Skyway Bridge, an engineering marvel

The Krishnarajapuram hanging bridge. — Photo: Sampath Kumar G.P.  

BANGALORE DEC. 25. When the cable-stayed bridge over the railway tracks at Krishnarajapuram is opened to traffic as scheduled on January 26, it will be more than a new landmark in Bangalore.

The cables themselves, the pylons supporting them, and the way the material was transported to the site are considered unique in engineering and logistics. The Garden City Skyway Bridge, as it has been named, passes over five tracks.

The technology of holding such structures with strong cables came from a design developed by Swiss engineers 50 years ago. Thousands of metres of wire measuring 12 mm in diameter was imported from Kobe, Japan, by Usha Martin Co. of Ranchi.

The company then subjected the cables to a "cold drawing" process in which they were stretched under extreme cold conditions till they got reduced to 7 mm diameter, according to Southern Railway engineers. These special, high-strength, and high fatigue-resistant wires were then transported here in phases. They are four times stronger than the ordinary commercial wires used as construction material. They were coated with water-soluble oil and converted into thicker cables at Yeshwantpur.

There, the wires were cut to the required length and then anchored to a "head", along with other accessories, including sheathing pipes. After this, "button heads" were created so that no amount of force used to pull the cable led to the detachment of the wires from the anchor-head.

From Yeshwantpur, the cables were transported to the site on special trailers. The cables were wound on a huge bobbin, using coiler machines, as the high-strength wires and cables were difficult to wind. Each assembled cable weighed between two and nine tonnes.

At the bridge site, the fixed end of the cables was connected to the top pylon using a pulley-block fixed at the top. The other end was connected to the deck. The cables were stressed to their maximum strength. Each of the cables on the bridge is capable of taking loads up to 1,000 tonnes. The cables have been cased in HDPE (high density polyethylene), which protects them from ultra-violet rays of the sun. While the cable-stayed technology has been used in other structures in India, the K.R. Puram Bridge is the biggest of them.

The 1,830-metre bridge will have four lanes for traffic. The cables take the entire load of vehicles passing on the bridge — up to 300 tonnes at a time — and will pass on the load pressure to the pylons which in turn will pass it to the ground. The cable inclination with the towers varies from 23.7 degrees to 74 degrees. The longest cable is 113 meters.

The bridge is a joint venture of the Union Ministry of Surface Transport, Southern Railways, and the State Government. The Railways have shared the bulk of the total cost of Rs. 47 crore. Even as the bridge was under construction, city planners and engineers from places such as Patna, Nainital, and Ahmedabad came here to study it and examine if the technology could be replicated.