Onsite

Modular housing hits the heights in north London

30 March 2017

Europe's tallest modular tower, Apex House in Wembley, will rise to 28 storeys and take just one year to complete. Stephen Cousins examines the structural and economic gymnastics required to make it stack up.

For a high rise that’s more than a third of the way into construction, there doesn’t appear to be a lot going on at Apex House, a 560-bedroom block of student accommodation going up in north-west London, just a stone’s throw from Wembley Stadium.

I have been on site for over an hour and during that time I have only seen a handful of operatives, working from mast climbers on the facade, a few site managers, and just two lorry deliveries, each carrying one piece of cargo.

The lack of site activity is a result of the volumetric modular method of construction. Due for completion in September, Apex House will be the tallest modular tower in Europe, rising to 81 metres, some three storeys higher than the current record holder, a 25-storey student residential block at the University of Wolverhampton.

It will comprise 679 separate modules, including 242 modules in two eight-storey wings on either side of the tower, produced by manufacturer Vision Modular Systems at its factory in Bedford (see panel below).

The self-contained units resemble shipping containers, each one delivered fully kitted out with a kitchen and bathroom, services, light fittings, switches, socket outlets, internal finishes, even the bases for the beds are installed.

“The only thing missing is the duvet,” jokes Rory Bergin, partner at HTA Design, the scheme’s architect, planning and sustainability consultant. “Moving as much work as possible into the factory you can control the quality, and there’s no need for any follow-on trades to enter the units.”

The rapid-fire development involves  modules being transported down the M1 to London, then lifted by tower crane from the trailer and into position in just 10 minutes. Up to 11 units can be installed per day, which has enabled the project team to slash the construction programme in half, to just 12 months, compared to the equivalent concrete- or steel-framed tower.

The project overturns the conventional view that modular is more expensive than traditional construction, claims Bergin: “The whole question of whether modular is a bit more expensive than traditional, in terms of up front cost, doesn’t matter a damn. What matters is that saving of 12 months of time, which a group of professionals, including clients, builders and manufacturers, could use to build another building, making the capital cost almost totally irrelevant.”

GRC cladding will form the facade on the tower

The use of “prefab” has long been dogged by issues related to cost, aesthetics and quality of construction, but evidence suggests it is starting to gain ground again.

The government’s £3bn Home Building Fund, launched last year, will direct some funding to modular housing. The £1.7bn Accelerated Construction programme, launched this January, will exploit offsite to deliver up to 15,000 housing starts on surplus public sector land during the current parliament.

Under the  latter, the Homes and Communities Agency (HCA) will provide support for local authorities that use offsite building methods and construct homes at “up to double the speed of a traditional development”.

Ramping up housing numbers

As it happens, representatives from the HCA and the Department for Communities and Local Government (DCLG) visited Apex House just before CM arrived on site. “They wanted to see how our system fits into the plan to ramp up housing production numbers,” explains Bergin. “Our model is appropriate for medium to high rise and we represent the highest end of production, where around 70% of all construction effort is in the factory, whereas a panellised or frame system might be around 50%. They want to apply the right system to the right place, rather than shoehorn the same system in where it is not appropriate.”

According to Bergin, anything above six storeys high hits the “sweet spot” in terms of economic feasibility, “it’s difficult to build low-rise housing with this type of system cost effectively at the moment”, he says.

Apex House is located at the intersection of Fulton Road and Albion Way, close to Olympic Way, the main pedestrian thoroughfare leading from Wembley Park railway station to Wembley Stadium.

Typical bay study

1. Glass balustrade
2. Aluminium secret-fix proprietary cladding system, dark grey
3. PPC dark grey aluminium proprietary window system and dark grey (perforated metal panel at opening lights)
4. Vertical dark grey GRC cladding panel
5. GRC cladding panel
6. Curtain wall

It is the fourth modular block to be built on a single parcel of land inside the Wembley Regeneration Area, by the same developer, Cork-based Donban Construction. The four projects were all delivered with variations of the same modular system, by a close knit construction team, led by Donban’s sister manufacturing arm Vision Modular Systems, HTA Design, main contractor Tide Construction and M&E supplier Red Electrics.

This integrated set up has helped increase the pace of delivery, from one project to the next, increasing the number of modules installed per day from five or six to 10 or 11. This improvement has made it easier to convince Brent Council that the approach is reliable and worthwhile, says Bergin.

Hybrid frame

The building frame is actually a hybrid, comprising a reinforced concrete core, concrete foundations, and a series of concrete columns, at ground level, that support a transfer slab on level two. The transfer slab provides vertical support for all the modules above, which are stacked one on top of the other.

The modules are produced in eight different configurations and vary in weight from around 12 tonnes to 17 tonnes. A typical module is a student room, containing a bedroom, study area and en suite. Half of the modules include a section of corridor.

Larger modules, installed at the corner of the tower, house common areas with a shared living/dining space. Every module is open at one end to allow services to be connected, including the soil vent pipe and electrics, and to fit a door onto the corridor. The concrete core provides lateral support for the modules, as do two steel bracing structures at the end of the two wings. Concrete floors in the modules act as a “diaphragm” to channel lateral loads into the core.

All units are welded together on site to form the rigid honeycomb structure, the most complicated weld junctions are where four modules come together on an external face. Supervising engineers photograph each weld detail to ensure quality control.

