Offsite Structural Options: The Case for Steel


James Finnie

September 25th, 2020
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This is my second article focusing on the three main off site structural options – steel.

(Funnily enough) The first point is that steel has tremendous versatility. While I was working at SOM, one of their US engineers gave an engrossing talk to the London office on the structural frame design for Frank Gehry’s Guggenheim in Bilbao.

How did they get all those curves to stand up? Rather than looking at reinforced concrete, they took a different approach. They broke things down and produced a flexible “mesh” with steel, which could be moulded to suit the free form shapes. It also sped up construction.

Guggenheim Bilbao Structural Steel

Image credit: Guggenheim Bilbao

I think this highlights steel’s versatility well. I have always loved steel and it is arguably the most versatile material of all three structural options. Although a technicality, the other materials almost always require steel for their fixings and connections.

Options when using steel

There are quite a few options worth considering when using steel for small to mid size residential developments. Site location, density and access can often be restrictive and therefore panalised steel systems might edge out other options. But that’s only a maybe. I have recently seen modular pods placed inside the shell of an existing inner-city building. This is a wonderful example which shows that if the correct approach is taken, there really are no limits.

You would think that trying to get pods to fit into an existing building (probably historic) on what looked like the main street of a busy town/city isn’t going to work in one of the most restrictive conditions you could imagine. Obviously, the planning of such a project is critical. The pods were delivered at night over a few days. Once they were located, only the services, windows and roofs had to be completed. City centres are notorious places to access, work in and get to. With time and space at a premium, a contractor using steel spends the least amount of time on site.

Other options are volumetric modules or pods, which I love. There is also the option of using recycled shipping containers. They make it harder to pull off a quality design that the residential market would find acceptable, but there are examples of high-end housing using containers – and one of the examples I use later in the article proves that there is also merit in this option.

Then there is straightforward steel frame, which is (probably) more appropriate for taller residential projects. These would typically use metal decking for the floor and infilled with concrete, but for this article I am taking a purer view and trying to promote the use of steel throughout. So I will discount this option on a technicality, but it is still a worthy option and could well be of use on a tricky site.

There are other hybrid options where steel has concrete infill floors and partially filled steel framed shafts to give the building rigidity and stiffness. However, if you’re looking for a purely steel building, fire-proofed steel floor cassettes can be used and the main building shafts can be stiffened with additional steel bracing, then fire-proofed with intumescent paint or plasterboard.

Cost and programme

As mentioned in previous articles, it’s very difficult to compare steel to concrete and timber to establish the most cost effective solution. Likewise, it’s also difficult within the use of steel to identify which is the best for your project. However, if a great deal of emphasis is placed on design and compatibility at the outset of a project, I believe the right product and approach will start to emerge from the design process.

For cost and programme, the cheapest and quickest would be using shipping containers. This approach tends to deliver the least design impactful solution. However, in the hands of a good architect, there is no reason why a well-designed building can’t be achieved.

The next most cost effective is probably the panelised approach. However, this does require more work on site to place the various elements, whereas a pod is more of a drop and go solution.

There are many specialist companies that can deliver the panelised approach, particularly as these are made of repetitive components. I would assume this approach would be more cost effective than volumetric pods, especially as the pods are a standalone structural module with its own floor, “roof” and walls, which means many of the elements are being repeated for each individual module.

From a programme perspective, I would imagine the pod approach definitely saves more time when being placed in position on site. Therefore, the pod would be the better and quicker system of the these two, requiring much less craneage time – whereas a crane would be required until all the panels are placed in position when adopting a panelised approach.

Impact on Design

It would initially seem that panelised and volumetric off site steel construction would deliver a more aesthetically pleasing result. However here are two relatively well known residential buildings which deliver, in my opinion, designs which would be more than acceptable in the main stream residential market.

La Aduana in Leon Mexico

Image credit: La Aduana in Leon, Mexico

Apartment building in Washington DC

Image credit: DC Shipping Container Apartments

These two examples dispel the myth that such buildings can’t be aesthetically pleasing, although they are the exception rather than the norm. Containers are often used for student accommodation and there are some fine examples of these, particularly in Holland. The narrowness of containers can limit their potential, although by removing wall sections, larger spaces can easily be achieved.

The container example I have chosen below is a wonderful project which exemplifies its optimum use. It’s a temporary social housing project called Meath Court by IsoSpaces for those waiting on the Ealing housing list. Once people have been housed, the building will be relocated to another site to carry out the same role – a truly sustainable approach to building. Although repetitive in design, I believe it’s more than fit for purpose.

Below are excellent examples of the three main types: container, panelised and volumetric:

Meath Court Ealing

Image Credit: Isospaces – Hope Gardens

399 Edgware Road

Image Credit: U+I, 399 Edgware Road

Town House Manchester

Image credit: Shedkm, Town House, Manchester

Panelised systems using light gauge steel framing (LGSF) as a structural element and can give plenty of flexibility in design and 399 Edgeware Road project exemplifies this. Some panelised systems can be a little repetitive, but in the right hands good design can be easily achieved.

