarchy defined in the Directive for risk mitigation or elimination.
To operate a Formtraveller including opening its formwork, launching from one segment to the next, closing the formwork, introducing the cambering, preparing the formwork to receive the reinforcement and casting, what team do I need?
The size of this team depends on the weight of the segment, the width of the deck and the cycle defined for the construction of a pair of segments; but in general terms this team includes between 8 and 10 people for the Formtraveller pair.
What is the normal launching speed of a Formtraveller during its relocation from one segment to the next?
Normally the launching speed of a Formtraveller is approximately 10m per hour.
Is it possible to increase the Formtraveller launching speed?
Doesn’t make sense to try to increase the launching speed since usually max segment length is only 5m.
What is the typical cycle for the construction of a deck pair of segments when using a Formtraveller?
The cycle depends on several factors, namely the time needed to assemble the reinforcement, the awaiting time needed to apply the prestressing, the segment weight, the area of the formwork, but in general terms the cycle is 1 week.
Is it normal to do load tests on the Formtraveller?
This is not normal because it is not easy to reproduce in practice the load conditions considered in the design of the Formtraveller steel structure, which correspond to the concreting and longitudinal launching stages.
Is the steel structure of a Formtraveller pre-assembled in steel workshop during its original manufacture?
Yes, normally during the manufacture of the steel structure of a Formtraveller, partial pre-assembly is carried out to ensure that the assembly takes place without difficulties on site. During the pre-assembly phase in workshop, it is normal to mark all the components of the steel structure with references that allow their position within the assembly to be identified.
Can the parts of a Formtraveller be loaded into containers?
Yes, normally the design of a Formtraveller considers the need to transport the different parts in 40′ containers or on TIR truck platforms.
Should the assembly of a Formtraveller on site be planned?
Yes, planning is essential to ensure that the lifting points of the sub-assemblies are the right ones, that during the different assembly stages the position of the assembly’s center of gravity is never violated and to ensure correct access to the lifting and transport auxiliary equipments.
Should the detailed weights of the parts of a Formtraveller be known rigorously?
Yes, it is important to know exactly the weights to be lifted with cranes to be able to plan the assembly and dismantling operations safely. Usually, this type of equipment is modelled in 3D software or represented in 2D drawings with indication of the detailed weights of the parts. If there are doubts about any of the weights, scales can be used to confirm the weights in question.
How to evaluate if a given Formtraveller is suitable to build a given deck?
When preparing a quote or starting a design for a Formtraveller, whether it is the original or a subsequent rebuild, it is essential to prepare a Design Criteria Document, which must unequivocally define the loads resulting from the concrete of the deck segments and launching of the Formtaveller, the safety factors, the wind speeds during launching, during concreting and under storm conditions, the materials, the maximum span, the live loads, etc, etc.
The Design Criteria Document is an essential piece of information since Formtraveller System cost and site performance deeply depends on these technical definitions.
What type of bolted connections are used at the Formtraveller?
The type of bolted connection are usually non-prestressed shear connections and bolts.
