Home Articles New Erection & Launching Systems for Segmental Bridges

Erection & Launching Systems for Segmental Bridges

1258
Civil Engineering
Vinay Gupta
President, IIBE President (Elect),
ICI Vice-Chairman, IABSE Director & CEO,
M/S Tandon Consultants Pvt Ltd

After completing his Civil Engineering from BITS, Pilani, India in 1983 Er Vinay Gupta has served Span Consultants Pvt Ltd and STUP Consultants Ltd before joining Tandon Consultants Pvt Ltd, India in 1987, where he is serving as Director & CEO. Er Gupta, has been involved in planning, designing and coordination of design, detailing and tender preparation of various bridges, flyovers, vehicular underpasses, long span and tall buildings, steel structures, chimneys and quality control documents. Some of the highlights of his carrier include design & planning of prestressed structures in India and abroad. 102m span Balanced Cantilever construction prestresed bridges with 70m tall piers of the famous Jammu Udhampur Rail Link (JURL) for first time in Indian Railways, Delhi Gurgaon Elevated Expressway, involving largest no of 1800 Pretensioned Prestressed Girders for the BOT project of NHAI, 500M long Prestressed Segmental Canntilever Construction Superstructure incorporating STUs and Dry Jointed Precast Segmental Prestressed Superstructures of 26km long LRT Metro in Malaysia are some of the pioneering structures, designed by him. He is an active member of various Codes and Standards Committees of BIS and IRC in India dealing with Earthquake Engineering, Special Structures, Loads & Stresses, Concrete Bridges, Bearings & Expansion Joints etc. His contributions include preparation of IRC:SP:65-2005 (Design and Construction of Segmental Bridges) and IRC:SP:71-2006 (Design and Construction of Precast Pretensioned Girders for Bridges). He has been honored with awards for the best papers titled “Seismic Design and Construction of Radisson Hotel, New Delhi” and  “Launching Systems for Segmental Bridges” both by IBC and “Prestressed Concrete Design Award-2013” by IEI (The Institution of Engineers, India). He has authored about 100 technical papers in civil engineering journals & conferences and has been lecturing as Guest Faculty in NITHE, CRRI, CIDC, ISDA, DPC, etc. Er Gupta is the President of IIBE (Indian Institution of Bridge Engineers), President (Elect) ICI (Indian Concrete Institute) and Vice Chairman, ING IABSE.

Introduction

India 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:

  1. i) Precast segmental superstructure, simply supported or continuous, internally prestressed or externally prestressed, epoxy jointed or dry jointed.
  2. ii) Balanced cantilever construction using precast segments
  3. iii) Spliced girder superstructure constructed using concrete stitch or epoxy joint, etc.

As a mater of information, until year 2005, Indian codes did not cover design and construction of segmental superstructures. Later, a new document IRC: SP: 65-2005 prepared by the author under the aegis of the IRC code committee, has been published. This document covers design and construction aspects of segmental bridges.

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 can not 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 Truss

Under slung 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.

This type of Underslung Launching Truss system is suited to sharp plan curvatures, where long straight launching truss can not 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.   Ref figs. 1 & 2for 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.

A more versatile and faster system is the one as shown in figs. 3 & 4, which uses a self launching type launching girder. (approximately 2¼ 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 trailor 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 figs.  5 & 6. This system has been successfully used for 26 km long LRT System 2 in Kuala Lumpur, Malaysia over 10 years ago, wherein Dry Jointed Precast Segmental Superstructure using External Prestressing was used.

Over Head Assembly Truss

This 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. 22 km long line 3C of DMRC has used ‘C’ shaped segments. Fig. 7 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.

Figs. 8 & 9 depict the over head 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.

Spliced Girder System

In this system of segmental construction, precast segments of concrete girder (RCCor 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 fig 10 for Spliced Girder system.

Segmental Balanced Cantilever Construction

When bridge height is so much that it is not practicable to provide ground supported staging for casting of bridge deck, this technique is used. Three span continuous superstructures of Jammu-Udhampur Rail Link (JURL), incorporating central span of 102m, pier height as much as 70m employed Balanced Cantilever Construction. Specially designed Form Travellers capable of supporting green concrete weight of segments as deep as 9m, with maximum length of casting 3.6m at each stage, before prestressing is imparted at each stage of operation were employed  (see fig. 11). The scheme gives a time cycle of about 10 days per set of two segments of about 3m length each.

In order to speed up the construction, use of Precast Segmental Superstructure constructed using Balanced Cantilevering was made for a Commonwealth linked project of Barapulla Elevated Road in Delhi. In this case a specially fabricated Segment Lifter was employed, to give a construction speed as high as 1 day per each pair of two segments of 3m length each.

Conclusions

A 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.

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