The principle of the incrementally launched bridge consists of building the superstructure segments in a casting yard located behind the bridge abutment. Each segment is matchcast against the previous one and prestressed to the section of superstructure already built. The entire superstructure is then jacked forward a distance equal to the length of this segment. This process is repeated until the bridge is in its final position. The secondary PT is then installed and the temporary bearings are replaced by the permanent bearings. This form of construction can be used for bridges having constant cross-sectional shape throughout their length. The bridge should be straight or have a constant horizontal and vertical curvature.
This method, following a suitable technological design with aspects of the ILM method projected to the superstructure design, brings a wide range of advantages and is very effective compared to other methods, despite generally higher material demands. This is virtually a “factory” for bridge construction in segments with advantages like maximum productivity, reinforcement assemblages along the whole length of segment, minimum formwork and others.
Having considerable experiences in this technology, MK4 Canada offers and successfully provides the optimisation of the load bearing structure design and stabilisation of piers against effects of horizontal forces, if required, as well as the design of technological equipment for launching and casting yard, in cooperation with the designer and contractor and according to the scope of required works. Particularly the casting yard, with respect to the control of the load bearing structure’s geometry with minimum tolerances, and safe launching device are crucial for successful realisation of launching. The MK4´s concept of launching based on flexible casting yard and hydraulic strand units maximally reduces the risks associated with this method.
The scope of works offered by MK4, besides the participation in design, includes prestressing works, delivery of special sliding pot bearings convertible to permanent ones, delivery and operation of the launching equipemnt and additional hydraulics related, and lifting works necessary for replacement or conversion of sliding bearings.
The incremental launching method is applied mainly to structures with high piers or where the formwork on solid centring cannot be used (above the existing build-up area, communications, water areas, in ecologically sensitive areas, etc.)
For the launching operation special “friction system” using hydraulic lift-push jacks is used. Alternatively, the launching can be done by means of hydraulic pulling jacks using strand or bar temporary tendons. A steel nose is installed on concrete viaducts or a temporary tower with stays on metallic decks.
TYPICAL SCOPE OF WORK (CONSTRUCTION PACKAGE) OFFERED BY MK4 INCLUDES:
- Construction design / alternative design.
- Supply and installation of temporary/permanent post tensioning.
- Supply and erection of special formwork and launching steel-nose.
- Supply of bearings (temporary/permanent).
- Launching operations and equipment.
- Supply of special sliding pads and devices.
- Bar prestessing for steel-nose and lateral guides attachment to the structure.
