Precast segmental construction is now probably the most common method of construction of long or multiple span structures particularly for roads or railways. In basic terms the superstructure is broken down into a series of elemental segments - usually the full width of a structural element over a partial length. Precasting of bridge decks involves an industrialised process which allows the mass production of standardised components.
Although relatively simple in concept; akin to building structures out of giant Lego blocks, the process requires careful thought, considerable planning and a high level of experience and expertise to be carried out successfully and safely.
There is usually more than one solution to any given situation and the final choice will depend entirely on the actual situation on site and any other prevailing circumstances such as availability of equipment and expertise.
Erection involves the lifting and positioning of the segments such that they can be joined using post-tensioning to form the final structure. Precast segmental bridges are classified by the way the segments are erected, since the erection method determines the segmentation and the prestressing layout. When cantilever construction is used, the segments are erected in balanced cantilever starting from a pier by placing segments on either side in a symmetrical operation. This method requires an equal number of segments cantilevering from the piers. For span by span erection, all segments for one span are placed on a temporary support truss or ground supports, are aligned, joined and longitudinally prestressed together in one operation to make a complete span.
Internal or external post-tensioning, or a combination of both, is used to stress the precast segments together. Internal post-tensioning runs and is anchored within the concrete shell elements such as the top slab, bottom slab or webs.
On the other hand, external tendons are exposed inside the segment box. At anchorage and deviator locations, heavily-reinforced concrete elements such as diaphragms and deviator blocks are required.
Depending on the design, epoxy glue may or may not be applied to the segment joints. Decks designed with internal post-tensioning will require epoxy glue, while those with external post-tensioning usually do not unless there is a particular concern relating to water ingress in the joint area. The epoxy serves two main purposes: during erection it acts as a lubricant that helps to join the segments, and in the final stage it seals the joint against water ingress from the outside and is therefore a corrosion protection barrier. When internal post-tensioning is used, the epoxy glue and the grout in the ducts are the only protection of the strand against corrosion. For external post-tensioning the epoxy is not required since the whole tendon is encapsulated in an HDPE pipe.
With epoxy glued joints the connection of the segments is achieved using temporary high strength bars which ensure that the joint has been fully closed and fitted and that a minimum compressive stress has been applied across the face during setting time of the epoxy. These bars are usually omitted when dry joints are used, and the segments are pulled together by the permanent post-tensioning.
Span by span structures are usually designed to be erected as a series of simply-supported isostatic spans which may or may not be connected at a later stage.
In some cases structural continuity over the supports is achieved during erection, and each new span is erected as a continuation or end span of the previously completed part of the bridge deck. During erection, the task is to assemble the individual segments and connect them to form a full span unit which can then be post-tensioned into a structurally-stable element. This can then be placed on or joined to the supports. There are several different methods of carrying out this task and these can be divided into three main categories: erection by overhead launching girder, erection by underslung launching girder and erection by crane on falsework.
Erection by overhead launching girder is probably the most common system, and involves the use of a self-launching girder situated above the bridge deck. The girder is launched into position above the span to be erected and is usually supported on the existing deck at one end and the next pier or a bracket/temporary tower at the other. The girder is usually fitted with at least one primary winch which is used to pick up the precast segments and carry them into their approximate erection position. It is generally accepted that all of the segments should be suspended within the girder to allow it to take up its fully loaded shape before beginning the process of aligning and joining the span together. Obviously the system for lifting, hanging and adjusting the spans is critical in terms of both the safety and the erection progress, and careful thought should go into this matter. Alignment and connection of the spans usually starts from one end with each successive segment being joined to the span length. If required epoxy glue is applied to the segment joints at this stage and the joints are stressed closed with high-strength temporary stress bars. Once the full span has been completed, the post-tensioning is applied in stages and the self-weight of the span is transferred either to heavier suspension points at the ends of the launching girder or to jacks on the final support structure. After this load transfer has been effected, the individual segment hangers can be released and the span can be lowered into its final position. The launching girder can then be moved forward to the next span and repeat the process.
The benefits of this method are that the equipment is relatively self-sufficient, it is flexible in terms of segment delivery - they can be delivered from the front, below, or behind for example - and it is capable of dealing with a wide range of variable span geometries both in terms of segment size and span alignment and curvature.
There is though a limitation in terms of deck width and span length. When an overhead gantry is loaded from behind, the segment must be delivered at 90 degrees relative to its final position. The segment is then rotated into the correct orientation while hanging in the gantry, above the span. This means that space is required between gantry supports and hanger bars. Very wide segments may create a problem as the width can be as much as half of the span length and hence segments need to be double-stacked to provide sufficient space for the rotation of the last segment.
The disadvantages are that the equipment is relatively heavy and complex and consequently more expensive both to manufacture and to operate.
Segment erection by the use of underslung launching girder follows the same basic sequence as for the overhead however in this case the primary supporting structure is below the segments. The most common system used involves pairs of self launching girders which are situated on either side of the span alignment below the wings of the segments, although there have been some cases where a single self-launching girder has been used below the main soffit of the segments. The launching girders are supported either on brackets or corbels at the piers and they are assisted either by a mobile crane or a lifting gantry to handle the segments, and often to launch the support brackets from pier to pier. Segments are loaded onto mobile adjustable supports in the gantry and once all of the segments are loaded the process of aligning, gluing, if necessary, and joining takes place sequentially along the span. Once the span is comp
