Pushing the Limits of U Shaped Viaducts

Rajan Kataria, Chief Engineer Design, DMRC (Delhi Metro) New Delhi, Yves Gauthier P.E. Head of Civil Eng. Dept, Serge Montens, Technical Director, Arezki Touat, Civil Engineer, Systra Paris, France

The Pragati-Maidan viaduct, is an extradosed railway bridge with a main span of 93m. The deckcross-section has a U–shape, which permits a perfect integration of the metro system in the superstructure. The extradosed cables are covered by a concrete beam that allows to consider them for internal prestressing. This beam also increases the stiffness of the main span. This bridge is the first of its kind to be erected using the cantilever construction method, and it is the first extradosed bridge ever built in India.

Introduction

For the extension of the U shaped viaduct of the 3rd Line of Delhi Metro (India), DMRC was confronted to the crossing of 5 railway tracks with very high constraints: sharp plan curvature, railway vertical clearance, impossible location of intermediate piers, no possible interruption of the railway traffic, and minimum span length of 93m. In addition to the technical difficulties, time was not an ally: the line opening date was scheduled within less than one year.

All these constraints have not allowed DMRC to use the typical structures of the line 3 viaduct, which had been designed by Systra [1].

Systra has then proposed a very innovative and economic solution, which will allow a very fast construction thanks to the use of the same cross section as for typical viaduct, and by means of only conventional materials.

The segmental extradosed bridge has been erected using for the lateral spans the same construction method than for typical viaduct (span by span), and cantilever construction for main span.

Design

Foundations

All the piers and pylons are founded on deep foundations, with cast in situ concrete piles. The impossible relocation of some utilities and place constraints, has led to design large pilecaps with 20 piles per pylon, as shown in the Figure 3.

Some utilities, which it was not possible to shift vertically, were embedded in the 3m depth pilecap. The remaining lateral piers are founded on 4 piles of 1.5m diameter.

Piles length was limited to 30m to save the time of pile testing (30m is the longest length for which the pile testing was already done).

Substructures and Pylons

The bridge pylons are supported on a portal with vertical circular shafts of 2.4 m diameter. The rectangular transverse beam is 2.5 m depth as shown in the Figure 4.

Above this transverse beam, the pylon segment is cast in situ to be rigidly connected to the pylon. The outer pylon on the curved part of the bridge is prestressed vertically, to reduce transverse bending moments due to the plan curvature of the deck. The evacuation walkway located in the current area on the top flange of the U–shaped deck, is deviated laterally at pylons location.

To resist uplift forces on intermediate piers during service stage, the superstructure has been connected to a thin wall, which will not restrain longitudinal movements due to temperature, and concrete long term effects.

This connection is done through a cast in place concrete between starter bars coming from the pier segments, and from the thin wall. During construction stage when this connection is not effective, uplift forces are resisted by vertical prestressing bars.

On extremity piers the superstructure is simply supported on elastomeric bearings.

Seismic Considerations

Transverse movements of the superstructure are restrained on all piers: at pylons and intermediate piers by the mean of rigid connections, and on extremity piers by the mean of shear keys. Longitudinal movements are free only on extremity piers. Horizontal earthquake forces were computed using a spectral analysis, and combined together following Newmark combinations. Vertical earthquake was not considered.

Superstructures

The deck has a constant depth of 2.14 m and is continuous with a spanning of: 24.714 + 31.25 + 93 + 24.8 + 22.58 m.

The bridge is curved on its major part with a radius of 302.5m starting from one extremity, to approximately mid of main span, and straight on the other side lateral spans. A transition curve connects the straight and the curved parts of the bridge.

.The concrete grade used for superstructure is 40 MPa on cylinder, corresponding to 50 MPa on cubes. The superstructure is rigidly connected to the pylons and simply supported on extremity piers. On lateral spans, the deck is connected to the intermediate piers in order to resist uplift forces, coming from the loading of main span.

The deck is composed of precast typical segments with the same cross-section as for the typical viaduct, and precast extradosed segments with thicker webs to locate the extradosed cables. The length of the typical precast segments is 3.5 m.

The prestressing layout of lateral spans is more or less similar to the typical simply supported spans, with 12T15 tendons in the slab and webs. The difference comes from the web tendons that are continuous from one extremity of the structure, to the pylons.

For the main span four families of cables are used: cantilever 12T15 cables located in the top flanges of the typical segments; 19T15 extradosed tendons anchored in the webs of the extradosed segments, 4T15 tendons located in the slab of all the segments and 12T15 continuity tendons in the central part of main span.

