As London architects and engineers work on what will be Europe’s tallest building, Jon Excell investigates the challenges of going high-rise after 9/11.
The mighty skyscraper. Stretching upwards, dominating the skyline, dwarfing its neighbours and inspiring awe. It’s hard to imagine a more potent architectural symbol of wealth and capitalist aspiration.
And if one of the aims of September 11, 2001 was to consign such symbols to dust then it has failed conspicuously. For the worlds’ architects, planners and civil engineers are apparently determined to scale ever more dizzying heights.
Today — from New York to Hong Kong to Dubai — an unprecedented number of staggeringly tall buildings are on the verge of making their mark on the skyline. And UK engineers aren’t about to be left gazing enviously skyward either — full planning permission is now in place for the 70-storey, 310m-high London Bridge Tower.
London’s latest high-rise project won’t exactly dwarf the proposed 541m-high Freedom Tower (the likely successor to the World Trade Centre) or the current holder of the world’s tallest building mantle (the 508m-high Taipei 101). But the Shard of Glass — as it is affectionately dubbed by those working on the project — will, at its projected 2010 completion date, be the tallest building in Europe.
Paid for by UK property developer Irvine Sellar and designed by Pompidou Centre architect Renzo Piano, the Shard is poised to enter its next design phase. A fifth of its occupancy is now spoken for, with part of the building a luxury hotel, part residential and part offices. A number of discussions are underway with potential major office tenants, and the project team is confident of the 2010target.
Clearly for any truly tall building the civil engineering and construction challenges are acute. But such structures also call for all kinds of cutting-edge technology — from the techniques used at the design stage, to the systems that will keep the building functioning and make it safe for occupants. The Shard, claim its developers, will be no exception.
As well as using far less energy than comparably tall buildings, the structure will also be one of the first skyscrapers built since the publication of the US National Institute of Standards and Technology (NIST) report into the World Trade Centre collapse. It will, say theShard’s engineers, showcase an approach to fire safety engineering that will leave many taller buildings standing in its shadow.
Mark O’Connor, a structural fire-engineering specialist at WSP Cantor Seinuk, one of the chief consultants for the Shard, explained that the building is being designed to withstand a complete burnout. ‘The NIST report said that instead of structures being designed for fire resistance you should be assessing the worst-case scenario and building structures to withstand complete burnout.’
To do this his team is applying an analytical process in which it examines a range of different fire scenarios, applies them to structural models and assesses the response of these models to the fires. According to O’Connor, this is radically different from the approach conventionally used on high-rise buildings. ‘Usually an architect finds out what fire resistance is required from a book, going to a table to see how much protection he has to apply,’ he said.
At the heart of the structure is a reinforced central concrete core containing the lifts and stairs. Surrounding this are reinforced floor plates. As the building tapers, the columns that support these floor plates are transferred to other columns, and much of the work done by O’Connor’s team has focused on the behaviour of these all-important transfer structures.
To calculate and simulate their response to different fires, the team has employed a number of techniques.
These range from traditional semi-empirical calculations to more sophisticated ‘zone model’ computer simulations that take account of the fact that a fire grows in size, and Computational Fluid Dynamics (CFD) software that can simulate the flow of air and hot gases around the scene of a fire.
The group has also employed Finite Element Analysis (FEA) software to look at those areas of the structure affected by a potential fire in even more detail. O’Connor claimed that while FEA analysis was used as a forensic tool by WTC investigators, very few engineers are employing it in this capacity.
FEA has proved particularly useful in the design of the structural elements at the base of the building which are especially vulnerable to attack by terrorists. O’Connor said that by analysing and simulating the behaviour of these structures, the design team is likely to specify the use of blast-resistant resin-based coatings of the kind usually applied in the offshore sector. ‘One of the problems with the WTC was that a lot of the fireprotection materials were quite friable — they were simply removed, and the structure was left unprotected. But epoxy-based intumescent coatings have some adherence and retain the fireprotecting capacity if subjected to a blast,’ he said.
While O’Connor’s team has been focusing on the behaviour of the structure, others have been looking more directly at the safety of the occupants. Alistair Guthrie, a director of building engineering at Arup, the other major consultant on the project, said that one of the most challenging aspects of developing the building is that that it has mixed usage, consisting of a luxury hotel, office space, residential units and public viewing galleries.
He explained that this presented particular problems when the design team met to decide on a strategy for evacuating the building.
‘You have to layer different usages of escape and of access all on to the same core,’ he said, adding that the evacuation and access problem has tended to mitigate against the construction of mixed-use buildings in the past.
