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the 17-storey, modular Paragon tower has brought volumetric modular construction to new heights. Developer Berkeley First decided the modular approach was the only way to deliver a mixed-use scheme in the tight deadline set by its client, Thames Valley University. Its mission was to deliver 1000 student bedrooms, key-worker units, teaching facilities and retail space by the start of term this September. But not only did the affordable housing division of Berkeley Group have to build quickly, it also had to contend with a tight and complex site.

Modules are lowered into place at Berkeley First鈥檚 17-storey tower. It will be completed in September, after just 21 months on site

The 2 ha site in Brentford, west London, is squeezed between the M4 and a network of residential buildings, a school and a church. "Because of the primary school, getting materials on site was difficult as the access path was closed when children were around. We also had to be careful not to overshadow the school premises. Everything was against us," says Tim Tolcher, director at Carey Jones Architects, the practice that designed the scheme.

Matthew Biddle, managing director of Berkeley First, adds:

"We had a contract that required the accommodation quickly.

So we developed a construction method that would allow us to build faster than usual. Because of the constrained site, it made sense to build off site."

The limited space on the ground meant the project team had to build high if it wanted to fulfil the university's brief of providing almost 1000 accommodation units. The site had previously been granted two planning consents for office blocks with the condition that they wouldn't exceed 18 storeys.

So, Berkeley First and modular manufacturer Caledonian 好色先生TV Systems spent six months from March 2004 designing units suitable for a 17-storey tower (the scheme also includes seven smaller buildings - see "How the development works", overleaf). The highest the manufacturer had gone before was nine storeys.

"The key with modular is getting the modular specialist on board early," says Biddle. "We had to have everything designed before we designed the foundations."

As such a tall modular building hadn't been constructed before in the UK, several structural engineering issues had to be resolved first. "The key challenges were stability, disproportionate collapse and differential movement between the steel modules and the concrete core," explains Rob Delahunty, structural at multidisciplinary consultant Capita Symonds. Lateral stability is taken care of by using a conventional concrete core but Delahunty says it wasn't just a matter of bolting the modules to the core. "You have essentially got two structures - a 17-storey high steel column next to a 17-storey high concrete wall," he says. "You have to allow the two to move together."

The development is on a tight site between the M4 and a residential district of Brentford. It is made up of eight buildings of varying heights. Six are modular constructions, including the 17-storey tower

In a traditional high-rise building, the core is constructed first, then the steelwork and floors, and finally the cladding is attached and the floors are fitted out. During this time, the concrete core moves as it dries out. "With a traditional building, by the time you install the finishes the initial movement has taken place," says Delahunty. But here, fully fitted-out modules were being attached to a core that was still moving, which could have cracked the finished interiors. The answer was to use connecters that allow a degree of vertical movement between the core and the modules.

The modules have also been beefed up to resist higher wind loads. Because the wind could cause the modules to twist around the core, they include additional bracing in the ceiling zone. The connector doesn't allow any horizontal movement between the core and the modules. The modules must be rigidly attached to the core so that the latter can provide lateral stability.

A stronger internal frame has been used as the modules near the base of the building have to cope with much higher dead loads than a low-rise building. This was relatively straightforward, as a conventional module already has eight columns contained within its exterior walls so it doesn't distort during transportation and erection. No additional columns were needed, but the column sections were made larger. The standard square columns were made rectangular so the finished wall thickness stayed the same, and a thicker steel was used for the columns. The modules passed tests for disproportionate collapse - the structure was modelled with columns taken out to see how the structure would behave.

This elevation shows the concrete core on the left and the modular units on the right

According to Dave Turnbull, director of Caledonian 好色先生TV Systems, the modular units were not significantly more expensive than on other projects because their rectangular shape could be easily standardised. The Paragon scheme uses a variation of 21 module types made of hot rolled steel with cold steel walls and plasterboard. They are built in 20 days, erected in one day and fitted out - carpets, final coat of paint and electrical wires - in two days. Two module types were used for the 12 m2 student accommodation units: one comprises two bedrooms with en-suite bathroom and a section of the corridor; the other is made up of the kitchen and the rest of the corridor. Each module comes with a built-in wardrobe, low-energy lighting and carpets.

Although building work could only take place at a certain time of the day - dictated by school schedules and the limited number of units stored on site - construction is going very efficiently. Robin Lackey, build director at Berkeley First, said: "We thought we would erect eight units a day but we ended up doing 15 a day, which is 24 bedrooms and four kitchens."

Besides the fast-paced construction - an adjacent five-storey building was erected in just six days - Biddle found that safety performance improved on the Brentford site. He believes off-site construction is less risky because there are fewer workers on site.

It might appear that the developer took a big risk by using modular units for high rise, but the length of time spent on research and an assessment from the Council of Mortgage Lenders on the building's life expectancy and stability provided real safety nets. Berkeley First says the 拢100m project has been "cost neutral because the time saving outweighs the money spent". The main achievement is that it will be delivered on time in September, just 21 months after the January 2005 start on site. "It took a year less than with traditional methods of construction," says Biddle.

For Turnbull, the project has proven that off-site construction was suitable for tall buildings. "There is a huge opportunity for modular build, especially in the city where there is a shortage of labour," he says. "Paragon has shown that it is more cost effective to build high rises with modular than with traditional methods."

As for architectural quality, Berkeley First and Carey Jones Architects say they aspired to make the tower "visually more interesting and richer to pull away from the traditional image of dull modular building". This does not mean hiding its origins, however. Biddle says: "We don't shy away from the fact that the buildings are modular. On the contrary, we emphasise the strong vertical and horizontal facades." As the tower nears its completion date, it is being clad in a lively combination of cream ceramic tiles, glazing, terracotta and aluminium. As Tolcher says: "It shows modular build doesn't have to be ordinary and repetitive."