Since last September's World Trade Center disaster in Manhattan, the US has been wary of tall towers. But elsewhere in the world enthusiasm to push the limits seems undimmed. In Hong Kong for example the 420m storey waterfront International Finance Centre, is already halfway to completion.
Across the harbour, foundation work is under way on an even bigger structure, the complementary 100 storey Kowloon 'megatower', a major feature of a cluster of residential and commercial tower developments above Kowloon station, the two columns forming latter day 'Hercules Pillars' at the entrance to Hong Kong's famous Victoria Harbour.
But Taiwan is going even further with a 507m structure that will reach the record books as the world's highest, at least for a while.
The Taipei Financial Centre (TFC) was due for a worldwide public launch on 11 April, but this was deferred because of a fatal crane collapse last month (see News) during a medium size earthquake. The quake recorded 5 on the Richter scale in the Taiwanese capital, and caused two cranes to fall from from the top of the half height tower, killing several workers. It will be two months at least before work can resume.
Initial accounts suggest the structure was not substantially damaged, but a detailed engineering inspection will take some time since the steel frame is currently at the 54 storey level.
Damage may be quite smallstrong seismic forces are one of the primary factors in design in Taiwan, one of the most active tectonic zones in the world. As one expatriate engineer commented, 'few countries' newspapers have a 'yesterday's earthquakes' column underneath the daily weather report'.
The section of the TFC already erected demonstrates how sturdy it will be, with a main structure of eight 'megacolumns' rising two on each side of the building, bigger even than those in Hong Kong where the region's greatest typhoon winds have to be resisted.
Each column is a rectangle 2.4m by 3m which continues to the top, 91st floor, of the 448m main structure. Each is formed as a box section from 80mm thick steel plate and then infilled with superstrength silica fume concrete reaching 70N/mm 2. 'The S570 steel is a new high specification type from Japan, with both high strength and ductility, ' says engineer CP Wang from the lead design firm, Taiwanese architect CY Lee which is working with structural engineer Evergreen Consulting Engineering, part of the four-firm team. 'It will be reinforced inside with both inward projecting fins and number 11 rebar, 35mm in diameter.'
The columns are connected with outriggers from a central steel core structure at every eighth floor, he explains, 'those floors being the mechanical and electrical floors. Each eight floor section will act as a single unit'.
The complexity of the structure is compounded by its stepped form, each section flaring outwards. The building is to be the centrepiece of the new Hsingyi financial and government district which Taiwan wants to make into an Asian Manhattan. Developer is a consortium of 15 major banks and companies, which is building the scheme as a BOT.
The 'upblooming petal style' symbolises economic prosperity, according to a handout.
Floor plates vary from 47m square to 52m square with a larger 63m by 63 in the bottom of the building.
'We tried many forms, ' says Wang, 'and most of them looked so ugly we nearly gave up.' But this one works, he says, particularly when the building is lantern lit from within at night.
The megacolumns will allow transparency 'though that meant a careful choice of glass to match Taiwan's stringent solar gain requirements, Wang adds. 'We have a double low E glass for the curtain walling', which is being supplied by the German Gartner Group.
The steel core is formed from large high strength steel I and H beams, tied via moment connections. There is also bracing in the core area.
None of the 95,000t of steelwork would function without proper foundations however, particularly important in the Taipei basin. A 60m layer of soft silty clays only serves to magnify earth tremors so it was necessary to pile at least 50m through the silt and down another 20m into relatively firm cemented sand.
Some 380 piles of 1.5m diameter support the structure, mostly grouped in clusters. They lie below a five storey basement.
'There is also a six storey podium section where we used single piles for the columns, ' adds Wang.
But all this will still not be enough for the 1/950 year seismic design event and the building will also have a tuned mass damper installed at the 88th floor, just below the observation deck. An astonishing 5.6m diameter solid steel ball will be the 'pendulum weight' for this device.
'We also have to think about typhoons, ' says Wang. Forces are a little less than in Hong Kong, but a 40m/s wind speed must be catered for. A particular problem is fatigue effect on the slender pinnacle where vortex shedding is calculated to cause more than 3M vibration cycles during its lifetime.
Floor areas in the last 60m of the tower are greatly reduced in size, the tower tapering to the top, 101st floor 'which may be a private observation platform', speculates Wang.
Until the recent accident, construction had been fairly straightforward, says KK Pang, construction manager from the Hochtief subsidiary Turner International, which is acting as project manager for the NT$50,000M scheme. Land value is about NT$26 000M and construction cost is a little over half (US$700M).
Contractor is a joint venture of Japan's Kumagai Gumi, in partnership with its own local subsidiary Kumagai Taiwan, Taiwan's largest firm RSEA Engineering Corporation and a fourth, local, partner.
Main site problems are vertical, solved by using a large Schwing pump for the concreting through two riser tubes, and four big cranes on atop the structure for much else. Alimak hoists carry workers and materials.
The cranes are from Favco - the largest M1250DXs originally designed for oil rigs and able to lift 100t at 22m radius, the weight of a two storey megabeam section weighs. Two smaller cranes are thought to be the ones that fell.
'Welding was our other problem, ' says Pang. 'There was a struggle in the fabrication shop in the south of Taiwan because the steel must be preheated to 100'C and it is hard to operate close to that.'
