Sonjoy Deb, B.Tech, Civil
“The concept of bridges spawned from the necessity to cross rivers, valleys, hills and other obstacles. While the concept might have been stolen from an inadvertent fall of a tree that enabled the crossing of an obstacle, advancement in civil engineering has enabled the construction of ‘Steel and RCC’ bridges. The complexity of these structures have since increase, becoming symbols of engineering capabilities of modern cities. Hence, this article conducts a critical analysis of the components and construction aspects of one of India’s most iconic bridges with the aim of furthering development in this sector.”
Evolutionof India’s MegaBridges
Before we begin with the analysis, it would be ideal to showcase the journey of India’s bridges, culminating to the point where we are today, in terms of technological capabilities. India’s history with bridges begins with the Attock Bridge across the Indus River. The bridge which took the Punjab Northern State Railway up to Peshawar (presently in Pakistan) and the frontier, is a marvellous spectale, even to date. The gradual development of history of Indian bridges can further be illustrated with the spectacular Lansdowne Bridge over the Indus River at Sukkur.
Of the last bunch, the most familiar of all was undoubtedly the Howrah Bridge across the Hooghly River in Calcutta. It did not carry a railway proper, only a tram-line, but it served as the only connection between the main railway termini of the city, one on the east bank of the river, with the west.
WorliBandra Sea Link Cable Stayed Bridge:
Worli-Bandra sea link bridge is an engineering marvel and an architectural wonder as well. The first -of-its-kind in India (first bridge to be constructed in open-sea conditions), the 5.6-km-long, eight-lane, approximately Rs 1,600-crore Bandra-Worli Sea Link (BWSL), which has now been renamed as the Rajiv Gandhi Sea Link, is an engineering marvel that aims to ease traffic in Mumbai, India’s commercial capital. It is for the first time that cable-stay bridges have been attempted on open seas in India. Coupled with the fact that the aesthetically designed pylons have an extremely complex geometry and one of the longest spans for concrete deck, the challenges encountered were indeed formidable. A total of 2850 workers and 150 engineers were employed to work on the project and, over a span of eight years, a total 2,57,00,000 man hours were utilised between 2001 and 2009.
Let us review various components and construction methodologies of this Mega Structure in following sections:
Bridge Layout Analysis :
The main cable stayed section of the bridge spans 600m in length, consisting of two 250m cable supported spans and two 50m conventional approach spans. The smaller cable stayed section is 350m in length and comprises of 2 smaller cable stayed sections with a 150m central span and 2 50m approach spans on either side (Refer Figure2).
The design is described as follows by the designer. “The overall tower configuration is an inverted “Y” shape with the inclined legs oriented along the axis of the bridge” (Refer Figure4). In total there are 264 cables attached to the towers, they form a semi-fan arrangement. The bridge deck is constructed of pre cast box girder sections which are identical those used for the approaches “the bridge is proposed to be built utilizing the concept of precast, post – tensioned, twin segmented concrete box girder sections”.
Bridge Structure Analysis :
Pylons – These are arguably the most important components of a cable stayed bridge. The main span bridge has 2 pylons, each with 4 legs, each tower is inclined towards the other by 10°, eventually merging at 98m above deck to become a single tower. Transverse and longitudinal post – tensioning is provided in the tower head to resist local cable forces. The single tower is tapered towards the very top. Beneath the superstructure of the bridge the 4 legs merge to 2 points which are carried into the ground through the pile caps.
As with most cable stayed bridges the pylons are very stiff. An A-frame adds torsional stiffness to the bridge, this is due to the natural resistance to twisting created by the closed triangle. The stiff pylon in conjunction with the slender deck and numerous cables means that the pylon will be subjected to high longitudinal moments due to the live loads on the deck and pylon itself.
Cables – The arrangement of the cables is 4 planes of a semi-fan arrangement. The Cable Stay system comprises 2,250 km of high strength galvanized steel wires which support the Cable Stay Bridge weighing 20,000 tons. Each deck section has 2 planes of inclined cables which are attached to the top of the tower in one plane. This layout of cables is suitable for the large spans as the inclined arrangement provides the lateral stiffness required. The advantage of this layout is that the deck can be slender as it does not have to account for the torsional inadequacies of a single plane of cables whilst taking advantage of the preferred aesthetics of a single plane attachment to the pylon. The cables are inclined due to A-frame pylons. The deck and the two planes of inclined stays behave like a rigid closed section in bending having this apparent closed section made by the deck, the inclined stays and pylon causes the rigidities of the deck and pylons to work together to make a rigid structure which acts against rotation in the deck. The inclination of the cables is such that clearance is not an issue for the passage of vehicles across the deck due to the spans involved and the height of the pylon, it means that the level of inclination is slight. The method of lateral suspension used in this case causes transverse bending moments with a maximum in the centre of the deck. There are points of maximum shear at the edges of the deck. It was therefore important that the design took into consideration the possibility that the transverse pre-stress in the deck and the anchorage for the cables may clash. Cable spacing is 6.0 meters along the bridge deck. Refer Figure 3 for cable spacing.
