Launching Systems for Segmental Bridges
Vinay Gupta, Principal Consultant, Tandon Consultants Pvt. Ltd. New Delhi.
IntroductionIndia has witnessed a large growth in the field of construction, with construction sector comprising 40% to 50% of India’s capital expenditure on the projects in various sectors. While the growth has been equally high in the field of real estate and infrastructure projects, a much larger growth has been witnessed in the field of segmental bridge construction. This is due to the distinct advantage of speed and aesthetics that this technology provides.
There are various types of segmental bridges, to name a few:
- Precast segmental superstructure, simply supported or continuous, internally prestressed or Externally prestressed, epoxy jointed or dry jointed.
- Balanced cantilever construction using cast-in-situ or precast segments
- Spliced girder superstructure constructed using concrete stitch or epoxy joint, etc.
In totality, there are many forms of segmental construction. However, the most popularly known form of segmental construction is precast segmental superstructure, constructed by span-by span method of construction. In this technique, precast box girder segments, aggregating a total length equal to approximately one span at a time are assembled and prestressed. Thereafter, segments of similar total length are assembled for each subsequent span. The assembled length can either be between two consecutive piers, each time or 1 ¼ span then ¾ span + ¼ span then ¾ span + ¼, and so on. At each stage, prestressing has to be carried out, in order to make the constructed structure self- supporting. It may be noted that precast segmental superstructure, so constructed, has to necessarily be prestressed, as untensioned reinforcement cannot continue through the joints of precast segments.
Assembling of precast segments is facilitated through either under slung assembly truss or over head assembly truss.
Underslung Assembly TrussUnderslung assembly system of assembling and prestressing the segments has been used in several important bridges and flyovers. In this arrangement, the assembly truss is located below the segments, wherein the segments may either be supported through the flange (cantilevering deck slab) or through the soffit slab. Former being more often employed, has the advantage that larger vertical clearance below the assembly truss for the traffic movement during construction is available. But, it entails a larger barricading width. It is worthwhile to mention here that the cantilever slab would require extra bottom reinforcement in order to be able to carry the segment weight. The system of providing assembly truss below the soffit has been depicted in the proposal prepared for the flyovers of Signature Bridge in Delhi, Figures. 1, 2 , 3 & 4.
This type of Underslung Launching Truss system is suited to sharp plan curvatures, where long straight launching truss cannot negotiate the curves, due to the transverse offset, that is created between the arc of the bridge centre line and chord line of the launching truss. In this system, short span straight segments of steel girders/trusses of 6m to 8m are placed over steel trestles resting over temporary spread footings at ground level. These straight girder segments negotiate the curvatures with kinks at their junctions. Over these girders, a set of jacks and trolley for manoeuvering the segments longitudinally, transversely and vertically, including the movement for dry matching is provided. Figures. 5&6 for depiction of this system.
This type of underslung system is slow but, it makes it feasible to construct segmental superstructures in as sharp as 70m radius of plan curvatures. In this system, the precast segments can either be fed from the forward end, which takes more time to slide the segments back or from sides, if space is available, entailing a faster construction. In another system of feeding of segments, over head Goliath Crane is provided, which makes it faster to move the segments, Figures. 7.
A more versatile and faster system is the one as shown in figs. 8 & 9, which uses a self- launching type launching girder. (approximately 2 ¼ span length) as used for Delhi-Noida bridge, wherin13 spans of 42.5m each, making a continuous superstructure of approximately 550 m were constructed. Here, the segments were fed through a 64 wheel trailer plying over the previously cast deck. These are picked up by a cantilevering portal to place it on trolleys to rest over the underslung launching girder. This type of system is faster but it is suited to straight or near straight spans only.
Another interesting system of underslung launching girder uses mid span articulation to negotiate the plan curvatures, as depicted in Figures. 10 & 11. This system has been successfully used for 26 km long LRT Systrem 2 in Kuala Lumpur, Malaysia over 10 years ago, wherein Dry Jointed Precast Segmental Superstructure using External Prestressing was used.
Over Head Assembly TrussThis system of launching involves assembly truss (launching girder) that rests on either pier, precast segment over pier or separate temporary supports taken from ground. In most cases, the launching girder is made self–launching type and it is about 2 ¼ span long. Figures. 12 & 13 depict the system used in Delhi Metro project.
The over head launching system depicted in Figures. 14 & 15 for construction of four flyovers in Delhi made use of very simple and light weight truss weighing 52t for a length of 41m, capable of lifting 30t precast concrete segments. These trusses were supported over temporary steel columns resting on temporary spread footings. Length of these trusses was made just adequate to cover the largest span. Not being self–launching type and light in weight, the trusses could be simply lifted using mobile crane and shifted to next location for construction of subsequent spans. Major advantage of this type was that it could be used for the construction of any span in any sequence, where substructure was available. It is well–known that in the urban areas unchartered utilities delay the construction of foundations and hence, the construction may not be feasible, sequentially. This advantage is not available with self-launching type of assembly trusses because they have to follow a particular direction of movement.
