Bridging by Segmental Box Girder
SEGMENTAL BOX GIRDERS SYSTEM
Segmental box girders (segments) system is used for building superstructure for bridges / other structure in replacement of conventional construction via pre-cast beams and cast-in-situ decks. Segments will be pre-cast at another location preferably at outskirts of city. New setup of casting shall be done specifically to carry out the segments casting works. Segments are cast using specific moulds as the shape and dimension are also specific. Moulds may be made out of cut and weld steel sections and other accessories to complete the moulds. The cast segments later shall be brought to site and erected at specific location via launching girder system (or other method) by continue joining segments to each other to form the completed decks in between of two piers. Post tensioning in form of stressed groups of strand wires shall keep the segments together and provide the strength required to support the required loading on the deck. Further works can then be carried out as per requirement prior opening to users.
Advantages of using segmental box girders system (comparing to conventional method)
The segments system reduces the environmental disturbance compare to the conventional method by carrying out the concreting works further away from the construction site where is usually located at city centers.
Quality can be maintained since the casting of segments shall be carried out at control environment as compared to carry out at site.
Aesthetic of using segments system is better due specific shape can be cast if so required by clients.
Since segmental box girders are pre-cast elements, casting of segments can proceeds while piling works, pile-caps, piers, pier caps construction are proceedings at site. Therefore, shorter duration of construction is achievable.
Method of casting segments.
Segments are being cast similar to cast of any structure via moulds to specific shapes and dimensions as per the approved drawings and specifications as required by the clients. Main components of the segments are reinforcement bars and concrete. Others cast in items such as HDPE pipes (for post tensioning tendons ducting), anchorages (for anchoring the wire strands), pipes for drainage outlets, etc are embedded inside the segments for specific purposes.
For this particular project, long line casting method is used. Long beds are prepared to act as the base of segment moulds. Outer shuttering is then fixed to the bed to form the outer face of segments. Rebar cage is formed via cutting, bending and fixing of reinforcement bars. The cage is formed for particular segment shape and dimension. The cage is then lowered down to the bed within the outer shuttering. Ducts are then aligned within the cage for specific alignment. Since the segment is hollow, inner shutter / core mould is wheeled in between the outer shutter and rebar cage to form the hollow section. To form the frontal and back of segment, bulkhead (part of mould which is form from by welding of steel sections) is used. From this bulkhead, the frontal side of segment shall be shaped what is called shear keys (shall be explained in detail at later part). After all of the moulds sections (outer shuttering, inner core and bulkheads) have been properly installed, concrete shall be poured in. Vibrating and leveling of concrete then be carried out.
Segment cast is allowed to cure prior to opening of moulds section. This is depending on the strength of concrete achieved as required by specification.
Similar process is being adopted for casting of next segment in the exception that frontal side of the next segment shall be cast against the previous cast segment. This term is match casting. The shear keys act as the interlocking shapes between the two segments. Once the next segment cast and cured, it can and shall be separated from the previous segment. Further segment shall be cast against the newly cast segment. This process shall be repeated until enough segments are cast to complete the span (distance between two piers at site). The interlocking between to segment via shear keys is specific. Match casting allows two segments share one specific face since they are cast against each other.
These segments shall be stored at the storing yard and waited for site requirement. Once the site is ready, the segments shall be transported and erection operation shall begin.
Segments casting works shall proceed for next spans to completion and store for site requirements. Each segment cast is therefore identified by span location and position in that particular span. Shifting segment position is impossible since it is match cast (further shall be explained on this matter).
Why match casting?
Match casting is necessary to ensure that when segments are joined back together at site during erection they shall fit exactly. If the segment is not fit exactly, post tensioning of the span will break the segments at the joints. Post tensioning forced the segments together to act as one complete structure. If it is not match cast the intention to force the segments shall fail.
Method of erection of segment.
At site, once the pier caps and area have been made ready for the segment erection works. Launching gantry system (which have been specifically fabricated earlier) shall be installed on the completed pier caps. Launching gantry system is the combination of fabricated steel sections, high tensile lifting bars, winch system, supporting hydraulic jacks, slider beams, trolley system, auto launching hydraulic jacks, etc. This launching gantry has been fabricated, fitted and installed as per the drawings and specifications.
This particular launching gantry is limited to erection of segments for 31.62 meter span maximum.
Once launching gantry is successfully installed, segments shall be delivered to site via multi-axle trailer. Segment shall be lifted one at each time via winch and attach to launching gantry via lifting bars and bring to position the slider beam. Next segment shall be connected to the previous segment by bar and jack system to ensure the perfect fit. Epoxy glue shall be used at the joint of the two segments. Similar process shall be carried out until all segments for the particular span have been erected, connected and glued. Launching gantry shall support the whole of segments for the entire span prior to next operation.
Wire strands shall be inserted through all of the segments via tendons ducts which have been embedded in all segments in a particular alignment as per drawings and specifications. Once wire strands have been installed, hydraulic jacks shall be used to stress the strands from one end to the other. This stressing works is being carried out also as per drawings and specifications.
Once all tendons have been stressed, span (connected segments) shall be lowered down from launching gantry to the pier caps. Load of span from the launching gantry shall be transferred to the pier caps. Once the launching gantry is free, auto launching may proceed. Auto launching is the process of bringing the launching gantry to the next span (next pier caps). Launching gantry travels forward on top of the completed span to the next span.
Once launching gantry has successfully traveled to the next span, similar segment erection process shall be carried out.