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 complete the post-tensioning is stressed in stages and the load of the span is transferred onto jacks at the support points. The girder is then launched out to the next span to repeat the procedure. If the geometry allows, the completed span can then be lowered on to its final supports, however this may not be possible in some cases until the launching girder has been launched completely out of the way.

The advantages of this method are its simplicity and relative lack of complex equipment. It can be used on a wide range of span lengths but is limited with regard to the curvature of the spans as the rear of the girder may be obstructed by the existing spans and the segment support point below the wing slab may get too far away from the web. As the brackets take the gantry support reactions down to the pier or even directly to the pile cap, the pier is not eccentrically loaded during deck erection. The relative simplicity of the segment support systems also tends to lead to slightly faster cycle times.

The disadvantages of the system are that it can only be used above certain levels of curvature in plan, it is also preferable that the deck supports are piers and not portals, which limit available vertical clearance between the wing slab and the portal in which the gantry and segment supports need to fit. Also this method usually requires significant support cranage for segment loading and bracket handling. However, the additional cranage can be used to support other works on site as it is not required full time by the gantry.

A final option for span by span erection, and possibly the most simple is to erect the span on falsework supported from the ground below. This is slower and more limited than the other methods but it does provide a useful solution for certain specific spans which might otherwise prove difficult or expensive to include within the scope of a launching girder. It is also a useful solution on projects where there are insufficient numbers of spans to justify the capital investment needed for a launching girder.

Erection takes place from one end of the span with each segment being progressively lifted and joined directly onto the preceding segment and then supported on a falsework tower from below. Considerable care is required to ensure that the alignment is correct during the segment erection such that the final span is in the correct position when the post-tensioning is applied and the load is transferred to the final support locations.

If this is not possible then the falsework needs to be equipped with sufficient hydraulic and mechanical sliding systems to allow the full weight of the span to be moved relative to the falsework. The advantages of this method are its simplicity and low capital cost. The disadvantages are the requirements for good ground conditions, the slow cycle times due to the need to manually relocate the falsework, and the limitations on the height of the falsework towers.

Balanced cantilever construction is generally used for longer spans, or for structures with more complex geometry. Precast segmental balanced cantilever construction involves the erection of pairs of segments on either side of a pier, by which process the contractor gradually builds up pairs of cantilevers extending from each pier - these cantilevers are then joined at mid-span to form the final bridge structure. If the bridge deck will be supported on a single line of bearings in its final state, it is necessary to temporarily 'nail' the bridge deck to the pier or to prop bridge deck with temporary support towers from the pile cap to stabilise against overturning during cantilever erection. Piers or at least foundations must be designed to be able to take associated out-of-balance moments. Here again there are three basic methods of erection: by crane, by lifting frame and by overhead launching girder.

Erection by crane is the simplest solution; site cranage is used to lift the individual segments directly to the front of the cantilever where they can be glued and temporarily stressed in place prior to the permanent post-tensioning being installed and stressed. This method has the benefit of relatively low capital investment, cranes usually being easy to rent, and high flexibility. Also it can support reasonable rates of production, for example two to three pairs of elements per shift, depending on circumstances. However this method is severely constrained by the availability of suitable cranage, the size and weight of the segments, the height of the piers and the general accessibility and ground conditions.

With the use of erection by lifting frame, the frames are placed on top of the cantilever being built, and are fitted either with winches or strand lifting units. These are used to lift the segment from a delivery point below the cantilever up to the final position. Once the segment has been lifted to the right level it can be glued and temporarily stressed to the face of the cantilever before the final post-tensioning is applied. Traditionally lifting frames were very simple structures mounted directly at the ends of the cantilevers and only able to receive segments directly below the final erection location however in recent years these systems have evolved to provide more flexibility, with some systems able to lift and carry the segments from the delivery location to the final erection location.

This system has the advantage of being fairly simple and light and thus is relatively inexpensive. The disadvantages are that the system has to be relocated from pier to pier by crane, it requires a relatively large amount of space on the pier head and even with the most flexible of the lifting frames, delivery is still required to a location very near the cantilever in question. This approach also results in fairly significant additional temporary erection loads on the ends of the cantilevers where they have the greatest effect.

Erection by overhead launching girders is generally the most impressive and mechanically-complex method of construction. It involves the use of an overhead launching girder which is capable of self-launching from pier to pier along the structure as it is built. The launching girder is usually equipped with two primary winches which are used to lift and place the pairs of segments such that they can be glued and then attached to the final cantilever with temporary and then permanent post-tensioning.

Initially relatively simple machines, modern balanced cantilever launching girders have been developed so that they are now capable of erecting a large range of structures including very long spans, tight radius curves, multiple parallel decks, and high levels of longitudinal and transverse gradient. Generally the launching girder is fully self sufficient and able to self launch its support beams and brackets. It is also usually possible to receive segments from below, within the span or in front of the pier or more commonly from the bridge deck which has just been constructed. This last feature is particularly useful when constructing the viaduct or bridge over difficult terrain or through urban areas where it would be difficult to deliver the segments at ground level.

In addition to the benefits described above, the use of an overhead launching girder usually applies the temporary loads from the erection system directly at the pier locations where they are easiest to manage.

The disadvantage of this system is mainly the relatively high capital investment required to purchase the equipment, although this can be addressed by ensuring, through careful consideration and design, that the equipment is sufficiently flexible to be adapted for use on other projects. It is not unusual for a well-designed launching girder to be used on five or six different projects, by which time the capital amortisation is less significant. Another consideration is that this equipment is relatively complex and requires considerable expertise and experience to operate it in a safe and efficient manner.

Neil Thorburn is manager of major projects at VSL Intrafor Asia Pacific

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