What a difference 30 years makes. High-rise apartment blocks have gone from upright slum terraces to homes for the upwardly mobile. But building tall towers on tiny city-centre sites is a tough challenge. We report on the new popularity of homes in the sky and the engineering and logistical solutions used on the Beetham Tower in Birmingham
Once reviled as the symbol of urban alienation, the residential tower block has returned in triumph as proof of a city’s self-confidence and virility. Take as an example the Holloway Circus area of central Birmingham. This is the epitome of the rundown city-centre pocket, with its down-at-heel 1960s buildings, roundabouts and dual carriageways. Yet it is here that developer the Beetham Organisation has chosen to build a sleek 40-storey tower. The reason is that people now perceive inner-city high-rise living as sophisticated.
The Beetham Organisation is not alone in its project. Research published by property consultant Savills shows that 87 residential towers of more than 20 storeys are either under construction or on the drawing board in the UK. And this new generation of towers are not like their forebears. They are sleek and glamorous, more stainless steel and glass than stained concrete festooned with satellite dishes. And they are not marooned in the middle of forgotten residential estates but in city centres, as part of mixed-use developments with shops and restaurants on the doorstep.
However, putting up these tall buildings is not for the faint-hearted. Developers have to negotiate two hurdles: the high cost of construction and the dreaded planners.
High-rise developers clearly think it’s worth paying extra to give their development extra kudos. What’s more, stylish schemes mean flats can be sold for a premium, especially those on the upper floors. Today’s high property prices have also made residential towers a more economic option. According to research carried out by EC Harris, the most economic height for a residential tower is 20 storeys. Above this costs rise until the tower is 40 storeys high, at which point they level off again.
Planners always take a close interest in high-rise buildings because of the impact they have on cities. But according to Simpson, they are behind the times. “Planners need to be more forceful in assessing the design quality of tall buildings but they are not geared up for this,” he says. “That’s why commentary from CABE is helpful.”
He says this is changing, though. “I think Manchester has got it right as they can see the potential these buildings have to regenerate cities and attract investment. Because of this they focus on the quality of the building.” Simpson adds that many cities now have a tall building policy. “I think in five years’ time people won’t even think about it,” he says.
Over the next four pages we report on how the Beetham Organisation’s 120 m high, Ian Simpson-designed Birmingham tower was built – and consider how the project team’s solutions can be used on other high-rise schemes.
Tall buildings are better: Ian Simpson Architects at Holloway Circus
Ian Simpson Architects has become something of a tall building specialist, with seven schemes on his books in cities as diverse as Leicester, Glasgow and London. For Simpson, elegance is the key. “The usual approach is to build a red-brick building with punched windows and a zinc roof and then stretch it until it becomes a tall building. They all have balconies you can’t stand on because it’s too windy,” he says. “You can either follow that model or create something a little more elegant and challenging.”
Simpson says the limiting factor is the size of the residential tower floorplate. It has to be between 550 m2 and 750 m2 so all the flats have good views and meet fire and building regulations at the same time. “Something about 150 m high gives you the right proportion,” says Simpson. “Then you take a more site-specific view and look at its context, its aspect and its relationship to other buildings in the city. It’s quite an aesthetically driven process.”
Ian Simpson Architects was responsible for the design of the 40-storey high Holloway Circus in Birmingham. It will be the tallest residential tower and hotel in the UK – for a short while at least, before other towers under construction catch up and overtake it. Originally the tower was going to be 45 m taller but the Civil Aviation Authority wanted it cut back to 120 m because of its proximity to Birmingham International airport.
The first 19 floors are taken up with a 220-bedroom hotel; 144 apartments capped with six duplexes and two penthouses make up the remaining 21 floors. Developers like hotel operators to take the lower floors of towers as it means they only have to raise funding for the residential part.
Simpson says it was a challenge to work out how these two separate uses would be expressed architecturally. “It’s about creating a uniform facade and reinforcing the verticals,” he says. “What would have been wrong would have been to express the floors as punched windows. The idea is to create something that has an identity and clarity from a distance but when you get closer it starts to express itself. When you look at the building it takes you a little while to realise there are two different uses.”
The front of the building follows the shape of the site with a smooth, curved glass facade. This is arranged as a series of coloured, patterned vertical strips with further vertical strips of opening louvres to ventilate the residential part of the building. “We have introduced colour and pattern to give a feeling of vitality in an area of 1960s buildings and a roundabout,” says Simpson. The rear of the building has two stainless-steel clad cores containing the stairs.
