By 2006 Hong Kong harbour will have two towering 'gateposts', each more than 400m tall. Dave Parker reports.
There is a pleasing symmetry about Hong Kong's two megatower projects. Both sit on reclaimed land, both dominate massive development projects centred around new rail stations. Similar structural solutions are adopted for each, for the same reasons. The only real difference is that the $413M, 88 storey tower currently under construction close by Hong Kong's Central Station will be a mere 420m tall, some 70m shorter than its 100 storey cousin across the water in Kowloon.
Site investigations for the latter have already begun (see box). Structural engineer Arup is well advanced with the design of what is prosaically dubbed the Hong Kong Station North East Tower, with construction of the central core approaching 10th floor level. So far the foundations have posed the biggest challenge, as Arup director Paul Tsang explains.
'Rockhead beneath the building is more than 35m down, approaching 47m locally and sloping from east to west. We decided to found the building on a 6m deep concrete raft poured directly onto bedrock - which meant a very large cofferdam.'
Dubbed 'the biggest hole in Hong Kong', the cofferdam was 61.5m in diameter formed by a 1.5m thick by 35m deep diaphragm wall stiffened by four insitu concrete ring beams.
Eleven main pours of C40 concrete were needed to form the raft - the first 1m deep, then 10 1m deep layers split into two halves with the pours in each layer orientated at 90degrees to the previous layer. Crushed ice batching and cast-in cooling pipes kept concrete temperatures to safe levels.
Designers of very tall buildings have a number of wellestablished options for resisting wind forces, invariably the biggest structural challenge.
From the point of view of the designer of a commercial office development in Hong Kong, many have practical drawbacks.
'The client wanted an all glass facade, which ruled out closely spaced perimeter columns, ' Arup associate director Alexis Lee reports. 'And external cross bracing as used in many tall buildings is seen as bad Feng Shui here, because the crosses stop the money flowing in. So we came up with the megacolumn and outrigger concept.'
At the centre of the 56m square floorplate is a relatively conventional 27m square concrete core with walls up to 1.4m thick housing lifts, stairs and services. Opposite each corner of the core is a perimeter 'megacolumn', giant 3.5m by 2.5m composite members made up of up to six steel I-sections fabricated from 90mm thick steel plate encased in C60 concrete.
Composite floors are supported by 457mm by 152mm steel universal beams spanning up to 12m between the core and megacolumn. Every 20 floors or so the core is linked to the eight megacolumns by fabricated double I-section steel outrigger trusses, three floors deep, which are fitted into recesses formed in the locally-thickened core walls.
'There obviously had to be many openings in the core wall, ' Lee says. 'So, to get full transfer of wind loads from core to column, the outrigger trusses had to wrap around the core.'
The ends of the outriggers are then linked by a steel 'belt truss'. Apart from optimising net lettable floor area - more than 200,000m 2- the 'notched square' footprint creates the maximum number of high premium corner office spaces.
These outrigger floors are used for services and plant as well as acting as fire refuges.
Practically, there is one tricky problem with the structural concept, as Lee explains.
'We calculate there will be a differential shortening between the core and the megacolumns due to load and shrinkage. So we had to design a 'movement joint' between the outriggers and the megacolumns, incorporating shim plates, to take up the difference.'
Detailing the 'very complex' transitional areas, where the steel cores of the megacolumns reduce in section at the connections with the outriggers, was another challenge, Lee adds.
'Steel connections were preferred due to the low price of steel at the time.'
Construction of the core is now past the 50m level with overall completion scheduled for the end of April 2003. By that time the North East Tower's taller cousin will be well advanced on the Kowloon shore. Hong Kong's already spectacular skyline will have an even more dramatic impact, one that will mark it out from other great waterfront cities of the world.