Steely grey in the early morning gloom, the Tsing Ma suspension bridge, to be officially completed today, stands like some surreal sentinel guarding both gateways to Lantau Island and urban Kowloon. Yet nobody who has seen the bridge can fail to be impressed by its elegant structure - more than 300,000 tonnes of steel and concrete soaring 206 metres into the air between Tsing Yi and Ma Wan. Spanning nearly two kilometres of open sea, the $7.2 billion structure is, at 2.14 kilometres long, the world's longest combined road and rail suspension bridge. Yet despite its tremendous strength, people gasp at what seems the bridge's flimsiness. They wonder in awe how millions of grains of sand, cement and stone bonded with a kind of water-based super adhesive can stand firm in slender columns that reach for the sky. When they see these towers apparently supporting thousands of tonnes of steel plate and wire that form the deck and cables, non-engineers must ponder how it does not collapse. Yet the bridge, properly maintained and barring unforeseen disasters, is designed to last 120 years and will not need replacing until 2117. It is truly a tribute to man's engineering skills. These have already been partly recognised by construction artist Tabitha Salmon who spent several months casting the bridge and its creators in oils for steelwork subcontractor, Cleveland Structural Engineering. 'It's a friend now, while in the beginning I didn't understand it,' she said a year ago, part way through the assignment. 'All the time you have a feeling of wires and strings, and sometimes there is a humming sound that resonates through it. And when the deck sections are lifted up by cranes, there is a sound of sighing.' The bridge has already featured in thousands of tourist photographs, played host to visiting dignitaries including Baroness Thatcher, acted as the backdrop in movies and become a marketing tool for local real estate agents. Its fame is set to widen as more than 30,000 travellers a day pass on or under it when it opens to traffic at the end of May. Top design and civil engineering prizes are bound to follow as international groups recognise the huge effort that Mott MacDonald, the bridge designer, and Anglo-Japanese Construction, the contractor, put into the construction. About 66,000 cubic metres of concrete were used to form the massive towers, the foundations of which were excavated in a socket of rock 28 metres long, 20 metres wide and seven metres deep. From ground level, technicians used a mould called a slipform to cast the towers which grew about two metres a day and reached their maximum height after about six months. In fact the towers act like props holding up a clothes line. It is the cables, anchored in massive 50-metre deep underground chambers behind the bridge towers, which effectively hold up the main steel deck via 500-tonne saddles secured to the top of each tower. One hundred and sixty-eight cubic metres of concrete were used to form the two anchorages. The first physical link between Lantau and Kowloon since the Ice Age occurred in 1994, two years after work started, when the first wire was pulled across the Ma Wan Channel to link the twin towers. This cable formed the start of a catwalk which staff used to help guide a pair of self-propelled wheels which pulled thin strands of wire across the great divide. About 33,400 strands of wire, enough to circle the Earth 4.5 times, were used to form bundles of thicker cable that was coated in red paint to weatherproof them against the elements. These bundles were then grouped together to create the one-metre thick suspension cables. Hangers, vertical cables which secured each bridge deck section, were clamped to the main suspension cables. With the hangers in place, technicians fixed the steel double-deck sections. Ninety-six segments each weighing 500 tonnes were made in a fabrication yard near Dongguan and floated down the Pearl River Estuary by barge to the Tsing Ma site. They were then welded together to form 48 units of 1,000 tonnes. The first combined unit was lifted into place in the middle of the 1,377-metre long main span in March 1995. The final segment was hoisted just over a year later in March 1996. A little over 13 months was allowed for final welding, road surfacing, installation of trackwork and associated services before Baroness Thatcher officially marks the completion of the structure today.