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Top of the Pods

The tallest cold rolled steel building in Britain is dropping into place in the East End of London. Jessica Rowson reports on the post-Egan world of modular construction.

Ten years ago Sir John Egan, with manufacturing experience from Jaguar, urged the construction industry to adopt the "lean" production line methods pioneered by the car industry in his groundbreaking Rethinking Construction report.

Now the futuristic dawn of assembling homes on factory production lines is a reality,
and in London’s East End one of the tallest cold rolled steel modular buildings to be constructed in Britain is taking shape. Here, instead of using hot rolled steel sections, steel sheeting is shaped to form the pieces which make up each module or pod.

A large percentage of the 11-storey student housing project is being assembled in
a factory in Gloucester by Unite Modular Solutions (UMS). The pods leave the site completely decorated.

"The finishing line is like a car production line," says UMS research and development manager Michael Braband.

"It moves slowly and during the length of the line all of the fit out is done; carpets, electrics, furniture, doors... A module comes off the end of the line every 38 minutes."

Modular and off-site construction has grown in the last few years. However, the design team is keen to emphasise the difference between this system and others.

"This system is off-site manufacture," says structural consultant Walsh Associates project director, Andy Stanford.

"Most projects now utilise off-site construction, but this is using manufacturing skills for construction."

Braband agrees: "The people are from manufacturing backgrounds and are applying manufacturing techniques. The people assembling the modules are semi-skilled and in-house trained. It’s not like taking carpenters and plumbers and putting them in a warm shed."

One of the key differences is that the main frame of the student accommodation modules is made entirely from cold-rolled steel.

"Lots of companies use hot-rolled structural posts in the corners. In ours, it’s fully cold rolled, with 1.6mm gauge steel forming 45mm by 75mm "c"-stud sections along the length of the wall. The walls are load bearing."

Unite has its own roll forming capability in-house, which means that it can buy in flat steel and form its own studs. Though this means the height that they can go to is limited, it gives Unite flexibility and speed as it is not waiting on a third party supplier.The stud walls are continuous through the structure with the floors spanning between the walls.

"The modules are simply stacked on top [of each other], there is no need for intermediate structure," says Braband.

"The walls run the full height and the ceiling and the floors are hung from the walls."

Who’s who
Principal contractor: Mansell
Structural engineer:
Walsh Associates
Modular supplier: UMS
Modular structural engineer: WSP Cantor Seinuk
Architect: DMWR

At 11 storeys, the new building is not much of a show stopper, height-wise. The skyscrapers down the road dwarf it, but it is still quite considerable in terms of the material it is using. Cold-rolled steel is traditionally used for lighter applications but it is not the strength of the material that is restricting the height of the structure.

"At the moment we are limited at 11 storeys by fire protection," says Braband.

"After 30m there is a step change. We would have to increase the fire protection from 90 minutes to 120 minutes. We could go higher if there was a fire-engineered solution."

The speed of construction on site is still very quick by traditional construction standards, though it can’t match the speed of production in the factory. It takes just two months to land the 10 storeys of modules onto the concrete transfer slab at first floor level.

"The bedroom is fully fitted out with carpets and furniture and once it is landed on site, all that is left is to fix the M&E connections," says Braband.

Something that can’t be manufactured off site is the foundations. Main contractor Mansell started on site in September last year and the groundworks were completed by December. The reinforced concrete transfer slab at first floor level supports the modules and was completed in May as the first modules began to arrive from Gloucester.

One of the most time consuming activities was waiting for the core to be built. The core, which helps stabilise the building and houses the lift, was slip-formed using concrete

"We’re looking at modular systems for cores but that’s more research and development at
the moment. We had to wait for three or four months for the cores to go up 10 storeys," says Stanford.

"Research and development is often cost prohibitive on a single project, but much more palatable when it can be spread over several projects, and so for us as a business knowing there is
a likely flow of work encourages us to invest in R&D."

The corridors are supported by the pods on one side and a steel frame on the other, where there is no adjoining pod. The steel frame is going up at the same time as the pods.

"Normally you would take the column all the way up, but if that was erected before the pods were here, it would leave you with some big spikes that would be a problem for the modules being hoisted in. By taking the beams across, the frame can be built at the same time as the modules, storey by storey."

Prefabricated modules have come a long way since the days of Portakabins. Walking around the site, one can imagine the bright open spaces that will be created in this student housing block. The concrete structure on the ground floor, with its large floor-to-ceiling opening, creates a spacious entrance to the block and the modules are grouped around an airy lightwell or courtyard. Looking inside the modules, the rooms are neat, well-finished and solid. Architect DMWR has ensured that the project remains simple – over-complication of the floor plan defies the point of using modules in the first place.

"One of the main difficulties is getting the architect to realise they have to stick to module dimensions," says Calvert.


Deflections

Deflections and on-site tolerance are critical when trying to make off-site manufactured pieces fit.

The modules sit on upstands on the first-floor reinforced concrete transfer slab.

"We cast the upstands that support the modules after the slab had been de-propped," explains Walsh project associate Matthew Calvert.

"We let the dead load deflection happen and Unite just needed to know the live load deflection. This helps to stop the modules leaning."

A plummet laser is used on site to throw a vertical beam straight so the modules can be lined up.

"With the laser, we achieved tolerances of 5mm in our last project," says Brabant. "When the modules are landed, there’s a laser target in the corners, which allows us to align it to the laser point."

It’s not just in the vertical that accuracy is critical. The modules are also being landed alongside concrete cores.

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