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EducationPhysicsChenab Railway Bridge: Engineering marvel in world’s highest overpass explained

Chenab Railway Bridge: Engineering marvel in world’s highest overpass explained

The ‘golden joint’ of the Chenab Railway Bridge, the highest single-arch railway bridge in the world, was inaugurated today. While building any bridge is an engineering challenge, when one stands 359 m above a treacherous riverine terrain exposed to gale-force winds between rugged mountain valleys, it’s a marvel. Mumbai-based infrastructure major Afcons constructed this railway bridge, which is 35 m taller than the Eiffel Tower.

The Chenab Railway Bridge is part of the ambitious Udhampur-Srinagar-Baramulla rail link (USBRL) project of Northern Railway. Its estimated cost is Rs 28,000 crore.

While the engineers at Afcons had to take into account the viability, aesthetics, innovation, technology and geography of the bridge, ensuring there is no risk to human life was the topmost priority in their minds. They worked on it at an impossible height, braving extreme winters, torrential rains and scorching summers, finally overcoming the odds.

No less awe-inspiring feat in the building of the Chenab Railway Bridge is the intricate design and geometry. The erection of the massive segments (weighing 34 metric tonnes) was done with the help of the world’s tallest cable crane.

What does the humongous load lift of the Chenab Railway Bridge rely on?

The cable crane is the backbone of the Chenab Railway Bridge. It is also the only source of erection activity in the project, owing to its location and height. Heavy rains, gale-force winds, thunderstorms and lightning may impact the cable crane operations. So, the engineers paid special attention to proper planning and time-bound activity.

How was the arch of the Chenab Railway Bridge aligned? How will it be maintained?

To control the geometry of the arch, the engineers aligned the arch segments with meticulous and regular surveys during the erection process. They calculated the temperature and monitored the wind movements while erecting the bridge and conducting the surveys, which were done early in the morning to avoid temperature variation. Arch-erection cannot be done if wind speed exceeds 15 m/s.

How much did the engineers face vagaries of weather during the construction?

The weather is unpredictable along the course of the Chenab, particularly where the railway bridge stands now. The arch-erection activity was always fraught with dangers. To lessen the risk, the engineers adopted adequate safety measures with additional activity time accounted for each arch-erection. The hostile climate is a clear and present danger for engineers.

Was it a singular process of putting the whole structure together with pre-fabricated parts as in modern flyovers?

No, the challenge was greater here. The engineers had to frequently move the platform for every erected segment and provide scaffolding for bolting and torquing (calculating the turning impact of the force the bridge would be subjected to) for every arch they erected.

Then they had to transport wind bracings to the erection location, which was challenging because of the length and weight of the bracings. The uneven mountainous terrain made these movements extremely difficult.

Thereafter, before they could erect the wind bracings, they had to provide platforms at the location for ease of erection with proper safety measures. 

The pre-assembly was restricted to ensuring a required degree of inclination to erect as per the design. Accordingly, they made arrangements to lift the wind bracings. After erection, they did in-situ welding at required locations, which is a difficult activity under windy conditions.

How did the engineers ensure it was safe before giving the bridge its final shape?

Before arch-erection, the engineers checked the geometry (stress less) at ground level so that they could modify and/or alter the parts at the ground level before erection. It’s extremely difficult and risky to rectify errors at such precarious heights after erection.

One of the toughest challenges the engineers faced was in erecting split segments in multiple lifts, as multiple arrangements including platform erection and lifting arrangements were involved. Due to eccentricity in loading of the split segment, the engineers made special lifting arrangements.

Other challenges

Torquing (a measure of the force that can cause an object to rotate about an axis) of the HSFG bolts was another task at hand. It plays a vital role in the erection of an arch. When the engineers moved the equipment for torquing at such height and location, it also took a lot of time for every manoeuvre. Working at such elevations is very risky and needs specialised teams and platforms with safety measures in place. They knew it would be impossible to retrieve any HSFG bolt if it unfortunately slipped and fell into the river from such a height.

Finally, they had to take care of the stay cable that would hold the cantilever arch. A cantilever (as seen in the famous Howrah Bridge of Kolkata) is a rigid structural element that extends horizontally and is supported at only one end. The engineers used the stay cables to control the arch erection activity by using temporary piers (erected on permanent piers) and a foundation on both sides of the valley. Afcons carried out this exercise for the first time.

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