The world's highest railway bridge

  The Chenab Bridge

                                                  
   The Chenab Bridge is a railway steel and concrete arch bridge under construction between Bakkal and Kauri in the Reasi district of Jammu and Kashmir in India. When finished, the bridge will span the Chenab River at a height of 359 m (1,178 ft) above the river, making it the world's highest rail bridge.

• Deck height (height above river): 359 m (1,178 ft)
• Bridge length: 1,315 m (4,314 ft), including the 650 m (2,130 ft) long viaduct on the northern side
• Arch span: 467 m (1,532 ft)
• Arch length: 480 m (1,570 ft)

Introduction and topography

Chenab Bridge construction

        Konkan Railways has undertaken the mega-project of constructing a new railway line across the Indian state of Jammu and Kashmir between the towns of Udhampur near Jammu and Baramulla on the northwestern edge of the Kashmir Valley. This project has been declared a national project in 2002. It is directed by the Konkan Railway Corporation Ltd..
The extraordinary challenge lies in a large number of tunnels (totalling 63 km in length) and bridges (7.5 km) to be implemented in highly rugged and mountainous terrain, with the difficult Himalayan geology. The most difficult part is believed to be the crossing of the deep gorge of the Chenab River, near Salal Hydro Power Dam, by the Chenab Bridge.
Another, smaller, arch bridge proposed in the new railway line was 657 m (2,156 ft) long, 189 m (620 ft) high Anji Khad Bridge between Katra and Reasi over the Raavi river. This proposal has been abandoned by the railways due to the specific geology of the location and a cable-stayed bridge is proposed.

Design

After many deliberations, taking into account aesthetics, economy, and availability of local expertise and construction materials, the Chenab Bridge was designed as a large span single arch steel bridge with approach viaducts on either side. The arch is two-ribbed, fabricated from large steel trusses. The chords of the trusses are sealed steel boxes, internally stiffened and filled with concrete to assist in controlling wind-induced forces on the bridge. Another advantage of concrete filling is that internal painting will not be required.
The number of bearings has been minimized, particularly on the approach viaduct, through the use of continuous construction. This is advantageous, as it reduces the maintenance and inspection efforts, and improves the riding quality. The viaduct piers are of concrete, while the piers near the arch are of steel.
The design of major arch rail bridges requires considerations of a number of additional parameters, such as fatigue, global stability, second order effects, composite action, etc. It also requires that such a bridge is designed to achieve a consistent level of reliability for all load cases, and that the design standards match the construction standards. Indian construction standards such as the Indian Railway Standards (IRS), the Indian Road Congress (IRC) and the Indian Standards (IS) were found inadequate for the large spans of the Chenab Bridge. For example, the Indian Railway Standards (IRS) is primarily intended for simply supported bridges with spans up to 100m (although these have been successfully used for higher spans up to 154m). The spans for the Chenab Bridge greatly exceed this limit, and are continuous. Therefore, to assure a safe design, Indian national standards have been supplemented with International standards such as British Standards (BS), International Union of Railways (UIC) and Euro. Also, many experts throughout the globe, based on their versatile and relevant experience, have been involved in order to make the building project a success.

Following are some of the design considerations taken into account:

• Limit state philosophy of design has been decided to be followed as per BS codes
• Computation of wind load effects as per wind tunnel tests
• Site specific seismic spectra developed by Indian Institute of Technology (IIT) Roorkee
• Provision of Euro code 8 for ductility detailing of very tall and hollow rectangular RCC piers
• Provision of long welded rail (LWR) over the bridges and resulting force calculation as per UIC – 774-3R guidelines
• Blast resistant design has been used
• Design checking for fatigue as per BS codes
• Deformation limits as per comfort criteria of UIC – 776-2R and UIC 776 -3R guidelines
• Redundancy provided in the structures, for lower level of operation during mishaps and against collapse in extreme cases of one pier failure

The Quality aspect has been emphasized, as the quantum of fabrication and welding is colossal. Mostly indigenous material compliant to IS codes has been planned to be used, whereas for the design, international codes have been referred, which means the Quality Control work is still difficult.
 
      

Construction

The Chenab Bridge was originally intended to be completed in December 2009. However, in September 2008 the project was halted due to fears over the bridge's stability and safety.
Work on the bridge restarted in 2010 with the plan to complete it in 2015.
The construction has been contracted to Afcons Infrastructure Limited, a part of the Shapoorji Pallonji Group, the third-largest construction group in India.
The erection scheme for the bridge is a project in itself. Two pylons (about 130 m and 100 m high) were erected on either side of the river, and two auxiliary self-propelled cable cranes (capacity of 80t each) were used to tow temporary auxiliary ropes across these pylons. The ropes were used to support the partly finished arch parts. After arch completion, the trusses will be added, finally the girder will be constructed as a horizontal sliding type platform.