The 14.5m high precast concrete wall panels used in L&T Factory at Bangalore are the largest single piece exterior cladding wall panels in India till year 1976. Each wall panel is 2.5m wide channel section having web thickness of 50mm. The exterior cladding of a factory having 50mm thick concrete presented the problem of how best to insulate this surface against solar heat. It was also not desirable to leave the exterior face of the panel as a smooth surface, since a known phenomenon is that wind can drive a sheet of rain water across the surface and into the joints between panels. An additional requirement stated by the Client was that they would like the factory to have as little maintenance as possible. The solution devised was to use half-bottles to cover the exterior surface which provided the necessary qualities required as follows:

• They would provide insulation to the 50mm thick concrete.
• They would provide vertical ribbing, effectively forcing rainwater downwards and preventing it from being driven by the wind into the joints between panels.
• They would provide a glazed and therefore maintenance-free surface, which would be washed clean with every rain.
• They were the cheapest form of external applied finish available at the time.
• They would provide a rich colour to the exterior of the factory, the colour varying depending upon the angle of the sun, and providing a contrast at the roof line with the blue or grey sky above.
• The waterproofing of joints between precast panels was achieved by strong inclined ribbing on each face of the panel which opened on the vertical construction joint. The ribbing would run from the inside downwards towards the outside so that any water penetrating the joint and forced into it by the wind would be caught by the inclined grooving and forced outwards and downwards by gravity. Standing inside the factory if one aligns oneself with a joint, one can see sky through the vertical joint, yet no water penetrates.

Precasting has significant advantages, particularly in terms of the economy of materials required for construction. The entire roof of the Hydraulic Excavator Factory at Bangalore for Larson & Toubro Limited is made of Pre-tensioned Hyper shells of 50mm thick spanning 20m and 2.5m wide.

It was the longest span in the world for this kind of element at the time of construction, 1975.

One of the difficulties with stressed skin construction is that the erection of the inclined trusses can be quite complex, and very often the finishing work (which is done up in the air, with little or no inspection) is of very poor quality.

Consequently, what was attempted for HAL in Bangalore was a system by which the entire roof covering area of a 90m x 45m was fabricated and assembled on the ground. All the joints were thoroughly checked, the structure was fully painted, and light fittings were also installed.

Thereafter, using ordinary 25 ton screw type truck jacks (of the type used by trucks for repairing punctures), one mounted on top of each column, the roof was jacked vertically into its final position for final seating and fixing. This arrangement provided the requisite quality of construction, while losing none of the advantages of lightness of the finished roof.

The principle of using northlighting for factories is derived from Europe, where sunlight is occasional, and the northern sky is the best place to look for shadowless lighting which is more or less uniform through the daylight hours.

In India, where direct sunlight is available most of the year round, it seems a pity to forgo the brightness available from this direct sunlight. Unfortunately the problem with traditional skylights in that they allow the sun to enter the factory in a way that produces a shifting patch of sunlight on the floor. Any worker who finds himself in that patch of sunlight feels uncomfortable from the direct heat.

If a means could be found of diffusing this direct sunlight, it would provide much brighter yet well diffused light over the entire floor. Suppose the skylight opening could be replaced by a lens as shown in the diagram. This would cause the direct sun’s rays to be scattered over the floor below. Obviously a single lens to cover an entire skylight opening is not practical. However, the diffusion would be equally well provided by a series of smaller lenses side by side. If instead of plain glass for the skylight we were to use a ribbed glass, which in effect provides a series of adjacent lenses it would have the desired effect. Note that if the skylight is in the form of a narrow strip, then it is essential that the ribs run parallel to the strip, because the ribs will diffuse light in one direction only and not in the other.

Above principle of diffused light was used first time in 1982 for Factory at Bangalore for Larson & Toubro Limited.

In India, given the relative costs of concrete and steel for columns and other compression members, it is much cheaper to use concrete rather than steel. For beams and roofs, which suffer bending, prestressed concrete is again equally competitive when compared with structural steel.

