THE towers and approach supports of one of the world's longest suspension bridges are now underway in Hongkong at the western edge of Tsing Yi island. The rectangular concrete approach supports in the foreground are being constructed using the ''climbforming'' method, while the Tsing Yi towers use the slipforming method. Now a beehive of activity, the site is alive with concrete batchers from Mitsui-Costain, earthmovers and foundation specialists from Gammon and tower erectors from VSL. By early 1994, the four main concrete towers of the Tsing Ma bridge will rise to their full height of 206 metres (taller than Jardine House), paving the way for the cable spinning process. 37,000 strands of five millimetre wire will be continuously spun back and forth across the tops of the towers and to the anchorages from the Tsing Yi to the Ma Wan side, until they combine to form 1.15-metre diameter main suspension cables. The bridge deck sections, which have enclosed lower decks to accommodate the MTRC's Airport Express and Lantau Line trains, are being fabricated at a site up the Pearl River in China, will be floated in on barges, hoisted into position, and hung by suspender cables.