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Of the many tall-building proposals vying to transform London's city skyline, KPF Architects'Heron Tower seems the most likely to be realised in the near future. With work projected to begin in 2007, the tower will stand as an exemplar of bold new approaches to tall-building design.

'We would like this to be the Chrysler Building for the City, ' says KPF's Fred Pilbrow about the Heron Tower: 'Beautiful, but not the biggest.'

What distinguishes the 46-storey building even more than its looks is the fact that it is certain to be built. Main contractor Skanska has been appointed, and construction is due to begin in July 2007. Yet since it first achieved planning permission in 2002 (permission for a revised design was granted in 2005), several other London towers have also received the go-ahead - but with little sign of plans to build. The dramatic scaling down of the Minerva Tower ( ajplus 01.08.06), is just the latest in a series of setbacks.

While Pilbrow, the director in charge of the project, credits its success to the design, he also thinks a lot of it is down to the developer, Heron Corporation. Heron's normal modus operandi is speculative development, whereas many London developers like to work on the basis of pre-letting. Pre-letting does not tend to work for tower buildings, Pilbrow believes, because:

pre-let tenants want a building delivered more rapidly than is possible for a tower; and

most pre-let tenants are looking for large oorplates, whereas towers tend to be slender with relatively small oorplates.

A developer like Heron, willing to take the financial burden of borrowing money for the three-and-a-half years of construction before seeing any return, will therefore have much more chance of seeing its project completed than one using the more traditional London model.

The building is also unusual, says Pilbrow, because it is designed for multiple tenants, making it the first such building in London. This means that problems with creating multiple lets that were not originally intended for subdivision will be avoided because they have been considered at an early design stage.

But Heron needs to be confident that when the building is completed, it will be lettable. This is the second of the goals that KPF has to achieve, the first having been planning permission. In pursuit of these aims, KPF has used a lot of thought and analysis to come up with innovations, ranging from a new approach to lift control to the largest privately owned fish tank in the country.

BUILDING FORM The City of London site, surrounded by roads, is constrained in its footprint, with an area of just 1,800m 2. Early in its analyses, KPF moved away from the idea of having a central core, since this would have given it a mere 9m ribbon of accommodation.

Pilbrow explained that the likely tenants for such a building will be either professional-services firms, such as lawyers, or banks and financial-services providers. The latter, even in relatively small units, are likely to want to have a trading area, which would require a greater depth than a 9m ribbon. And lawyers tend to want a mixture of cellular offices and meeting areas, that again would be constrained by a narrow width.

KPF's solution to this was to offset the core to the south of the building, giving larger uninterrupted spaces. This relatively solid space helps to restrict solar gain. Also, the site boundary is most irregular on this side of the building and the core is able to absorb some of this irregularity.

One effect of this strategy and of the three-storey glazed spaces is that the building looks different from every side: most closed on the south face, most open on the north face, and in an intermediate condition on the west and east faces.

DISPOSITION OF SPACES The architect has subdivided the space vertically into a series of three-storey 'villages' consisting of a full floorplate for the lowest level, with the two above it centred around a three-storey atrium that is glazed on the north elevation, forming a social space on the ground floor. This approach allows the three-floor 'villages' to be let as a single space, giving an option to tenants of renting a total space of around 4,000m 2 and so widening the constituency beyond those who would be content with one floor. But floors can still be let individually, in which case the 'balcony' floors would be glazed with fritted glazing that allows views out but not down, protecting the privacy of those on the bottom floor. Indeed, floors can also be divided between two different tenants.

This approach of linked floors is one that KPF has previously used successfully on its Thames Court building. Tenants taking a whole 'village' can put stairs between the floors (KPF has mocked up a number of options). This also removes the need to have a structure that can accommodate the later punchingthrough of stairs between floors by tenants who want to make a connection.

Another advantage of this arrangement is that it brings natural light into the heart of the building, meaning that nobody is that far from daylight. This improves the working experience and cuts down on the use of artificial lighting. And, because this glazing is north-facing, there is little need for solar shading.

LIFTS One of the keys to making a high-rise building work financially is to maximise the proportion of lettable space. The higher the building, the greedier the core becomes. Heron Tower's net-togross ratio is, says Pilbrow, excellent, and one way that this has been achieved is by keeping the number of passenger lifts down to 10. This is in spite of the fact that it is essential to provide a good service to the building users. People in the City of London are more impatient than New Yorkers - in New York's financial district, people will tolerate waiting 45-50 seconds for a lift.

In London the maximum acceptable time is 25-30 seconds.

At Heron Tower, where KPF has worked with elevator consultant Lerch, Bates and Associates, efficiency comes from the use of two relatively new technologies that have never, says Pilbrow, been used together before. They are:

the use of double-deck lifts, with passengers boarding lifts at both groundfloor and mezzanine levels (which are linked by escalator); and

a destination-control (also known as hall-call) system. This means that passengers make their desired destination known before boarding a lift, and that the control system then directs them to the appropriate lift. In addition, customer destinations can be read from security swipe cards on entry to the building.

