Viaducts Use in LRMTS

The LRMTS Priority (Green) Line will be elevated in outer, less congested sections of the alignment stretching:

• In the South from Hamza Town to Model Town North, a distance of approximately 11km
• In the North from Data Darbar to Shahdara, a distance of under 5 km

For such a long viaduct (totalling about 16km) supporting a rail system, it is of paramount importance to select a design concept that takes into consideration the following aspects:

• Speed of construction: The structure needs to be erected as quickly as possible in order to achieve the contract schedule and open the line on time. Moreover, the disruption of roads and traffic as well as mobilization on site are limited if construction time is minimized.

• Cost-effectiveness: The cost of the Civil Works is a major component of the total Capital Cost necessary for such an important project. An economical viaduct structure complying with all design standards will reduce the cost and make the technical project more competitive.

• Minimum disturbance on road traffic: The LRMTS viaduct will follow a main arterial road of the city of Lahore, Ferozepur Road. The design of the viaduct as well as the construction method implemented on site should minimize, as far as possible, the impacts on the road traffic and avoid any traffic blockages at all times.

• Limited visual impact on urban environment: The service life of the LRMTS viaduct will be approximately 100 years. Therefore, the visual aspect of the structure must be carefully designed as it is very difficult to modify the architecture of the viaduct after the construction. The appearance of the structure will have to be both a landmark along the route for future generations and to minimize the visual intrusion on those affected.

• Full integration with system: The concept adopted will have to make provision for all the system requirements of the LRMTS system and to be as flexible as possible to cater for possible future changes in the system.

For both speed of construction and cost-effectiveness, the structural concept most suitable for this kind of long viaduct is a pre-stressed concrete viaduct using a precast segmental construction method.


Pre-stressed concrete structures are nowadays much more economical than other structure types while being reliable and durable.

This kind of structure is generally associated with precast segmental construction. The method involves prefabricating the structure parts in a separate casting plant yard. The members are then transported on site and assembled by pre-stressing to complete the structure.

This standardisation of the construction has several major advantages:
better quality of structure as it is made separately in a plant with a better QA control than on site;
• increased speed of construction as only the erection of structure parts is done on site;

• disturbance of traffic drastically reduced as only the erection of segments is done on site.

Precast segmental construction is also associated with the span-by-span construction method using auto-launching erection devices, called "launching girders".

For LRMTS viaduct, the bridge will consist of short spans, typically 25m long, simply supported on single piers and typically formed in the central median of the roadway, with a multiple pile foundation system.

The viaducts should be designed to a recognized international bridge design code taking into account railway loadings. Additionally international requirements for railway structures (UIC – Union Internationale des Chemins de Fer) shall be taken into account, as shall Pakistan Standards for seismic loading. The decks and supporting structures shall provide a minimum headroom clearance to the road below of 5.3m or equivalent Pakistan National design standard (whichever is the greater dimension).

For LRMTS viaduct, the bridge will consist of short spans, typically 25m long, simply supported on single piers and typically formed in the central median of the roadway, with a multiple pile foundation system.

The viaducts should be designed to a recognized international bridge design code taking into account railway loadings. Additionally international requirements for railway structures (UIC – Union Internationale des Chemins de Fer) shall be taken into account, as shall Pakistan Standards for seismic loading.

The decks and supporting structures shall provide a minimum headroom clearance to the road below of 5.3m or equivalent Pakistan National design standard (whichever is the greater dimension).

The "U-shape" viaduct system shown above is proposed for the elevated sections of the LRMTS.
The "U" shaped structure was first conceived by a system-wide multidisciplinary value engineering team of SYSTRA experts. The value-engineered structure integrates economically all components of the system in a very convenient and practical way, considering both installation and maintenance.

The so-called "U-shape" viaduct uses a U-shaped open channel girder as superstructure instead of the traditional box girder (or other beam supported types of decks). The tracks are placed directly on the bottom slab of the "U" girder.
The top of the "U" flange is used as a walkway at vehicle floor level, facilitating passenger evacuation from the train. This is a major advantage of "U" girder design with regard to safety compared to other common viaducts.

Signals, signalling boxes, telephones, if any, can be installed on the "U" girder flanges in a convenient position for operation and maintenance as they are installed at a correct height for train drivers. Cables are routed on both sides of the "U" girder in cable trays fixed to the sides.
The other main advantages of the U-shape structure are:

• overall standardization of the project;

• optimization of construction methods and schedule;

• optimal integration with the environment;

• and will help to reduce the overall projects risks.

Safety requirements for railway viaducts include an anti-derailment device. In conventional structures, this function is met by provision of a concrete anti-derailment wall or counter-rail. With the U-shape viaduct, the rail vehicle is maintained between the flanges and does not need other additional device.
The environmental advantages of the U-shape viaduct are:

• reduced noise to pedestrians and occupiers of adjacent property, as the sides of the "U" are also used as a noise barriers; and

• reduced visual impact on the urban landscape as the depth of the viaduct deck is about depth of conventional viaduct deck