In contrast, the internal spaces, with their careful detailing and lighting design, reflect the human scale. The voids round the piers afford views between the different levels and ensure that the building remains comprehensible, despite a floor area of 410,000 m². On the north side, sections of the structure are cantilevered out by up to 40 metres to cover the large drop-off area. The roof, therefore, forms a new public baldachin for Mumbai.
The design responds to the challenges posed by the tight confines of the site, which is located at the heart of the metropolis. The proj-ect also reflects the fact that the new complex had to be built immediately next to the existing terminal – which had reached the limits of its capacity – without interrupting operations. The X-shaped layout facilitated a division of the development into a number of sections and allowed the work to be executed step by step around the existing building. In turn, the latter could be successively demolished to make way for the next stage of construction. The most far-reaching constraint imposed by the confined site, however, was that the four main functions for regulating passenger traffic – access and drop-off, check-in, security checks and passport control – had to be accommodated beneath a single roof, where the needs of both domestic and international flights would be met (ill. 3).
This structure, almost 70,000 m² in area, is borne by only 30 composite mega-piers, which are laid out in five rows (ill. 10). The layout reflects the roughly trapezoidal plan form of he roof. A broad column grid at 64 × 34-metre centres divides the hall into areas that are large enough to accommodate all in-built elements and functional processes without divisions. The grid was adapted to the specific needs and loading of each level: starting at 4 × 9-metre centres on the lowest level, the spans are increased in the luggage hall above and even further in the shopping area on the next floor up. Finally, in the departures hall, there are broader spaces without intermediate columns. Here, the composite piers swell like mammoth trees to 40 m in diameter where they merge with the ceiling (ills. 1, 2).
To construct what is one of the world’s largest roofs without expansion joints, the composite piers and the steel space-frame roof were completely separated from the plinth structure (ill. 7). The roof can thus move independently in response to wind loads and temperature changes. The large openings formed in the concrete construction around the piers – as part of this separation of roof and plinth –
also allow natural light to penetrate to the lower floors. To facilitate a rapid system of construction, the load-bearing roof structure was nevertheless divided into modules. A further aim of the overall concept was to minimize the amount of steel used. The structure consists of large-span steel girders in an orthogonal layout and functions in a similar way to a flat-slab or “mushroom construction”. The thickness of the girders increases towards the heads of the piers, where additional members were incorporated at an angle of 45° to the orthogonal system. The dimensions of these diagonal members, which connect the rectilinear girder grid to the piers, vary according to the specific loading (ills. 8, 9). As the moment diagram shows, the pier heads subject to the greatest loading occur in the cantilevered areas at the edges of the roof. Even here, though, it was possible to maintain a uniform girder depth of only four metres, which is surprisingly little for spans of this size (ill. 11).
The 30 mega-piers are the key to the structural efficiency, the open spatial character and the iconographic monumentality of the building. They bear not only all vertical loads from the roof, but also horizontal loads caused by wind. Nevertheless, they are designed with extremely slender dimensions. This was made possible by a composite form of construction: a cruciform steel section encased in concrete proved to be the most effective solution (ills. 12, 13). The reinforced concrete provides compressive strength and protection against fire, while the steel bears tensile stresses and resists bending moments. Each pier is 40 m high, and the diameter of the concrete core is 2.7 m. The concrete casing terminates at the level where the girder grid bears on the piers, allowing the steelwork in these two elements to be connected. The large piers also served as primary lifting members during the assembly of the trussed framework. As a result, it was possible to erect the entire building without tall tower cranes.
For visitors, the synthesis of architecture and load-bearing structure is most clearly evident internally in the cast glass-fibre-reinforced concrete cladding. A 64 × 34-metre column grid with 24 × 12 elements to each bay, resulted in dimensions of 2.66 × 2.83 m for each rectangular soffit unit. The rigidity of the elements is attributable to additional bracing along the edges and the three-dimensional form, with a constructional height of 540 mm. The rectilinear edge extends in conical form towards the circular opening above, which has a diameter of 442 mm. A circular pane of satin-finish laminated safety glass was point-fixed on top. This allows daylight to enter through the roof and also serves to diffuse artificial light. Between the upper and lower panes of glazing, are segments of film of different hue 125 mm in diameter that lend the top lighting a restrained bluish-green coloration from the heads of the piers up to the horizontal areas of the roof (ills. 15, 18).
To achieve a perfectly smooth geometric transition from the horizontal roof areas to the vertical casing around the piers and rainwater pipes, 80 different double-curved elements were required for each square pier head. These range in form from narrow two-metre-long lozenge-shaped units to six-metre-long triangular elements (ills. 16, 19).
The design provides a high degree of transparency and natural lighting, which not only help to create a sense of orientation and wellbeing among passengers, but serve to increase energy efficiency. The numerous roof lights are complemented by a 900-metre cable-net glazed facade – the longest in the world – which runs round the entire edge of the roof, lending it the appearance of a floating structure. Between the approach area and the check-in hall, the cable-net facade extends from floor to ceiling, forming a filigree transparent membrane that allows interior and exterior to be experienced as a unified whole. Along the edge of the sloping roof that rises to the south over the wing structure, the cable network is designed as an uninterrupted clerestory (ills. 23 –25), thereby obviating the need for artificial lighting during the day and resulting in an energy saving of 25 per cent.
