The 5 main construction types for building concrete bridge decks
Bridges and viaducts are the infrastructure par excellence of civil works. In fact, they enable transport connections (whether vehicles, rail or pedestrian) between two locations in urban or extra-urban areas.
There are many types, categorized according to the material used, length, shape, height and many other variables.
In this article, we will analyze those bridges that have an in-situ concrete deck: We will see which are the most common types and how to proceed with their formwork.
First, how can concrete bridge decks be categorized?
Classification by geometry
Concrete bridge decks can be categorized by geometry according to 5 main types:
Straight bridges: These are all those bridge and viaduct decks that are straight and with no big slopes. This typology is geometrically the simplest to solve at the formwork level, although it is necessary to analyze the section type (see next paragraph): the constant section does not present particular complications, while a variable section must be studied carefully.
Bridges with curved geometry: There are some further complications in the case of curved geometry, or when the deck of the bridge or viaduct has a certain curvature and, therefore, a certain camber. In this case, a more in-depth study will be required of the formwork surface and the solution, which will have different heights in different directions.
Arch bridges: The most complex type is undoubtedly the arch bridge, where the arch has a load-bearing function for the piles and the roadway above, discharging all the forces on the foundations through the arch. This type is chosen to span long distances when support on the ground is not possible.
Suspension bridges: Suspension bridges allow the spanning of very large distances and feature piles that transmit the loads to the foundation supporting the deck by means of traction elements (tie rods or steel cables).
Classification by section
There are many sections that may be present in cast-in-place concrete bridge decks, and the choice of design may depend on many variables: design loads, span length, width, and many other factors.
Among the most commonly used types are the following sections:
Simple box section: This is the least complicated section to design and construct as the section has a simple and constant geometry. In general, the box section can provide high resistance to torsional loads, usually due to the eccentricity of loads due to vehicle movement.
Single cell box section (constant or variable section): The need to reduce the transverse dimensions of substructures (columns and lintels), which is an important factor especially for urban and extra-urban overhead structures, has led to the frequent design of box sections with a reduced core in relation to the total width of the deck section.
Lightened gull wing section (constant or variable): The lightened gull-wing section (generally with porexpan cylinders) allows for a reduction in thickness while maintaining the resistance to the main loads.
T-beam section: The open T-beam section is the simplest section from a structural point of view but also the one with the lowest performance from the point of view of structural weight and torsional strength.
The superstructure or deck can be braced using different methods depending on the characteristics of the deck itself and the underlying orographic situation.
There are 5 main types of concrete bridge deck formwork construction. Let’s look at them in detail:
- Continuous formwork
- Porticoed formwork
- Advancing carriage
- In-situ prefabricated beams
- Beam launcher
Continuous formwork is a structure formed by towers of multi-directional elements braced together, which have the function of transferring the loads acting on the ground to the ground, i.e. the weight itself, the weight of the formwork and the weight of the concrete.
This system allows significant heights to be reached by supporting the deck formwork at the required height. The deck formwork is typically composed of a double framework of metal and timber beams covered with a phenolic mantle (see last paragraph).
Continuous formwork can be optimal in the following conditions:
- Heights up to 20 meters;
- Possibility of support on the ground;
- Low number of spans.
Porticoed formwork is a version of the previous one that allows the spanning of distances up to 20 meters without support on the ground. It is used in cases where it is necessary to ensure the passage of vehicles during the construction phase or in cases where it is necessary to overcome an obstacle or a watercourse.
The gantry is created by using different metal profiles depending on the span length. For spans of up to 12 meters, it is possible to use HEB-400 profiles supported on HEB-300 and HEB-140 profiles.
For larger spans, it is necessary to use lattice girders supported on high-capacity towers (no longer formed by multi-directional scaffolding but by metal profiles).
Porticoed formwork can be optimal in the following conditions:
- Heights over 30 meters
- Crossing a watercourse or road to ensure the passage of vehicles, not exceeding 20 meters in length.
- When there are many spans to be covered, there is a gain in terms of performance, compared to moving (with assembly and disassembly) the entire formwork volume used in each span.
The advancing carriage system consists of movable metal structures that support the cantilevered formwork, concreting symmetrically on both sides of the piles. These structures allow formwork to be erected, stripped and moved using jacks supported on the newly concreted bridge span (or, in some cases, by means of large cranes).
The construction of bridges using this method is considered during the design phase, since it is necessary to determine the advance length, advance angle (the weight of the bracket creates a deflection that must be considered so as to connect with the rest of the bridge), length of the supporting section and the necessary strength of the concrete, among many other calculations.
The process starts at the pile, cantilevering on both sides at the same time so as to maintain balance when concreting. After the first concreting, the steel structures are assembled on each side, resting on the already concreted section with 3-5 meters of overhang.
The formwork project must be studied for each concreting since they are designed to be adaptable to several sections.
The advancing carriage can be optimal in the following conditions:
- Heights greater than 20 meters;
- No possibility of support on the ground;
- Arch bridges.
In-situ prefabricated beams
One option that can be used in some cases is to prefabricate the deck beams in situ, so that they only need to be placed with the crane at a height on the columns, and then the topping slab is placed. This process avoids the transport of prefabricated beams from the factory, an operation that could be very complex due to their length, and speeds up the construction process compared to a non-prefabricated solution.
On the other hand, it is still necessary to study a formwork solution for the subsequent concreting of the topping slab, and furthermore, this solution does not allow for excessive curvatures and/or inclinations.
The system with in-situ prefabricated beams can be optimal in the following conditions:
- Bridges / straight viaducts
- Short construction times
- T-section design.
The beam launcher is a machine that should be treated as such rather than as a formwork system. It consists of a support structure formed by a huge longitudinal beam which acts as a support structure and as a guide for the movable transverse structure that supports the formwork.
It can be placed on top of the deck (in this case the formwork is hung) or under the deck (in this case the formwork is supported). The machinery is driven by hydraulic cylinders, which move from the freshly concreted section to the next pile, thus resting on 2 consecutive piles and on the freshly concreted section; therefore, its length has to be at least twice the length of the section.
It is a slow assembly system (between 1 and 2 months, depending on the length) and costly, so it is justified only in very long bridges (minimum 7 or 8 spans) and/or heights such that formwork on the ground would be too expensive (above 20 meters), and also when the orography is not favorable for placing formwork.
A variant of this system is the beam and segment launcher in which the longitudinal structure is very similar, but instead of moving the formwork structure over it, prefabricated elements are moved.
The beam launcher can be optimal in the following conditions:
- Large number of spans
- Significant heights from the ground
- Bridges / straight viaducts with constant section.
The deck formwork is formed by a double span of beams, one of wood and one of steel, resting on the underlying structure. It can be continuous formwork, porticoed formwork, or another type of formwork.
The deck coating in contact with the concrete consists only of wood panels or, more often, phenolic board panels.
While the creation of a solid slab requires only one concreting operation, the box section requires at least 2.
The construction of bridges and viaducts would require much more investigation, which due to space limitations it is not possible to address in this article.
We will definitely do this in other articles. Follow us to find out more!