GLIMPSES OF STRUCTURAL DESIGN OF DELHI METRO

Mahesh Tandon*, Tandon Consultants Pvt Ltd., India

29th Conference on OUR WORLD IN CONCRETE & STRUCTURES: 25 - 26 August 2004, Singapore

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29111 Conference on OUR WORLD IN CONCRETE & STRUCTURES: 25 - 26 August 2004, Singapore

GLIMPSES OF STRUCTURAL DESIGN OF DELHI METRO

Mahesh Tandon*, Tandon Consultants Pvt Ltd , India

ABSTRACT

The Delhi Metro is a Designer's Delight. Apart from a significant technology upgradation of bridge construction in urban situation, the sheer variety of construction is a pleasure to behold. The viaducts span across major arterial roads by continuous bridges built by cantilever construction or ground supported framework. An integral bridge, which eliminates bearings and expansion joints and incorporates a skew of as high as 700

, is another landmark structure. Some stations are constituted completely by precast elements resulting in high quality a nd finish with least disturbance to traffic. These selected structures of the Delhi Metro are described in the paper.

Keywords: MRTS, Stations, Viaduct, Integral Bridge, Precasting.

1. INTRODUCTION

A typical phenomenon of the Indian metro city scenario is the escalating chaos in the movement of its vehicular traffic. And Delhi is no exception. In its concrete jungles, the commuter is prey to the lurking predators of pollution, traffic snarls and road rage. Luckily however for its citizens, Delhi's Mass Rapid Transit System (MRTS) promises to bring about a sea-change in the existing traffic patterns and to lend credence to the capital city being the crowning glory of India.

As one of the brightest jewels of Delhi, the MRTS will be a most valuable asset of the city. Destined to improve the quality of life of its residents, it will directly or indirectly promote their health, welfare and safety on a day to day basis . Be they commuters on the way to office, housewives on the move picking up and dropping their offspring to school or business-men trying to reach a meeting in time, the MRTS promises to get them there quickly, safely, reliably and without hassles.

Delhi Metro Rail Corporation (DMRC) has undertaken to provide the MRTS for the city in 3 phases by 2020, and adopted techniques suited to construction of fast track projects. The first phase is scheduled to be commissioned in 2005. Its route length of 50.1 km comprises 11 km of underground stretch, 11.7 km of surface corridor & 27.4 km of elevated rail corridor. There will be 10 stations underground and 38 stations on or above ground. While the underground will run in the north­south direction, the surface and elevated corridor will span across the mid-riff of Delhi in the east-west direction.

The viaduct alignment is dotted with unique station buildings at every 1.0km with a large variety of striking features, which characterise their Architectural conceptions .

The loading from the rolling stock is fairly heavy- 18t per axle. Delhi is located in a moderately high seismic zone with ground acceleration under Maximum Considered Earthquake (M.C.E) conditions reaching as high as 0.24g. The structural design is based on design speed of the rolling stock as 80 km per hour.

This paper describes the following structures which form part of the first phase now fully operational:

Major Crossings over important traffic arteries. Integral Bridge Flyover crossing Grand Trunk Road Station Buildings employing Precast Elements

2. MAJOR CROSSINGS

A large variety of major crossings over road traffic arteries, railway tracks and existing flyovers were required along the alignment. Cast-in-situ construction with 3 spans was adopted for these major crossings. The sequence and methodology of construction was tailor-made toe nsure least traffic disturbance at ground level.

Consideration to aesthetics formed a major input during the design stages, as the structures were visible from various angles to the traffic at ground level. The final results were fairly successful in this regard, Figs 1 and 2.

The major crossings can be identified as follows:

A. Cast-in-situ superstructure over ground supported staging at:

a. ISBT CROSSING span arrangement: 33.4m + 46.2m + 33.4m

b. MORI GATE CROSSING span arrangement: 33.4m + 46.2m + 33.4m

B. Cast-in-situ free cantilevering method at major road and railway crossings at:

a. OUTER RING ROAD CROSSING AT MADHUBAN CHOWK span arrangement: 38.5m + 55.0m + 38.5m

b. RING ROAD CROSSING span arrangement: 38.5m + 55.0m + 38.5m

c. ASHOK VIHAR CROSSING span arrangement: 33.5m + 46.2m + 33.4m

d. KADAMBARI MARG CROSSING span arrangement: 33.4m + 46.7m + 33.4m

e. SHYAMJI KRISHNA VERMA MARG span arrangement: 33.4m + 46.2m + 33.4m

f. PULBANGASH RAILWAY TRACKS span arrangement: 41.5m + 60.5m + 41.5m

g. ROSHANARA ROAD CROSSING span arrangement: 38.5m + 55.0m + 38.5m

Structures in category (A) above had major constraints during construction as can be visualized in Figs 1 and 2.