Deck – The deck of the BandraWorli Sea Link consists of a hollow concrete box section with 3 cores, the dimensions of the deck varies throughout the length of the bridge. The precast segments vary in length from 1.5m to 3.1m. Each section of bridge deck will be post tensioned following installation. The idea behind having a very slender and lightweight deck is to reduce the longitudinal stiffness, it is therefore advantageous to provide a very flexible deck. Because this bridge utilises a lateral suspension, bending within the deck is reduced and torsion in the deck is not normally a critical case. For flexible decks the dimensions of the deck are determined by the transverse moments and the size of the point loads at the anchorages, this is therefore governed by the separation of those cables. For the type of box section used at BandraWorli Sea link the top slab is continuous over the webs and props. The use of webs and props creates a multi-box section allows the large width which is required for each direction of traffic. Refer Figure 4.
Foundations – The drilled shaft method of construction was used for the shafts. The shafts vary considerably in size, depending on the bedrock. “Rock encountered at the site included highly weathered, fractured and oxidized volcanic material with RQD’s of less than 25 percent and unconfined compressive strengths of 1 MPa”. Foundations for the towers comprised of 52 2m diameter piles arranged in an H shape to capably support the legs of the pylon. They are up to 34m in length. The loads on different parts of the structure vary considerably and this was reflected in the variability of the shaft sizes to accommodate loads from 2-25MN. Refer Figure 5.
Construction Methods – The precast concrete sections of the deck were launched incrementally between the pillars using a truss system, known as the balanced cantilever method. The bespoke girder (Refer Figure 6) spans between two piers with the girder being supported on the outside pier by a temporary support. This allows the central carriage to move between the two piers to install the precast deck sections by picking them up and winching them into the appropriate position. The precast sections were then epoxied together and given a certain degree of prestress to hold them in place. Once each span had completed and geometrical adjustments made the primary continuous tendons were stressed to the required level. Once the deck section was in its requisite position cable connection was initiated, using the following method :
The cables were delivered to site and uncoiled using a winch, fixed to their anchor at the upper tower, winched towards the deck connection and guided into the anchorage then stressed to the required level using hydraulic jacks.
One major obstacle which had to be overcome during the construction process was how to move the large truss from the Bandra end of the bridge to the Worli end without having to dismantle the truss which would be too time consuming on such a high profile structure with a strict timescale. A decision was made to use a large sea crane to lift the truss into its new position, however, the depth of the water at low tide meant that the crane could only be used at instances of high tide, this meant that the operation took a number of days. The span by span method was used for the construction of the approach sections of the bridge. Where each span was constructed up to its nearest point of contraflexure to the pier, the formwork is then shifted to the next span and the cycle is repeated. All of the rebar and formwork for the pylon, the diaphragm and the pier table were constructed off site and the pouring was carried out in stages. Due to the inclination and height of the main tower temporary longitudinal and transverse compression struts were required during construction. Geometric control is required to ensure as-built accuracy, for this reason reference points are taken at the base of towers, anchor point and completed structure, these point are used to set formwork.
A programme of works for the cable stayed section of the bridge as used by the contractor can be seen below :
– Construction of Foundation
– Construction of Tower /Pylon below the deck
– Construction of Tower / Pylon above the deck
– Construction of Pier Table
– Construction of Diaphragm
– Construction of Tower/ Pylon above the deck Erection of Deck and Stay Cables
– Stressing of Stay Cables
– Wet Joint Construction
– Continuity PT and Grouting of Cables
– Force adjustment and fine tuning.
Having studied the BandraWorli Sea Link in depth, we can appreciate that it is a worthy representation of current bridge engineering technology and a good example of what is possible in the current climate. The optimised execution of the inverted Y design of the pylon is a solution that is both aesthetically and technically successful. The use of tensioning mechanisms has provided an efficient compromise between deck sizing and costly strengthening methods. Due to this bridge being a new build it is unlikely that there will be any durability or serious maintenance issues for some time, however, the these issues must be accounted for and any possible inadequacies in durability highlighted so that there is a prior awareness of any shortfalls. It is useful to look to precedents prior to design to see if there is a trend in the type of damage caused to cable stayed structures over time which can be alleviated through design. It is important to undertake comprehensive inspections both periodically and after extreme events such as floods or storms.