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Bandra-Worli Bridge


Published on Jan 10, 2017

Abstract

The construction of the Bandra-Worli Bridge is part of the Bandra-Worli Sea Link project necessitated by the exceptionally large amounts of traffic which currently use the Mahim Causeway travel from the western suburbs of Mumbai to the commercial hub of the island City. This affects the efficiency and links to the economic metropolis and the internationally used harbor. As an upshot of Mumbai’s rapid growth in economy and population since its independence, traffic has stalled in the city Centre.

It is thought that 120,000 PCU’s use the Mahim causeway every day. Therefore for the sea link to make a substantial difference to the level of traffic on the mainland it is required to be able to accommodate large amounts of traffic. 8 lanes will be provided. It is the first bridge of its kind to be built in open sea, which has brought up a number of engineering issues.

The infrastructure within the city is completely saturated and there is little room for expansion on the mainland, there was no other alternative than to construct over the bay. With this solution come a number of other benefits, such as reduced pollution within the city, reduced vehicle operation costs and journey times as well as the city obtaining a coastal landmark.

This paper will be mainly concerned with the larger two twinned tower cable stayed sections of the bridge which were designed by consultants Dar Al-Handasah. The bridge is expected to be opened to traffic on the1st of May 2009.

Bandra-Worli Bridge

PROJECT DESCRIPTION AND NEED

SALIENT FEATURES OF THE BANDRA-WORLI SEA-LINK (BWSL)

 An 8-lane bridge with 2-lanes dedicated for buses to ease traffic congestions.

 Length of bridge = 5.6 km, Width = 25m

 Single tower supported 500m long Cable Stayed Bridge at Bandra Channel and Twin Tower supported 350 m Cable StayedBridge at Worli Channel for each carriageway which is made of eco-friendly material.

 An intelligent bridge with state-of-art systems for traffic monitoring, surveillance, information and guidance, instrumentation,emergency support etc.

 Development of promenade and landscaping to enhance the environment.

NEED FOR THE PROJECT

 Bandra Worli Sea Link Project is one of the most highly recommended projects of all the transport studies done for themetropolitan region during the last forty years.

 At present, Mahim causeway is the only link connecting western suburbs to island city of Mumbai. The existing north-southwestern corridor is highly trafficulated and congested during the peak hours results in a bottleneck on the Mahim causewayindirectly raising the noise and pollution level of the city.

FORM

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 Fig 2. 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”. 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”.

Bandra-Worli Bridge

AESTHETICS

It was the intention of the designers to create a striking landmark in the design of this structure. Although large span cable stayed bridge design is fairly restrictive due to designs of this size being fairly standard and there being a finite number of cable layout options. There are a number of subtle design choices used for this bridge that have helped to achieve an impressive design. I will analyse the aesthetics of the bridge bearing in mind the 10 areas of importance in bridge aesthetics suggested by Fritz Leonhardt. These are:

 Fulfilment of function.

 Proportions

 Order within the structure

 Refinement of design

 Integration with the environment

 Surface Texture

 Colour of components

 Character

 Complexity in variety

 Incorporation of nature.

As with most cable stayed bridges its structural function is clear and the structure seems to be fulfilling its function well and efficiently. It is clear without performing calculations, when looking at the spans involved compared to those in the approach section the purpose of the cables in taking the load of the bridge deck. Due to the structural form of cable stayed bridges the interaction between separate functional parts is very pronounced which creates clarity and simplicity in design. Although there are a lot of cables, the Colour and semi-fan layout blurs them when looking from a distance, giving, in my opinion the look of a translucent surface. The bridge is well proportioned. Although the towers are massive they do not appear to be disproportional

when looking at the span that they have to cater for and again from a visual point of view there appears to be little redundancy in design. The deck is very slender; the aerodynamic shape of the box girder sections allows the edges to be thin. The final design shows that the edges will be finished in a light colour, drawing attention to the deck. The width of the deck is so great it would have been easy to end up with very large towers. A graceful solution has been found whereby the 4 columns supporting the deck meet to form one column above the deck and 2 piers below. Disguising the fact that there is such an expansive deck. The deck appears sufficient in completing the triangle created by the towers and the deck.

There is order in the repetitive nature of this bridge and although the two cable stayed sections of the bridge are different, efforts have been made to mirror structural elements to create order. The cable stayed sections are of the same proportions, the greater span of the Bandra end bridge is created by merely increasing the number of cables, the tower height and cable spacing is similar for both cable stayed sections.