The pier segments on intermediate and extremity piers, are prestressed transversally using 19T15 units (2 units for extremity pier segments and 3 units for intermediate ones). The extradosed cables are considered as typical prestressing tendons, as they will be covered by a reinforced concrete beam; therefore there is no maximum stress limitation due to fatigue problems.

The extradosed cables are deviated in the pylons by the mean of steel pipes and are tensioned in two stages: first during cantilever construction, and final tension is given only after casting of the concrete beams covering the cables. This is in order to introduce a permanent compression in these beams: during service stage, when the main span is loaded the concrete beams remain in compression.

The total vertical deflection under one loaded track (17 t per axle) including dynamic impact and torsional effects, is equal to 19.4 mm where the maximum allowed value is equal to 38.8 mm (L/2400).

Construction

Substructures

The main issue for foundation works was the disturbance caused by the embedded utilities for first pylon, and the temporary deviation of a concrete sewer for the other pylon

Segments Precasting

The same precasting lines of the typical viaduct were used for the extradosed bridge. The segments were precast and match cast using long beds (long line method), and the pier segments were precast in short cells. Each span was precast on a single line except main span, for which two separate long lines were used.

Special considerations were given for the geometry control of the main span segments.

Typical segment weight is 45 t, extra-dosed segment weight is 55 t, and intermediate pier segment 80 t.

Lateral Spans

The lateral spans were erected using the same launching girders that were used for the typical simply supported spans.

The first lateral span was erected on simply supports (on lateral and intermediate piers), whether the second lateral span was simply supported on intermediate pier and connected to the pylon through a cast in place joint.

After the connection with the pylon, the two lateral spans were made continuous by the mean of a cast in situ joint, and by tensioning continuity tendons and prestressing bars located at intermediate pier segments.

Main Span

The main span was erected using two segments launchers of 105 tons each. The first cantilever segment on each pylon was oriented following an angle in elevation, so that at the end of the construction the two cantilevers will be at the same required level.

The typical segments are maintained by internal cantilever cables, and the extradosed segments by the extradosed cables tensioned from the lateral spans only.

Starting from the fifth cantilever segment, in addition to the internal prestressing, a temporary external tendon was tensioned between the pylon and the deck.

After completion of the two cantilevers, a reinforced stitching segment of 0.4 m length was cast in situ to connect the two cantilevers. Continuity 12T15 tendons located in the webs, were tensioned after the stitching segment has reached the required strength.

The railway traffic (200 trains per day), was not interrupted during all the construction period. Following the removal of the segment launchers, the concrete beams covering the extradosed tendons were cast in place. Once the concrete beams have reached the required strength, the extradosed tendons were tensioned to full load, to introduce a permanent compression in the beams. The final operation consisted to connect the deck to the intermediate piers.

Geometry Control

As the entire main span segments were precast, geometry correction in case of deviation would have been very difficult. For this reason geometry control during cantilever construction was one of the main preoccupations.

Due to level difference between pylons and non symmetrical bridge slope, the first cantilever segment has been connected to the pylons through a cast in place joint with an orientation, so that at the end of the cantilever construction the two cantilevers will be at the same required level.

Construction Schedule

The target date was achieved by following the schedule detailed hereafter:

• Design studies: mid December 2005 to end March 2006;
• Piling works: mid February to end April 2006;
• Piers and pylons: March to July 2006;
• Segments precasting: April to July 2006;
• Lateral spans erection: mid May to beginning August 2006;
• Main span erection: end July to mid September 2006;
• Concrete beams: end September to mid October 2006;
• Track and system works: mid September to end October 2006;
• Test and commissioning: mid October to mid November 2006;
• Line opening: 12 November 2006.
• The main contractor for the extradosed bridge construction was GAMMON India Limited.

Conclusion

Thanks to the use of precast segments and only typical construction materials, an innovative extradosed bridge was built in less than one year, including design. This is the demonstration that these economic structures are not so difficult to build, and have a big potential of development for the next years.

This shows also that the U shaped deck concept, already used by Systra for numerous MRT and LRT viaducts [2], can be extended in order to build longer spans.

References

• D. Dutoit, P. Arnaud, “Delhi Line 3 and Santiago Line 4 precast segmental viaducts”, FIB symposium, New Delhi, 2004.
• D. Dutoit, S. Montens, J.C. Chuniaud, P. Arnaud, “U shape prestressed concrete decks for LRT/MRT viaducts”, IABSE symposium, Shanghaï, 2004.