Chenab Bridge - Special Considerations

Special Considerations

The structural concept of the bridge over the river Chenab is a large span arch over the river, with approach viaducts on either side. The proposed arch is a two-ribbed arch, fabricated from large steel trusses. The chords of the trusses will be sealed steel boxes, 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 boxes will be stiffened internally.

The numbers of bearings have 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 in 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.

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 proposed for this bridge greatly exceed this limit, and are continuous. In order to provide these additional features, it will be necessary to augment the design with additional international codes and standards that enable the designer to produce a safe design.

Following are some of the special features of structural design for the bridges:

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