The simultaneous use of these two systems will mean that passengers should be assigned to lifts that will take them to their destinations with the minimum number of stops en route.

This will not only make the users happy, but also means that the lifts can return to ground more rapidly.

Five of the lifts will be medium-rise, serving approximately the first 19 oors, and the others will be high-rise, serving the upper levels. The architect has made use of all the space that is freed up by lifts that only serve some levels. So above the low-rise lifts it has created some informal meeting/reception areas. Behind the high-rise lifts on the lower oors, where there is no need for a lift lobby because nobody will be getting out, it has placed the bathrooms, which are separated from the lifts by frosted glazing. This allows some natural light to penetrate which, is, says Pilbrow, 'marvellous, because these spaces are so often entirely dependent on artificial light'.

STRUCTURE The steel structure is a perimeter tube, but in order to allow uninterrupted glazing on the south side, the bays to either side have diagonal bracing. This bracing, which extends up three storeys at a time, articulates the 'villages' externally.

SERVICES STRATEGY Each of the three-storey villages is separate in terms of fire compartmentalisation and, as far as possible, in terms of servicing.

There is a plant room contained in the core for each village, and service lifts are adjacent. Plant on the top of the building consists solely of elements that have to be sited there, such as heat-rejection units. Central chiller units are sited in the basement.

ENVIRONMENTAL PERFORMANCE Given the relatively long genesis of the project, one might have expected it to struggle to satisfy the requirements of the latest version of Part L, launched this year, which require environmental performance some 28 per cent better than in the previous version from 2002. However, says Pilbrow, the building in fact exceeds 2002 standards by about 40 per cent. This is down to several factors:

siting the plant rooms locally on the floors rather than at the top or bottom of the building increases efficiency by reducing the distance that air has to travel. Making the building as energy efficient as possible has the added benefit of keeping the plant rooms small, thus increasing the lettable space;

the free (adiabatic) cooling that results from the evaporation of water in the cooling towers will drive down the water temperature to about 15-16ºC. This low-grade cooled water will be used in the chillers. Alternatively, tenants will be encouraged to install their own chilled-ceiling or chilled-beam systems, for which this water will be at an ideal temperature. Although this would involve the tenants in increased cost, it may appeal to those with a CSR (corporate social responsibility) agenda and will create an extra bonus for the building's performance;

the east and west facades are triple glazed with clear low-iron glass. Coupled with the full-height glazing on the north facade, this enhances the amount of daylight entering the building and so reduces the need for electric lighting;

although, as one would expect, the building will be air conditioned, there will be openable windows, giving occupants the opportunity to operate in mixed mode. Again, the ventilation will operate on a threefloor 'village' basis, exiting at the top of the zone and therefore taking some advantage of stack effect.

Wind, which can be a great problem in tall buildings - while we want fresh air we don't want our paperwork blowing away - will be ameliorated in a buffer zone between two of the three layers of glazing; and photovoltaic cells will be integrated in the south-facing glazing, taking up around 30 per cent of the total area. Pilbrow believes that this makes it the largest installation in the country. He is confident that the cells chosen will 'look very beautiful' and they have the added benefit of providing some solar shading. They will only generate about 2.5 per cent of the building's total energy requirement, and at around £550/m 2 will roughly double the cost of the curtain walling. The payback period will therefore be 'rubbish' says Pilbrow, but he believes their inclusion is important, not just as a demonstration of willingness but also because largescale commissions like this will help to drive down the cost of PVs and so make them more affordable.

PUBLIC SPACE There will be public space at both the top and the bottom of the building. At the top, a restaurant will be publicly accessible and served by its own dedicated lifts. At the base there will be a three-storey arcade reaching along Bishopsgate, a kind of public 'village' at the base of the building. The arcade, scaled to reect nearby St Botolph's Church, gives access to public facilities on the three lower levels. A public concourse connects the arcade to a café on Houndsditch Street and shops on the lower-ground and first oors.

The office foyer has been designed to be as open and publicly permeable as possible, so that it can connect views across to St Botolph's Church and to a new garden space on Houndsditch Street. It will also include an aquarium, believed to be the largest in the country in a private building. Built from 360mm-thick acrylic to support the weight of the water inside it, its inhabitants will include a 2m-long conger eel.


Client Heron Corporation Structural engineer Ove Arup and Partners M&E Foremans Quantity surveyor Davis Langdon & Everest Planning consultant Montagu Evans Contractor Skanska Number of floors 46 Number of office floors 33 Total height 183m Total area 56,355m 2Initial planning permission granted 2002 Amended planning permission 2005 Contract let 2006 Construction starts July 2007 Construction ends December 2010

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