Arch bridges are constructed using different methods depending on the material used (concrete or steel). The key principle is to support the arch during construction until it becomes self-supporting.
Concrete Arch Bridge Construction
Concrete arch bridges typically employ a falsework system.
- Falsework: This is a temporary structure, often made of timber or steel, that supports the weight of the concrete during the curing process.
- Forms: Forms are molds that give the concrete its shape. They are placed on the falsework.
- Casting: Concrete is poured into the forms and allowed to harden.
- Removal of Falsework: Once the concrete has gained sufficient strength, the falsework is carefully removed, leaving the self-supporting arch.
Essentially, the falsework acts as a temporary bridge beneath the arch, allowing the concrete to set and gain strength before being subjected to its own weight and external loads.
Steel Arch Bridge Construction
Steel arch bridges frequently utilize the cantilever method.
- Cantilevering: Each side of the arch is built outwards from the supports (abutments) towards the center.
- Temporary Supports: These cantilevering sections are supported by either:
- Temporary Cables: Cables anchored above the arch to provide upward support.
- Falsework: Similar to concrete arch construction, but supporting steel members instead.
- Meeting in the Middle: Construction continues until the two sides meet at the crown (center) of the arch.
- Final Connection: The two halves are then joined, making the arch self-supporting and allowing the temporary supports to be removed.
This method allows construction over large spans without the need for full ground-based support. Another technique involves assembling the arch on land and then lifting it into place using cranes or flotation methods, especially for bridges over water.
In summary, the construction of arch bridges relies on temporary support systems (falsework or cables) that are removed once the arch can support its own weight and any anticipated loads. The specific method depends on the materials and the span of the bridge.