The third example is “Town House” designed and developed by Shedkm and Urban Splash, both of whom have created niches in the market as well as leading the field.

Town House has been cleverly developed to meet customer choice, offering owners many options so their house can feel like “theirs”. From different layout options, alternative bathroom and kitchen pod arrangements and a range of finishes, there are even two or three storey options.

What I like about off site is the thinking that takes place way before anything happens on site. So often the external works are the last thing to be completed. However, with a different mindset and off site approach, the landscaping can be completed before the buildings arrive. Although this takes a lot of careful planning, I love how offsite is not just pushing the building design, it’s infusing itself into the whole process.

What happens around a building could make or break a development. The arrival sequence and landscaping are such important parts of the whole design that if they can be completed by the time owners take possession of their new properties, a perception is created that the development oozes quality, which is a good advert for the developer.

To take things a step further, the designs are being continually tweaked and fine-tuned to make it more buildable and cost effective but even better designed.

In the modular field, there are several developers, housing associations, specialists’ contractors and architects pushing for quality design as well as coming up with more and more innovative solutions.

Buildability / Benefits

The buildability of shipping containers almost speak for themselves, being a very quick turnkey solution which is used more and more – typically on lower budget buildings – but as illustrated above, they typically have a place in the student accommodation and social housing market.

Insulation can be an issue and does reduce the internal dimensions, although slightly if it’s placed internally. It could be placed externally, though the wonderful icon element would be lost.

Panelised systems do tend to be used more on student accommodation and there are fine examples around the UK. The panelised systems are quick and are typically built off a concrete ground floor structure. Once this is in place the outer and inner walls can quickly be erected. The floors can be either metal sheet with infill concrete, or slightly deeper steel tray panels with troughs which can receive a boarded floor finish above and two layers of fire-proof board to the underside as a ceiling finish. This forms a cassette which can come in panels. They are often filled with rock or glass wool insulation to provide improved acoustics.

The external façade can come with windows and cladding or be applied later. Standard steel roofing systems are relatively quick to erect and install, so there is less need to provide a cassette system. This can also be done, though greater attention to detail is required.

It should be said that light gauge steel framework is being used in many building types, from housing to medical, commercial and leisure sectors.
The buildability of volumetric modules also continues to be refined. I am a big fan of this approach since visiting the Murray Grove social housing project for the Peabody Trust over twenty years ago. It should actually be called Shepherdess Walk but is affectionately known as Murray Grove.

The range of designs that are emerging are one of the benefits resulting from steel’s buildability. Using some basic architectural design principals allows the design to break away from its repetitive nature. For example, from the clearly modular Murray Groove approach to the Shedkm town houses and more densified projects such as Mountreal Gardens below by Pocket Living bring a crisp modern aesthetic while using traditional material.

Modular definitely requires a different mindset shift from all parties when approaching the procurement style. It also needs more team collaboration which certainly seems to be happening as enthusiasm and going the extra mile push these projects forward.

I love how this approach embraces the belief that the design, detailing and construction can still evolve, be refined and get better. It’s a true example of feedback being plugged in at the end of a project, eager to be implemented in the next.


Steel is sustainable as it can be recycled, whereas it’s a little trickier to recycle concrete and timber. Steel buildings are also lighter and therefore leave less of a footprint on the environment.

Interestingly, in the UK there is a push to use “green” steel that relies on recycling scrap steel. Steel is expensive in the UK, 20% more than Europe, so it’s difficult to compete with European manufacturers. Those who have invested in green steel have been lobbying the government to reprioritise so that there is a better chance to be competitive. The UK imports 6.5 million tonnes of steel every year but export 8m tonnes of scrap. These numbers don’t make sense and there does seem to be a missed opportunity here.

How suitable is steel?

Steel is a totally suitable option as an off-site structural material due to its versatility. It’s also light weight so can be easily handled while being assembled in the factory. Panel components can be prepared and fixed by hand; once the pod fame has been bolted together, the rest of the module can also be worked on manually. There is less reliance on craneage in the factory, whereas modular concrete is heavily reliant on automation.

There are several options on how best to assemble on site – volumetric, panelisation or reusing shipping containers, as well a the more traditional steel frame with metal deck floors.

Steel also has all the usual off-site benefits. Reduced waste as the steelwork is cut to suit in the factory. Speed of construction on site, and as a consequence greater health and safety due to the reduced time on site.

About James Finnie

James has been based in Cape Town for the last 14 years, trained in Scotland, and has worked with some prodigious practices over the years. He recently started his own architectural practice where the philosophy of Kind Design puts kindness at the centre of the creative process.

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