‘An exoskeleton and a spider’ | working with the combustibles ban

Kelly-Harrison, Whitby-Wood
Kelly Harrison
Whitby Wood
Dave-Lomax, Waugh-Thistleton
Dave Lomax
Waugh Thistleton Architects

Dave Lomax, associate director at Waugh Thistleton Architects and Kelly Harrison, director at Whitby Wood, consider the implications of recent fire safety regulations for mass timber construction. They suggest some of the solutions being developed in response to these challenges. 

“But I thought timber had been banned?”

It’s a phrase heard in a thousand meetings since Dame Judith Hackitt published the Review of Building Regulations and Fire Safety in 2018.  It is understandable that simple, easily digestible headlines were required as outcomes of the Review. But the truth, as ever, is more complex.

The first point to make is that the ban refers only to residential buildings, and only to those above 18 metres in height. This height is measured up to the finished floor level of the highest occupied storey, although there is consultation to reduce this height to 11 metres.  

Furthermore, no specific material is banned.  The legislation requires that:

any external wall or specified attachment of the building only contains materials of European Classification A2-s1, d0 or A1.”

There are multiple complexities we might dwell on here. But, put simply, external walls to mid-rise residential buildings must be made from products that are non-combustible. The only materials named specifically are those that are exempt. This includes some surprises, such as uPVC windows, but we digress.  In principle, low-rise timber housing and any size of timber commercial projects can still be delivered.

A new model for mid-rise buildings

So, what about the mid-rise, carbon-conscious corners of the housing market?  In the past, we have always, in the UK at least, designed mass timber housing projects with CLT loadbearing panels in the external walls.  It is clear that timber cannot be defined as ‘non-combustible’, and therefore it cannot be placed in the external wall. 

The answer here is fairly prosaic: design the building without wood in the wall.  There are lots of other technologies that habitually do not require load-bearing elements to be part of the external wall. Typical RC frames are one simple example, especially where a curtain wall solution is applied.  In the US, this is already how mass timber housing is constructed, using glulam columns and CLT slabs to create an avatar of a typical RC frame but in wood.  

An excellent example, albeit for student housing, is the Brock Commons centre in Vancouver. This rises to 18 storeys, with prefabricated external wall panels hung from the frame rather than vice versa.

So, the core (if you’ll excuse the pun) of a solution exists. But we have two key problems to solve. Firstly, if you sit the external wall on top of the slab, then the slab must encroach on the façade – placing timber back in the external wall.  

Secondly, we are accustomed to having flat soffits in our concrete residential buildings (and our timber ones, for that matter).  A solution is therefore required that does not require downstand beams which, in glulam, can become a significant barrier to the distribution of services.

Waugh Thistleton and Whitby Wood practices have both worked with enlightened local authority clients who, having declared climate emergencies, were keen to discover potential solutions.  This has given us two excellent case studies to conceive of solutions to these challenges.

Whitby Wood | Model of a 'tall timber' building

Whitby Wood | Model of a ‘tall timber’ building

A steel exoskeleton

Let’s take the façade challenge first. It is perfectly feasible to construct a non-combustible steel or concrete frame which is entirely structurally independent of the building beyond, tasked only with holding up all the things we need to keep the weather out and the heat in.  However, this seems incredibly inefficient – essentially building two significant and independent structural systems right next to each other.  

But what if we were to reverse the problem? If combustibles cannot be inside the façade, it does not mean the non-combustibles can’t be outside of the façade.  In our design, the steel ‘exoskeleton’ extends inwards, supporting the floor edge and avoiding the doubling up of structure.

The second step is to recognise that the steel exoskeleton is, in fact, a stiff tube. The other place we find stiff tubes in our tall residential buildings is in the core. We identified that it is possible therefore to use the steel façade to deliver the stability required for the building, and the need for the core to perform a structural function in this way is negated. This gives an opportunity to take more efficient advantage of the ‘overperformance’ of the steel frame, as well as mitigating its cost.  We omit a costly component of the typical structure to offset the capital cost increase for an atypical façade.  

The Rothoblaas spider connector

Now we move on to looking at the beams.  Usually, when dealing with post and beam structures we have precisely that – both post and beam.  We do not often design for mass timber with a connection solely between a column and the slab, given the challenges in resolving the forces at play.  

However, technology is progressing in this field at pace.  In the past we may have designed a bespoke steel connector to do this work only to be met by queries from clients, approved inspectors and warranty providers as to how we could demonstrate its performance.  Another common challenge for mass timber is that the material is held to a higher standard – a welded flanged plate to do this job must have been designed a thousand times on steelwork projects without much consternation.  

Timber specialists Rothoblaas have recently brought to market their ‘spider connector’. This product recognises the need to deliver clear soffits to open up different mass timber technologies to new and important markets. Better still, the spider comes complete with testing results, certification and warranties. A comparatively small piece of steelwork could be key to unlocking the delivery of mid-rise residential construction in mass timber in the UK – and with it, the conservation of millions of tonnes of embodied carbon.

What’s next?

Of course, in this article a number of necessary simplifications have been made.  An emerging update to BS9991: Fire Safety in Residential Buildings will land on our desks in late 2022, posing new questions to the sector. And the London Mayor’s recent blanket ban on combustibles in the façade at any height for grant-funded housing is a stunning blind spot, given the context of the climate emergency.  

But herein lies the problem. We simply can’t let this lie. It’s far too important. Both Waugh Thistleton and Whitby Wood are working hard to real-world, workable solutions – not dodges or workarounds – to keep mass timber on the drawing board and construction site. Over the next year we plan to release more formal and peer-reviewed research, design and information to support the mass timber residential industry in the UK.  Watch this space…