According to Simpson two-thirds of the development was sold off-plan. He says there are a multitude of reasons why people want to live in tall residential buildings. “They are safe because of the concièrge. The light and security is fantastic – nobody will break through your window and the city is on your doorstep.
We have found that there is a distinct demand for this type of living. Conventional housebuilders will catch a cold – why would you want to live in one of their buildings for the same price?”
It’s wind and logistics: Constructing a 40-storey residential tower
“It’s wind and logistics,” says Barry Hawkins, the project leader for main contractor Laing O’Rourke, summing up the challenge of building Birmingham’s Beetham Tower. “The building has a very small footprint and is sandwiched between a car park, a hotel and a main road in a city centre. It also gets very windy when you get up that high. Apart from that, it’s quite straightforward.”
Hawkins is not exaggerating about the footprint. The building may be tall but it has a tiny cross-section, which is typical of residential towers. It sits right on the pavement edge and is jammed right up against its neighbours. Its sheer size means a large movement of materials is needed to sustain construction but there is only space for one tower crane. There is one compensation for Hawkins: “Once you get away from the ground, the building process is fairly repetitive.”
Because logistics was so crucial to the success of the project, a careful strategy was put in place. Hawkins asked suppliers to tender on the basis of a logistics plan. “If anything is late it has a knock-on effect on other trades. It’s all about being engaged with the supply chain and explaining if there is a problem,” he says.
The company also put a contingency plan in place in case of any exceptional problems by renting a nearby warehouse containing emergency supplies. “This contained crucial materials needed for the concrete frame, including rebar and column moulds,” Hawkins says. “If we had a major disaster such as a supplier going bust this would give us time to find another.” The cladding is also critical to the programme. Because it came from Austria, Hawkins ensured this was delivered to a holding yard in good time in case of delivery hiccups.
Having only one crane and no scaffolding necessitated the use of construction methods that weren’t heavily crane-dependent. The cores were built using self-climbing jump-form shuttering, and the post-tensioned concrete floors followed three to four storeys behind. A screening system was used to help build the post-tensioned concrete floors. This combines edge protection with a working platform extending beyond the edge of the building. This enabled workers to post-tension the steel strands running through the slab at the edges, and secure these using grout.
These constraints didn’t slow the contractor down. “Once we were out of the ground we got 40 storeys up in 10 months,” says Hawkins. The team got the time to construct each floor down from five or six days at the hotel level to just three-and-a-half days nearer the top. This was because, whereas the square columns used at the lower hotel level were cast using conventional shuttering, the residential part of the building had round columns that were cast using disposable cardboard tubes. These were much quicker to place, then easily removed after the concrete had cured.
The cladding team is also independent of the tower crane. “Scaffolding isn’t used on high rises and the tower crane is busy looking after the concrete works,” says Dean Walton, director of GSL International, which specialises in installing cladding on high-rise buildings. Unitised cladding systems are the norm on high rises – this one was made by Austrian firm Bug-Alu-Technic. This is installed using a monorail installed two floors above the area being clad. A hoist attached to the rail lifts the panels up from ground level to the work area, and moves around the building as work progresses. “One team fine-tunes the panels that have just been fitted while the other loads up the panels on the ground – it’s perpetual motion once you get going,” says Walton. “We can do a panel in 15 minutes, from lifting it up to fixing the gaskets.”
The team may have got round the construction constraints, but the wind is one element it cannot control. “It could be calm on the ground and a nice sunny day but you can’t stand up at the top,” says Hawkins. He has borrowed strategies from the aircraft industry to stop things blowing off the building, such as attaching chinstraps to workers’ hardhats, but on some days this isn’t enough. “We have had to close the top floors a few times, as even stepladders could blow off,” says Hawkins.
Laing O’Rourke is making sure that the specialist knowledge gained on this project isn’t lost. The company is developing a series of “toolkits” containing best-practice information on how to do specific jobs from dry-lining to putting in post-tensioned concrete floors.
“The problem is we are inventing the wheel each time we do a job,” says Hawkins. He says people are already coming from other parts of the company to learn from this project – which will be useful because, thanks to the tower boom, it won’t be long before Laing O’Rourke is building more.