After analysis the group took the unusual move of making the lifts and elevators central to this evacuation strategy. While the contract for the lifts is yet to be awarded, Guthrie said that the challenges of the skyscraper will call for an extremely rigorous design with emergency generators, and a fully firerated ventilated shaft. The building will use 13 double-deck lifts, and 31 singledeck lifts, all of which will travel inside the central core and serve different areas of the building. The double-deck lifts, featuring sets of doors that open simultaneously on adjacent floors will be used for evacuation, he said.
The design of the elevators is also likely to be influenced by the building’s tapered shape, said Guthrie: ‘As the towers get higher the central core becomes a larger part of the floor plate, and the most important thing on this type of building is to minimise the area taken up by the elevators — otherwise there’s nothing left to rent.’
Clearly, though, getting into a lift may not be the first thing someone decides to do in the event of a fire. After all, it’s been drummed into most of us that we should use the stairs in the event of a fire, and a key element of designing the elevator system has been factoring in the kind of choices that terrified occupants will make in the stress of the moment.
Anthony Ferguson, a fire specialist at Arup, has been charged with modelling and anticipating the behaviour of the building’s occupants in such scenarios.
He has been using a range of techniques from simple hand calculations to computer modelling. This includes use of innovative Arup–developed software that simulates the 3D movement of people.
Known as Simulation of Transient Evacuation and Pedestrian Movements (STEPS), this software enables engineers to examine the flow of virtual people around a building in different fire scenarios. It is typically employed in conjunction with 3D smoke and fire modelling using CFD software. As well as helping to verify the lift-evacuation philosophy it has also, claimed Ferguson, enabled the designers to eliminate potential bottlenecks by widening corridors and moving doors.
But such sophisticated software tools are not without their limitations, and the unpredictability of human behaviour means there’s often no replacement for traditional expertise.
‘Although these software models are getting a lot better than they were, if you have a situation where people have choices it’s difficult to model. You can tell the software every third person will decide to do such and such but in the end you find yourself setting up so many such rules that all the computer is doing is drawing you a picture of what you told it to do,’ said Ferguson.
The building is still at the relatively early design stage and, understandably, to convince doubters and gain the necessary planning permission, the focus has been on safety. But the project’s engineers have also found time to develop a building that is impressively energy efficient. Indeed, it is claimed that it will use 30 per cent less energy than other tall buildings.
‘Every City of London office block generates more heat than it needs,’ claimed Guthrie. Many buildings spend vast amounts of energy on rejecting excess heat — much of which is generated by the cooling systems. The Shard, in contrast, will route the heat generated by the cooling systems into a heating circuit that will feed into the hotel and residential units.
Any excess heat will then be dispersed by a giant radiator that effectively forms the top 27m of the tower. This radiator, which will account for around 60 per cent of the building’s heat rejection over a year, will consist of a matrix of largediameter finned pipes and tubes through which the wind will blow and remove the heat. Guthrie said that it’s likely that these will be designed and built by Arup.
The preliminary designs are impressive, the artist’s impressions are stunning, but the future of London’s embryonic new landmark now lies in the hands of the property men who must quickly find some more tenants.
In the meantime, the design team is chomping at the bit to further develop the systems and technologies that will take their building to the next level.
The rise of the elevator
Advanced construction techniques, high-strength building materials and innovative architectural approaches have enabled civil engineers to scale ever more dizzying heights with buildings. But the success of the skyscraper owes a huge debt to the attendant advances in lift technology.
While the gearless traction electric elevator that made skyscrapers feasible first appeared by 1903, today’s elevators have perhaps more in common with the latest theme park rides than their sluggish forebears.
These advances are encapsulated by the system in place on Taipei 101, which at 508m high is the world’s tallest building. Two of the 61 elevators installed in the skyscraper run at a staggering 1,010m per minute (60.6kmh). These high-speed double-decker demons are, by a considerable margin, the world’s fastest.
Developed by Japanese engineers at Toshiba Elevator and Building Systems, the lifts feature the world’s first elevator-based pressure control system. This adjusts the atmospheric pressure inside the lift by using suction and discharge blowers, preventing occupants experiencing ear popping.
At the design stage the elevator’s developers called on techniques and stylings more familiar in the rail industry, carefully analysing and tweaking aerodynamic designs to reduce the whistling noise produced by the lift running at a high speed inside a narrow hoist-way.
Such high speeds also generate vibrations that can’t be banished by conventional vibration-proofing construction. Toshiba’s engineers, therefore, developed an innovative active control system that harvests vibration information from a series of sensors and cancels out these vibrations by moving a counter mass in the appropriate direction.
According to the company, the system is particularly effective at reducing the vibrations caused by the wind pressure when one lift passes another.
And just in case the worst happens, the lifts are also hooked up to the building’s sophisticated seismic and wind-monitoring systems and will automatically stop at the nearest floor in the event of dangerous conditions.