For BHEL Factory at Hyderabad, all structural elements used are precast concrete.

Purlins and cladding runners are 12m span precast pre-tensioned concrete. These elements were longest in India at time of construction, 1982.

Roof Trusses are post-tensioned spanning 30m. Conventionally the truss shape has a straight bottom tie. Particularly when the tie is prestressed, it is invariably straight. But the logical shape for a tie with constant stress would be to follow the shape of the bending moment diagram so a funicular profile was adopted, and the prestressing cables follow this profile. The trusses were cast flat on ground, tilted and lifted into position.

Columns are double branched concrete columns. Traditionally double branched columns are of the Vierendeel type. Since the columns were to be precast, these were made of the trussed type resulting in a reduction in the weight of the columns. Each column was 17m high, 1.85m wide and 0.9m deep and weighted 29 tonnes. Branched columns support post-tensioned concrete gantry girders of 12m span.

The elevated viaduct in Dwarka sub-city is located on a 30m wide strip of land made available for MRTS. The elevated viaduct caters for two tracks with a centre to centre spacing of 4.10m. The alignment crosses six major roads, one major drain and an oil pipe corridor. The soil investigation revealed that open foundations located at about 1.8 m below ground level could be adopted. The main consideration for span configuration was the clearance required for road crossings and bearing in mind that open foundation have to be used.

A 61m module with a span configuration of 13.5m + 16.5m + 16.5m + 13.5m proved to be the best choice as this met with the requirements of various road crossings and could be used typically for the entire length so as to obtain maximum repetition. Typical cross-section consists of two full span post tensioned voided girders integral with slender circular piers and cap cables for providing continuity.

The entire stretch of 6.47 km long viaduct in Dwaraka City, Delhi, is free of bearings, due to integral form of piers & deck with expansion joints located typically at 61 m. The viaduct is built using full span precast post tensioned voided girders.

Roof Trusses are post-tensioned spanning 30m. Conventionally the truss shape has a straight bottom tie. Particularly when the tie is prestressed, it is invariably straight. But the logical shape for a tie with constant stress would be to follow the shape of the bending moment diagram so a funicular profile was adopted, and the prestressing cables follow this profile. The trusses were cast flat on ground, tilted and lifted into position.

Columns are double branched concrete columns. Traditionally double branched columns are of the Vierendeel type. Since the columns were to be precast, these were made of the trussed type resulting in a reduction in the weight of the columns. Each column was 17m high, 1.85m wide and 0.9m deep and weighted 29 tonnes. Branched columns support post-tensioned concrete gantry girders of 12m span.

This flyover 5.75 km in length running through Jaipur. About 1.75 km of the flyover is a double deck construction supported on single central pier. Double track metro rail runs on the upper deck and a four lane road traffic runs on the lower deck. Span configuration varies to suit the alignment with maximum spans of 37m reducing to 25m on sharp curves. One of the challenges was to evolve a solution that would permit the same erection machinery to be used for construction of both the decks.

The solution finally chosen was precast segmental construction for both the decks. Amongst other benefits span lengths could be increased to 37 m which was an important consideration considering cost of pile foundations. The metro rail would be supported on standard DMRC segmental box design used in Delhi and the lower deck would be supported on precast segmental spine girder. The dimensions and weights were comparable to that of the standard DMRC segmental box and therefore same erection machine could be used for construction of both the decks. Precast cantilever arms would then be attached to the spine girder by prestressing and cast-in-situ stitch concrete. The spine girder is 5.6 m wide at the bottom tapering to 4.6 m at top and is 2.6 m deep. The segments are typically 3.0 m long except for end segments which are 2m m long. The weights of the spine girder segments for four lane flyover varies from 45 to 80t and weights of metro box segments varies 40 to 50 t. Weight of 3m wide precast cantilever arms is maximum 17.5 t. The cantilever arms are integrated into the spine girder by post-tensioning and cast in place stitch concrete.

This is first application of double deck flyover in India.