Thinking laterally: The structural engineer’s solutions on Birmingham’s Beetham Tower
Structural engineer WSP Cantor Seinuk is one of the world’s leading high-rise specialists, and was responsible for the structural design of Holloway Circus. The problem faced by designers of tall buildings is that they have to resist high lateral and gravity loads, which means the building structure takes up a lot of room. “Maximising the net-to-gross floor space is crucial,” says director Kamran Moazami. “The first step is to find the most efficient system then work with the architect to arrange the column layout to suit the functional characteristics of the building.”
Concrete is a popular choice of structural material for residential towers. Moazami says: “We went for concrete at Holloway Circus for the thinner structural floor zone it offers – you can get two to three additional floors and better acoustic characteristics with concrete floor construction.” The post-tensioned floor slab is just 225 mm thick and is ideal for residential buildings, as these don’t need a deep service zone under the floors. Moazami says close and early co-ordination with the services engineer is vital to ensure that openings for services are cast into the concrete.
Residential towers have smaller cores than commercial buildings because they don’t need as many lifts. This means additional structure is needed to resist the lateral loads. A number of solutions can be used, depending on the specific building. At Holloway Circus, the cores are located at the very back of the building side by side and are coupled together with a shear wall. This arrangement takes the lateral loads, and columns around the perimeter of the building help to take the gravity loads. The thickness of the concrete walls and the number of columns was also carefully optimised, resulting in a net-to-gross ratio of 80%.
Accommodating cantilevers in tall buildings is also an issue. The top two floors of Holloway Circus are cantilevered over open balconies by using a short beam, and extending the floor plate. This is not technically that difficult but it does create overturning forces that are balanced out by the cores and the column arrangement. The Beetham Tower in Manchester, a 47-storey building that will house the Manchester Hilton, has a much more challenging cantilever that extends outwards 4 m halfway up the building.
Very tall towers can move in strong winds, and cause sleepless nights. “How much movement is okay is governed by the use of the building,” says Heather Stanley, director of structural engineer Yolles. “You feel movement more when you are lying down.” Moazami says vibration is only an issue in buildings of more than 50 storeys. WSP Cantor Seinuk designed the 258 m Trump World Tower in New York, the highest residential tower in the world. This has a 600-tonne mass damper at the top of the building to lessen vibration.
Under pressure: Engineering services at 40 storeys up
Tall buildings present services engineers with specific challenges. “A key issue is the distribution of water, as the pressure is always a problem,” says Brian Doran of multidisciplinary consultant Buro Happold, the services engineer responsible for Holloway Circus. If water were supplied to a tank at the top of the building, the water pressure would be too high at the lower levels. The solution is to install a series of water tanks and pumps at regular intervals and “cascade” the water down the building. This means heat exchangers are needed on the hot water circuit to transfer heat between the different building levels.
Heating is also tricky. “In the past, high-rises had electric heating but this is no longer possible because of the Building Regulations,” says Doran. Individual gas boilers in each apartment are also impractical, because the sleek appearance of the building would be ruined by a multitude of boiler flues billowing out water vapour all the way up the tower. Doran adds that tall residential buildings are not necessarily energy-efficient because they usually have large areas of glazing to allow residents to enjoy their panoramic views, and there are potentially high air infiltration rates because of the high winds affecting tall buildings.
Two solutions have been adopted at Holloway Circus. The hotel is fully air-conditioned and is independently serviced by a plant room that takes up all of the fourth floor. The non air-conditioned apartments on the upper floors are heated separately using a hot water loop running around the residential part of the tower. This is heated to between 20ºC and 30ºC by gas boilers located on level 19 and supplied to a heat pump and fan-coil unit located in a cupboard in each apartment. Heated air is then distributed throughout the apartment. Heat pumps are used because they can either raise the temperature of the water supplied by the central water loop, or return excess heat from the flat to the loop, which saves energy.
Keeping the temperature of the central loop low also means the boilers operate more efficiently. “It’s an efficient form of heating that allows us to exceed the requirements of the Building Regulations,” says Doran.
developer The Beetham Organisation
architect Ian Simpson Architects
contractor Laing O’Rourke
structural engineer WSP Cantor Seinuk
services engineer Buro Happold
post-tensioned flooring StrongForce
cladding manufacturer Bug-Alu-Technic
cladding installation GSL International