The spacing of the piers for the approach section is concurrent however it could be argued that the spans could be made larger as from some anglesthere appears to be a forest of piers Fig. 3due to the bridge curving on plan. As already mentioned the order of the cables seems to work well, the semi-fan layout compliments the structural form of the towers because the density of cables connected in one plane increases as the 4 columns become one. This is an improvement of a precedent to this design in the Rion Antirion Bridge in Greece, where in my opinion the four columns which join appear too slender in comparison to the larger pier beneath the deck. There are a number of refinements that have been used to assist and improve the elegance of this structure. Most of these refinements tend to promote the simplicity of the bridge rather than adding to its complexity. The most worthwhile mentioning is the connection between the stays and the deck.

The connection is hidden beneath the deck which offers a very neat interaction between the stays and the top side of the deck. This solution is both graceful and practical as the connections are still accessible for inspection. The piers supporting the approach deck are widened at their juncture with the deck so that there is dimensional coherence between the two elements. There is a gentle taper to the columns in the towers which is continued through the deck, this is pleasing to the eye without being overly apparent.

As a rule of thumb cable stayed bridges look fitting over a large expanse of water, in that sense I think that it was a good design choice for its natural environment but the more important factor with this bridge is its sympathy to its urban environment. It is in a very prominent position and needs to look appropriate for its stage. The designers had the tricky task of designing something that would befit a business district and a harbour town.

Whether this had been achieved is a matter of opinion. I think that the designers have achieved in making something that from a distance is stylish and intricately simple in appearance but for the commuters who will use the road it will feel as though it is part of the city and merely an extension to the road network.

The primary observation about texture is that the concrete in the towers is notched making it appear much darker than the concrete used in the deck. Whether this was a detail based more on practicality than aesthetics I am not sure but it does serve to emphasise the much lighter cables and deck which is a virtuous property in bridge design. The piers also appear to be darker, this may be due to them being shaded by the deck, this helps in making the piers merge with the sea. It is difficult to analyse how colours will look on completion.

It is also difficult to define the character of this bridge because of the nature of the large span cable structure and the similarities to many other bridges of this type. My opinion is that the character of this bridge is defined by its use and the fact that the function of this bridge says a lot about the surrounding area and the congestion in Mumbai, the fact that the solution to connecting two areas which are not separated themselves by water, using a bridge over the sea is testament to the level of traffic in Mumbai and the desperate need to relieve this with such an ambitious solution.

STRUCTURE

Pylons

Pylons 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.

The salient characteristics of the pylon tower that make it complex and challenging from the point of view of constructability are as follows:

 The section decreases gradually with height;

 There are horizontal grooves at every 3m height and vertical grooves for circular portion that requires special form liners as well as it requires attention for de-shuttering;

 The tower legs are inclined in two directions, which creates complexities in alignment and climbing of soldiers.

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 andpylons 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 center 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.

Deck

The deck of the Bandra Worli 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 pre-cast 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.

Bandra-Worli Bridge

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 Bandra Worli 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.

BENEFITS OF PROJECT

 Savings in vehicle operating cost to the tune of Rs.100 crores per annum due to reduction in congestion in the existing roads and lower vehicle operating cost on the bridge ultimately reducing pollution.

 Considerable savings in travel time due to increased speed and reduced delays at intersections at existing roads. Which results in lower fuel burn ultimately saving precious fuels like Petrol, Diesel and LPG?

 The bridge would result in lower traffic congestions thereby improving the environment especially in terms of reduction in carbon monoxide, oxides of nitrogen and reduction in noise pollution in areas of Mahim, Dadar, Prabhadevi and Worli.

 Project to have no adverse effect on fisheries, marine life and livelihood of fisherman.

 Proper landscaping measures along the approaches and promenade along waterfront to enhance environment of the area.

CONCLUSION

Having studied the Bandra Worli Sea Link in depth I 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.

REFERNCES

[1] HCC. Bandra Worli Sea Link Project - Construction Methods. HCC India Website. [Online] 2008.

[2] Dar Consultants. Bandra Worli Sea Link. [Online] 2008.

[3]Bandra-Worli Sea Link - Mumbai, India. Waymarking. [Online] April 2009.

[4] Walther, Rene, et al. Cable Stayed Bridges. s.l. : Thomas Telford, London, 1988.

[5] Conrad, W. Felice and Brenniman, Henry. (2004) Drilled Shaft Construction For the Bandra Worli Sea Link Project., pp. 84-95.














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