22 km long line 3C of DMRC has used ‘C’ shaped segments. Figures. 16 Depicts lifting system of such type of segments. In this case, the launching girder rests on the pier cap using a specially fabricated steel frame, placed over the pier cap.
Figures. 17 & 18 depict the overhead launching system employed for 8 span continuous superstructure of 10 km long Bangalore-Hosur Elevated Expressway (a BoT Project). In this case, the 250t, 80m long overhead launching girder (LG) rests on specially designed and fabricated steel frames that rest over temporary steel brackets attached to the respective piers. During hauling, the LG rests on three supports. Thereafter, the middle supporting frame is removed and replaced by a precast segment. This pier segment is moved over double span launching girder and placed over the middle pier. Subsequently, the pier segment is temporarily nailed down to the pier cap and then launching girder rested on the pier segment including temporary nailing down.
Figures. 19 & 20 depict the structural system required for lifting the segments (in the case of over head launching) and imparting temporary prestressing. It may be noted that deck cables in all the precast segments need to be arranged in such a way that they clear the holes required for the lifting system.
Derivative of Over Head Launching SystemThis system uses launching girder overlapping with the segment depth and they are placed on either side of the segments, see Figure 21. An auxiliary steel frame is used to lift the segments, like in the case of conventional over head launching and out riggers to transfer this load on the adjoining parallel launching girders. In this system of launching, larger vertical clearance for the road below can be made available, during construction.
Incremental LaunchingThis is also a form of segmental construction, where segments of half the span length are cast and stressed each time and moved forward through pushing or pulling mechanism, see Figures. 22 & 23.Needless to mention that there is temporary steel launching nose at the forward end, which is dismantelled at the end of the launching operation.
Balanced Cantilever ConstructionIn this system of construction, first a 5m to 10m long segment of box girder is cast over the pier cap and prestressed. Thereafter, a pair of Cantilever Construction Equipment (CCE), as shown in Fig. 24 is fixed on top of the already constructed pier segment. While the forward support of the CCE transfer downward reaction on the deck, the rear end is anchored to the deck to take uplift forces. The cantilevering part of the CCE suspends formwork including necessary function platforms for casting 3m–5m long segments on either side. After prestressing of the newly cast segment, the CCE is moved forward for constructing the next pair of segments. Once the cantilevers from both sides reach close to each other, as shown in Figure. 25, one of the two CCEs is removed and the other one made to bridge the gap between the two cantilevers, in order to cast the closure segment. And on the other side, either ground supported staging or a centering truss supported over the cantilever and end pier/abutment is used to construct the end portion of the outer spans. Thereafter, the CCE and centering truss are removed. This scheme can also be applied to multi span structure employing cantilever construction.
Spliced Girder SystemIn this system of segmental construction, precast segments of concrete girder (RCC or PSC), smaller than span length are cast, in order to restrict the length and weight of the segments to be handled. These girder segments are placed on permanent piers and temporary steel trestles for assembling through either concrete stitch or epoxy jointing. These segments are then post tensioned to make them a full span or multi span unit, as the case may be. Subsequent to removal of temporary trestles, deck slab and diaphragms are cast. Alternatively, post-tensioning is carried out after casting of deck slab and diaphragm, in which case the temporary trestles are removed later. See Figure 26 for Spliced Girder system.
Launching of Full Span Box GirderIn case it is desired to launch a complete span of precast box girder, (RCC or PSC) it can be done in a simple manner. A rail is placed between precasting bed (situated on any one end of the bridge under construction) and the nearby abutment/end pier. This rail continues on top of the already launched spans. On the forward end, there is a steel launching girder (LG), whose height is same as that of the box girder. The LG is also provided with rail on top, so that to be launched precast box girder can be moved on a pair of steel trolleys. After the box girder reaches the intended location, it is picked up by two nos. A–portals, fixed to two pier caps. After moving the launching girder forward, the A-portal lowers the box girder on to the pier cap. This way, multi-span box girder bridge can be launched. Figures 27 & 28 for the system.
ConclusionA careful planning of erection technique for launching of segmental bridges can lead to desired speed and economics. Speed of construction is of paramount importance because the infrastructure facility is always a prerequisite to development of an area. More so, in the BoT projects, the concessionaire has a commercial stake, wherein he has to start collecting toll as soon as possible. Use of precast concrete makes the structure amenable to better aesthetic appeal, due to better finish and adaptability to innovative designs.
NBMCW May 2009
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