Patent Application
20120036811
The present application relates to a method and apparatus for constructing overhanging or cantilever structures. In particular, but not exclusively, the invention relates to the construction of concrete bridge elements cast in situ using the free cantilever method.
Bridge decks and other spanning, cantilevering or overhanging structures are often constructed by casting concrete in situ, using a temporary structure of shuttering or formwork to define a volume into which concrete is then poured. A structure of reinforcing steel is usually assembled in the volume, or placed into the volume, before the concrete is poured. Once the concrete is sufficiently cured so that the structure can support itself, the formwork is removed.
Instead of building a full set of formwork for casting an entire concrete structure in situ, which may require an extensive arrangement of supporting scaffolding as well as a firm, free area underneath the structure, form travelers are commonly used for cantilever structures. This is of particular advantage for the construction of span structures such as bridges, which by their nature are usually situated above water or terrain inaccessible for construction work. A conventional form traveler consists of a section of formwork which can be advanced in the direction of construction while being supported by the part of the structure which has already cured. A form traveler generally comprises a frame which provides support for the formwork and some means, such as rollers or rails, enabling it to travel forwards incrementally to each new section.
Conventional concrete bridge structures may comprise, for example, a number of piers supporting a bridge deck having an open cross-section with webs (vertical load-bearing members), for example in a “double T” or a “U” arrangement, or a closed cross-section such as a box section, having a deck slab, one or more webs and a bottom slab. In a “U” section structure, the deck slab is the bottom slab.
Conventional form traveler designs include the under-slung traveler and the overhead traveler. As its name suggests, an under-slung traveler is suspended underneath the bridge structure already erected, and extends beyond the end of the structure to support the formwork where the next section of the structure is to be cast. As construction progresses, the under-slung traveler is advanced underneath the developing structure.
An overhead traveler, on the other hand, is generally a frame mounted on top of the structure already erected, and it can be advanced forwards, on rails or rollers for example, to extend over the region where the next section is to be cast. In the case of an overhead traveler, the formwork hangs from the extended section of its frame.
In both cases, the weight of the construction elements, including formwork, traveler, reinforcement and uncured concrete, together with all the necessary access gantry structures, is supported on the part of the structure which has already been built. When each section of concrete has cured sufficiently to bear its own weight and the weight of the traveler, the traveler can be advanced to the next section.
It is essential that the traveler framework is highly stable and rigid, and that the formwork does not move significantly under the weight of the concrete as it is poured, or during the curing period. To this end, conventional overhead travelers comprise a multi-truss framework with a truss frame aligned with each web element of the deck structure. The frames are transversely braced, for example using cross trusses between the frames, to give the traveler framework transverse rigidity.
In the case of under-slung travelers, the framework is located either below the wings of the deck slab or below the bottom slab. The former arrangement has the disadvantage that the reactions into the bridge deck from the static weight of the traveler, the formwork and the concrete are not introduced directly into the webs (the webs being the parts of the deck structure with the greatest load-bearing capacity). The latter arrangement, on the other hand, can only be used on a structure where the traveler's path is unobstructed by objects beneath the structure. However, under-slung travelers do have the significant advantage of allowing virtually unrestricted access to the construction space from above. This means, for example, that pre-fabricated steel reinforcement can be lowered whole into the construction space. Reinforcement steelwork cages for the entire web, bottom slab and top slab of a bridge deck, for example, can be pre-fabricated and then lowered into place by a crane on the already-constructed bridge deck. In this way, on-site reinforcement assembly work can be saved, thereby significantly speeding up the on-site construction process.
Overhead travelers, by contrast, enable the introduction of the static weight reactions directly into the webs, and do not generally suffer from the obstruction disadvantage of under-slung travelers. However, conventional overhead travelers do have the disadvantage that their braced, multi-truss frame structure significantly impedes access to much of the construction space from above. Pre-fabrication of reinforcement cages for the main webs, the bottom slab and the main part of the top slab is therefore not feasible, or is only feasible for small sections, which significantly increases the amount of on-site assembly work required before each new section can be cast.
The object of the present invention, therefore, is to provide a method and apparatus for incremental construction of overhanging or self-supporting structures, which enables the static weight reactions to be introduced directly into the webs, which is not obstructed by piers or similar elements underneath the structure, and which permits substantially unrestricted access from above to the construction space within the formwork.
The above object is achieved by the invention as set out in the accompanying claims 1 and 8.
Throughout the following description and in the accompanying drawings, the same or similar components, will be referenced using the same reference numerals for the sake of clarity.
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this invention. The drawings serve to illustrate embodiments of the present invention and, taken together with the description, serve to explain the principles of the invention. However they are not intended to limit the scope of the invention, which is defined by the accompanying claims.
A prior art overhead form traveler is depicted in schematic form in
Using the conventional overhead traveler of the prior art, the reinforcement required for each new section must be assembled in situ, since the structure of the traveler does not allow complete prefabricated reinforcement cages to be lowered down into the construction space. Such prefabricated reinforcement can also not be raised from below the bridge once the traveler has been advanced, because the traveler and the formwork obstruct access from below to the region where the reinforcement is required.
Note that, while this description has concentrated on the example in which the cantilever structure has two or more longitudinal webs, it is also possible to use the overhead traveler and method of the invention for structures which have only one longitudinal web. In such a case, the proximal ends of both load frames are secured to the same web, and the load frames are splayed outward so as to afford access to the construction volume of the next section, in the same way as for structures with more than on longitudinal web.
Whereas previous travelers comprised several relatively lightweight load frames braced together in a single structure, the load frames according to the present invention are each individually constructed to support the vertical load of the formwork and the concrete when it is poured, but also to resist any rotational or torsional forces on it due, for example, to the wind, or to non-vertical loads occasioned during the construction process. This strength is achieved, for example, by constructing each of the individual load frames as a three-dimensionally triangulated structure, as partially indicated in
When they are installed in their operational position, the load frames extend out over the next section to be constructed, but rotated at such an angle to the longitudinal axis or the structure that substantially no part of the frame is directly above the main load-bearing region (2) of the next construction section. The installion of each load frame at a splayed angle to the longitudinal axis of the bridge deck structure, and the absence of traveler components over the main load-bearing parts of the next section of the structure, mean that the reinforcement elements for these load bearing parts (webs, top slap and bottom slab) and also for the central part of the deck slab (1), or bridge deck, can be pre-fabricated and positioned (by lowering from the deck by crane, for example) in the construction volume, thereby saving significant time assembling the reinforcement in situ before pouring concrete.
The angular position of the load frames would normally be set once for each specific structure being built. For example, for the bridge depicted in
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2009/054470 | 4/15/2009 | WO | 00 | 10/11/2011 |
