BUILDING ON AIR.

When the turnpike was extended into the center of Boston during the 1960s, it not only used up a great deal of land, but left an odd shaped parcel of almost 10 acres lying fallow along its right of way. The master plan - worked out by the architects and engineers and at times modified after public meetings - placed the hotel towers on the land parcels and the low rise part of the complex spanning the turnpike, the ramp and the railroad. The configuration of the lower structural system was dictated by the geometry of the site and the spots available for driving the piles. Some of the plate girders are as long as 102 ft (31 m) and weigh up to 120 tons (1,088 kg). They vary from 3 to 10 ft (1 to 3 m) in depth and no two are identical. Both to speed construction and to permit minimum floor heights in the parking levels below, the mall floor was constructed first, supported directly on 50 ft (15 m) columns. The garage floors are reinforced concrete flat plates supported by concrete column casings. More than 50,000 people enter the complex each day to shop, work, eat or patronize the hotels. Neiman-Marcus heads the list of more than 100 specialty shops. There are also 13 restaurants, a nine-screen cinema, banks and landscaped plazas.