2

In some of the structures mentioned in (8) above, the side spans were constructed on ground supported while in others all the three spans were constructed in free cantilevering . An example of the former is shown in Fig 5.

At one of the locations (Madhuban Chowk) the problem was made extremely complex with a vehicular underpass being simultaneously constructed perpendicular to the alignment, Figs 3 and 4.

3. "INTEGRAL" BRIDGE

A rather difficult bridge from the point of view of its geometrics was conceived for the MRTS curved flyover traversing Grand trunk Road. The f1yover is an "integral" bridge, i.e., the piers and abutments are monolithic with the deck thereby eliminating the bearings as well a s the expansion jOints, Fig 6. The six-span curved bridge of 117m length has a very large skew (varying from 72 degrees at one abutment to 60 degrees at the other).

Fig 7 shows the structural modelling adopted for the full structure. Fig 8 shows the construction stage just before casting of the deck while Fig 9 shows the construction nearing completion. Of particular interest is the fact that the structure is not assumed as fixed at the foot of the piers and abutments but that the pile cap and piles are included in the idealisation . The soil resistance is idealised by springs of appropriate stiffness. This type of structural modelling ensures that the creep, shrinkage and temperature rise/fall, all of which are of considerable significance in an "integral" bridge can be taken into account in a realistic manner.

Seismic forces in longitudinal direction and earth pressures on abutments can thus be transmitted to all elements of the "integral" bridge structure with a good simulation of the actual situation. The highly indeterminate structure would result in improved seismic performance as compared to conventional bridges with bearings and expansion joints.

The textured surface of the abutment and return walls combined with the circular free­standing piers were specially conceived to improve the visual impact of the bridge.

It is accepted now that the best arrangement for bearings and expansion joints of moderately long concrete bridges is that in which these elements are absent. This statement is not only true from the point of view of aesthetics but also from purely engineering considerations. Improved durability, maintenance-free character and enhanced seismic resistance are the main advantages that can accrue to the project by what are called "integral bridges".

4. STATIONS EMPLOYING PRECAST STATONS

Stations which are located on the central verge of an existing roadway are always difficult to tackle without disturbing traffic. This issue is of much greater significance in stations as compared to the viaduct, which has a limited lateral coverage.

A typical station building about 185.0 long over a 6-lane divided carriageway at ground level is shown in Fig 10. The first floor level is the concourse which is generally half the total station length and houses all the technical and commercial activities. The second level (or the platform level) is reached from the concourse by stairs or escalators. The concourse as well as the platform level consists of precast Double Ts which span 15.5m and constitute the slabs.

The two tracks are supported on a composite slab constituted from precast U-girders topped by a cast-in-situ slab. The U-grade transmit the load directly to the caps of the pier located at the central verge as can be seen in Fig 11.

Precast post-tensioned cross girders support the platform as well as the structural steel roof above, as can be seen in Figs 12 and 13 respectively.

Precast post-tensioned L-beams supoprt the entire concourse and transmit their loads to the pier through brackets, Fig 12.

3

The completed structure of the concourse viewed from the road level is shown in Fig 14.

One unit each of all precast units was load tested to assess the behaviour under working load as well as ultimate load conditions. The predicted defelctions matched well with those observed during the test loading. Fig 15 shows the arrangement during load testing of the cross girders.

Fig 16 shows the individual weights of the elements and their numbers per station. On assessing the repetitions involved, and economies of scale, it was decided to build 6 stations of this type broken into two separate contracts of 3 stations each.

5. CONCLUSION

Meticulous planning characterize the construction of the structures of the Delhi MRTS in densely populated and congested areas. The construction was required to be done swiftly and in the least obtrusive manner given the severe constraints of both time as well as space. While standardisation in designs and construction was attempted to the maximum extent possible, site constraints demanded improvisation Imodifications in substructure and foundation designs at various locations. Aesthetics and durability were deemed essential for structures that loom prominently in the public consciousness, and aesthetically pleasing external profiles were provided to the superstructure and substructure without compromising on functional & structural requirements .

The project has employed state of the art technology in the design and construction of the concrete bridges and the Delhi MRTS would possibly be the precursor to similar systems adopted in other cities with significant urbanisation.

4

FIO.1 DELHI METRO: 3-SPAN CONTINUOUS STRUCTURE

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DELHI METRO: STATIONS EMPLOYING PRECAST ELEMENTS UNDERSIDE OF CONCOURSE d.r:;-o

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