Here’s One I Made Earlier: Taking Prefab to New Heights

Image courtesy of HTA.

The world’s tallest prefabricated and prefinished construction is being built in Singapore. Alex Love explores the challenges and benefits of prefabricated prefinished volumetric construction, and how this method could be used in the future to create sustainable buildings

With an increasing global population and ever-expanding cities, developers are seeking ways to build more quickly and to greater heights.

Despite prefab buildings being around for several decades, the modular concept has not been used for high-rise towers until fairly recently, with the world’s first only completed in 2016. Yet within a decade of its completion, the record height of modular towers is set to double.

New York’s 461 Dean tower was the first prefab tower built, standing at 109m. But this is set to be comfortably eclipsed by Singapore’s 200m-tall Avenue South Residences, due for completion in 2023.

Prefabricated prefinished volumetric construction (PPVC) presents an opportunity to build faster. An estimated 80%-95% of construction can be performed away from the building site, reducing dust, noise and traffic in the surrounding area. Factory-built units are delivered to the construction site and moved into position by crane, then joined to the rest of the building. This also reduces the potential for weather to disrupt the project schedule. The appeal of this concept may increase in the wake of the coronavirus, as fewer workers are required on the construction site.

“You can build buildings very quickly, you can build them high-quality; and you can build them on sites that you might struggle to build any other kind of building,” says architect Rory Bergin, partner at HTA Design – the firm that designed Croydon’s 101 George Street prefab towers in the UK. “It’s a radically different approach, but it’s still quite embryonic in terms of the industry.”

Clement Tower in Singapore will be the tallest prefabricated tower in the world when it is completed. Image courtesy of ADDP.

The sky’s the limit?

The record for the world’s highest modular building could change hands several times in the next few decades. The current record holder is reportedly Singapore’s 140m-high Clement Canopy pair of towers, finished in 2019. Before that, the record was held briefly by the 135m-tall 101 George Street prefab towers in Croydon, UK. And in 2023, provided all goes according to plan, Singapore’s 56-storey Avenue South Residences is set to take the title.

Designed by Singapore-based architects ADDP for developer UOL Group Limited, Avenue South Residences will consist of two towers made fromsix-sided concrete modules manufacturedby a factory in Senai, Malaysia.

“ Its designers do not expect to build a prefab tower in Singapore taller than 56 storeys any time soon due to the heights that concrete needs to be lifted. 

From there, they will head to a facility in Singapore to be furnished and fitted out, then delivered to the Avenue South Residences site for positioning. Once complete, the tower blocks will consist of around 3,000 modules, with approximately 1,074 residential apartments in one to four-bedroom configurations, and sufficient parking spaces for occupants.

The exterior of the towers will feature balconies, sun shades, and plants and trees. Greenery will also be a feature of the area surrounding the development, with designers describing it as an “oasis-like community space”.

Its designers do not expect to build a prefab tower in Singapore taller than 56 storeys any time soon due to the heights that concrete needs to be lifted, although taller prefab towers may be possible with lighter materialsand as the techniques become more refined.

Image courtesy of ADDP.

astudio created a 360-degree virtual environment for the Ebury Bridge Estate project that enables clients and residents to get a feel for the space before the project commences. Images courtesy of astudio.

Image courtesy of ADDP.

A brief history of prefab towers

The world’s first high-rise prefab building was the 109m-tall 461 Dean in Brooklyn, New York. Finished in November 2016, the tower stands at 32 storeys, has 363 apartments and is made up of 920 steel modules.

Construction was not without its problems, with a lengthy legal dispute between developer Forest City and contractor Skanska, and arguments surrounding the design. Project works began in 2012, but were delayed by two years and subject to cost overruns. This was hardly the best advertisement for the high-rise modular concept.

However, construction of other prefab towers has gone much more smoothly elsewhere. Atthe 101 George Street towers –44 storeys and 38 storeys apiece – the last of 1,526 modules was moved into position just 35 weeks after construction began and three weeks ahead of schedule.

Meanwhile, construction of Singapore’s Clement Canopy double towers was completed six months ahead of schedule. The towers consist of a combined 1,899 concrete modules and 505 apartments. Traditional construction methods for a similar-sized development would take 30-36 months, whereas PPVC can achieve completion in 24-30 months.

“ Over time, as you do more of it, the costs will come down. And it will become more and more normal. 

Although PPVC can be 5%-10% more expensive, there is confidence within the industry that once the benefits of this building method become more widely known and understood, high-rise prefab buildings will increase and prices will fall.

“Over time, as you do more of it, the costs will come down. And it will become more and more normal. We’re just at a difficult point where so far it’s not really normal, it’s still quite innovative,” adds Bergin.

In spite of the concept’s potential to help solve the global housing crisis, the construction industry is known for being slow to embrace change and – perhaps understandably – more comfortable with established practices. This means that manufacturers of prefabs units for towers are still relatively few and far between.

“There are only a handful of people around the world with the skill and experience to do this,” says Bergin.

Image courtesy of ADDP.

101 George Street in London’s Croydon was the world’s tallest modular residential building when completed in 2019. Image courtesy of HTA.

Materials and sustainability

At present, there is noindustry-wide agreement on the best materials for constructing the modules.

There has been talk of using timber for some of the frames; furthering the concept’s sustainability credentials, but this remains untested for high-rise prefabs.

Singapore uses concrete for its modules and cores,while Croydon’s George Street towershave a concrete core and steel frames. But the higher a prefab building is, the greater the compression can be for hybrid units.

“In Singapore, they’re building in concrete mainly and that’s partly because of local culture. That's just what they're used to,” says Bergin.

“Whereas here in the UK, we’re building in steel because that’s what we’re used to here, and we’ve got a lot of technical competence insteel engineering. It's interesting: around the world people are trying different methods. I don’t think there’s a consensus yet about the pros and cons of one versus the other.

“I think we are pretty convinced about the benefits of a concrete core and steel modules. That seems to work very well. But I would also be interested in a prefabricated steel core and steel modules, because you avoid the problem of slight differences in movement and shrinkage within concrete and steel. So the taller you go, the more significant that compression is.

“ It's interesting: around the world people are trying different methods. I don’t think there’s a consensus yet about the pros and cons of one versus the other. 

“It happens already in a concrete building; but because the whole building is concrete, it doesn’t really affect anything. Whereas in a hybrid building, it does.”

It has been estimated that using steel frames instead of concrete for the modular units could save more than 40% in CO2 emissions to manufacture and transport, making them more appealing for eco-conscious developers. In addition, steel is far easier to reuse and repurpose than concrete. Steel used for modules could also come from a greater variety of industries as part of a circular economy.

“The way that the concrete modules are being built in Singapore means you just have to demolish the building as though you're demolishing a traditional building; you have to you have to blow it up or knock it down,” says Bergin, who also heads HTA’s sustainable futures team.

“Whereas the way steel modules are fabricated, you can dismantle them and refurbish them, remove them, repurpose them more easily. I suspect in the long-run, that’s a better bet.”

Prefab units are also considered more energy efficient due to being manufactured under stricter controls in factory settings.

“Factory control means that stuff is done correctly,” explains Bergin. “Certainly in terms of quality control and things like air-tightness, factory production just gives you a better result.”

Image courtesy of HTA.