The yard discovered a system which would allow welding to be carried out from a distance of 2m. 'But even so the inside of the boxes is hard to do.'
On-site welding, by subcontractor Nippon Steel with China Steel Structure, also had problems with pre-heating, though this had been overcome and floors were going up at the rate of four a week before the earthquake brought work to a halt.
Going strong in Hong Kong
Hong Kong's $413M north east tower for the International Finance Centre is at roughly the same stage as the Taipei project, with just over 50 storeys of steelwork completed, though it will reach a 'mere' 88 storeys and 420m full height (NCEI July 2001).
Even more than the TFC the structure uses 'thundering great outriggers kicking out of the building at three levels', explains Craig Gibbons for structural engineer Ove Arup. These act like 'skier's arms' to transfer forces into eight mega columns, giving clear open views for the offices, which will command a premium rent from the financial institutions which are expected to be the tenants.
The Hong Kong building uses much more concrete than Taipei.
The core section is a complex slipformed grid and the building's eight mega columns are a composite of six steel H sections encased in concrete.
Arup thought through its design strongly in terms of constructability, says Gibbons, and part of this process was devising a way to construct the outrigger 'belt and bracing', which is tied into the central core, without stopping the forming work.
The outrigger's steel sections each extend over three storeys with the extra steel and core wall thickness reaching over five levels.
'Normally this would interrupt the core work while the steel was integrated into the walls, ' says Gibbons, 'but we devised a way to reduce the wall thickness and continue above. The steelworkers then move in and weld the steel elements into position after which the core wall is poured to full thickness around them.'
Full thickness is 1.2m and the critical three storey section has to reduce to 300mm as the cores rise - easier said than done.
Contractor E Man Snaffled JV Construction had to develop a special climbing form for the core, expalins project director Kenneth Mo. Since the face of the concrete is inset, the form had to adjust.
'And it had to be supported with special steel elements underneath during these pours because the outside frame no longer has concrete beneath it.'
Additionally there are five storey high steel posts embedded in the core at the corners which the formwork has to pass inside. An adjustable - and suitably sturdy - was devised in conjunction with SGB says Mo.
Sufficient strength is available from the internal cruciform walls of the core to allow the form to keep climbing on upwards while the steel welders move in, and to ensure that the form remains stable even during the typhoons that beset Hong Kong each summer. A design wind speed of 64m/sec is used for the permanent structure.
Working in the inset space was complex for the steel contractor NKK Corporation, says its project manager Takashi Aihara. Not only did welders have to keep out of the way for the concreters but they had a problem bringing in the big steel sections, some in 100mm plate and over 1m deep, because the inset created an overhang. Big Favco cranes are favoured here for lifting, as in Taiwan - transferring to block and tackle for final positioning. 'And it has to be very accurate, ' says Aihara.
Fixing dense reinforcement around the bracing elements was also complex and meant coming back for re-concreting says Mo, which has complicated programming.
Eight further specially developed climbing forms from VSL are in use too; one each for the megacolumns to pour infill and encasing concrete around the H sections.
Where the steel outriggers are connected into 'windows' in the columns, differential axial shortening from concrete shrinkage and creep, and from loading effects, can be quite complex, ' says Gibbons. Despite 'a lot' of tests to devise low shrinkage mix designs the outrigger connections have to be adjusted at intervals.
The links into the columns are therefore adjustable by jacking and shim plates until construction is completed.
When NCEI visited the site, the second of three outriggers was almost installed with a third lighter 'belt' due to go in above. Client Central Waterfront Properties, should receive its elegant Cesar Pelli & Associates designed tower in 2003. Local firm Rocco Design was joint architect.
Many of these same problems will beset the 474m high Kowloon megatower which will have a similar structure to the IFC northeast tower, though with four layers of outrigger rather than three. But for the moment attention is focused on the foundations which unusually will rely on friction piles.
'Unlike the Hong Kong side where we could sit on bedrock 20m down, ' says Arup project manager Jackie Yau, 'here we have a fault creating a deep valley in the bedrock granite which is 130m down depth.'
A nasty surprise from the detailed site investigation was that the valley extended further than anticipated, covering most of the footprint, and the only option is to use friction piling.
Arup is currently working with Bachy Soletanche testing a number of 70m deep barrette structures for these; the 1.5m by 2.8m rectangular sections give more surface area for friction.
'Actually we take out the first 20m of fill and use the space as basement' say Yau so the piles drop below that. A total of 245 piles at 2m spacing is forecast as the likely design.
The foundations will affect the behaviour of the entire structure, he says. The friction effect means the piles and ground are 'flexible'and will move and turn along with the building in response to typhoon wind load. Instead of considering a structure 474m high, the whole 630m of the building, including the foundations, must be allowed for. This means factors such as inter-storey drift, which can affect the cladding - 'and resonant frequencies, are different' he adds.
'The resonant period of the building is longer and that can mean greater interaction with certain types of gusting where wind loads become cumulative.'
Also, the building tower has a squarer section without the rounded corners of the north east tower, he adds. This means vortex shedding is greater and building vibration will increase.
These factors are currently the main preoccupations of the team as the design is finalised.
Foundation work is in progress and the main contract is likely to get going in about 15 months time.
Client is Harbour Vantage Management part of the Sun Hung Kei Group. Cost of the structure, without cladding could be around HK$2bn.