I’m not allowed inside an installed module, for safety reasons, but we take a look round a room in the Novotel Hotel next door, built using the same system. The floors feel solid – each module has a thick concrete base – there’s no sense of flimsiness or issues with noise from outside. Apparently, one yardstick for acoustic performance was the ability to sufficiently mute a Metallica concert playing at nearby Wembley Arena. The only deviation from a conventional room is an unusually thick internal partition wall, created where two modules join.

A bedroom in the neighbouring Karma House, which was built by the same team as Apex House.

Crucially, the hotel doesn’t look like a carbon copy – a criticism often levelled at factory produced prefab schemes — and, once the cladding is installed, Apex House will express a strong architectural presence. The wings will feature terracotta striped cladding panels of varying tones, the tower will feature a mix of elegant white GRC cladding, a full-height strip of glazing on the corner, and retail units on the ground floor, set back from the chunky concrete columns.

Bergin comments: “We’re not interested in making schemes that look like they are modular, we are interested in good townscaping, good placemaking and good urban design... sometimes we have to push the manufacturer to do things that are a bit less efficient to improve the architecture, but they recognise that we want to deliver high-quality buildings that don’t look like boxes.”

Sky's the limit

The tallest modular tower in the world is a 32-storey residential tower, in Pacific Park, central Brooklyn, but Vision believes its system is capable of going even higher, potentially into the mid-30s.

Kieran White, managing director at Vision Modular Systems, told CM: “We are looking at other schemes that will take the height up into the mid-30s, fundamentally you are working with a steel structure so we could go higher.”

The modular approach suited the tight nature of the site, where there was no room for materials storage, apart from a small courtyard to the rear, and just one side road for deliveries.

Above: Typical floorplan at Apex House. Below: CGI of Apex House

Using modular resulted in a small workforce, of just 22 on site staff, eradicating the need for large areas for welfare facilities and site offices, normally associated with a traditional build. It also benefited health and safety — by removing the need for scaffolding and allowing all external work to be carried out from mast climbers.

In economic terms, off-the-shelf volumetric modular is more expensive than traditional build. However, it works out cheaper when taking into account the reduced build programme, says Bergin.

 “If you’re talking about a student building with 500 rooms, each with a rental cost of around £800 a month, then completing 12 months early could mean around £4m additional income for the client,” he says. (The rooms wouldn’t be 100% occupied so its not a totally straightforward calculation).

However, the technology also has limitations. Module sizes are limited by the width transportable by road — on this project the maximum is 4.5m-wide. While that’s not an issue for student accommodation, modules for build-to-rent projects would come close to the maximum allowed, says Bergin.

The size of GRC cladding panels was restricted by the weight that could be supported on the mast climbers, and means there are more joint lines on the facade than the architect wanted.

In addition, Apex might have gone up faster if the factory was capable of producing more than 11 modules a day. But boosting productivity would require a larger pipeline of work sufficient to justify greater investment in factory processes and people.

That could happen soon as the team working on Apex House has two projects in planning, in Hammersmith & Fulham and Willesden, and and another four projects at an earlier stage of development.

Modular's wider impact

Whether modular schemes like Apex House can trigger a paradigm shift in UK construction will depend on wider strategic political and economic goals.

The ongoing housing crisis, coupled with the possible impact of Brexit on numbers of migrant workers, could make fast-build systems, which require fewer on site operatives, very attractive.

A report by Arcadis in February estimated that the industry as a whole could result in a reduction of as many as 214,000 workers in the event of a “hard” Brexit, by 2020.

Bergin comments: “A lot strategic people in different organisations know that if we are going to have long-term future, we need to look at this as a long-term solution, as we are going to run into labour supply problems. Modular will help future-proof the industry.”

Adopting it will also require a change in mindset, as many staff in local authorities and housing associations remain wary of prefab as a result of negative experiences in the past.

“The Council of Mortgage Lenders has not been very welcoming of modular,” says Bergin. “Even with the minimum manufacturing quality standard in place, we understand that not every mortgage lender will accept it.”

Structural innovations allow more storeys

Two key structural innovations were required to build the highest modular tower in Europe, at 28 storeys.

A special coupling arrangement, between the steel modules and the reinforced concrete core, had to be designed to accommodate differential movement, caused by the gradual settlement of the concrete structure over time.

Kieran White, managing director at Vision Modular Systems, told CM: “We had to develop steel plates, cast into the core, form a horizontal tie with the modules and allow for some vertical movement.”

In addition, the modules each incorporate thick steel corner posts, designed to transfer vertical loads down into the foundations. The arrangement differs from Vision’s modules for low-rise properties, which include vertical studs at 600mm spacings designed to handle uniformly-distributed loads.

The walls of modules are mostly infill structure, including boards for fire protection and internal finishes. The walls on the facade incorporate windows and insulation, ready for application of the GRC cladding panels on site.

“The units are entirely sealed off from the elements so that, once craned into position, internal works can proceed in parallel with external works. You don’t have to wait for the cladding contractor to give you a dry building before you can progress,” says White.

Precision engineering role module

The 679 modules installed in Apex House are being manufactured at Vision Modular Systems’ factory in Bedford, in a process similar to that used on a car production line.

A total of 150 skilled and semi-skilled factory workers are employed across the production process, including welders, plumbers and carpenters, plus managers for production, supply chain and logistics.

Each module is assembled in the following stages:

Once on site, the module is connected to the power and water supply, then a commissioning process is completed to ensure that heating and hot water, sprinkler systems and fire systems, and every switch and socket, are fully operational.

Moving most of the site work into the factory helped cut waste to just 2% and, alongside other factors, was key to the project achieving a BREEAM Excellent rating.

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