While interest in tall timber buildings continues to grow, there still remains one obvious concern: combustibility. So how safe are timber structures really? Arup Connect spoke with Robert Gerard, a fire engineer in Arup’s San Francisco office, to find out how high-rise wood buildings take fire safety into account.
Can you give a brief overview of recent developments in wooden buildings?
In the past 100 years, the use of wood has generally been limited to light timber framing, generally no more than three to six stories high — structurally, the practical limit for that type of construction. But in the past 10 to 15 years, the development of new heavy timber products — cross laminated timber (CLT), glued laminated wood (glulam), and laminated veneer lumber (LVL) — has opened the door to taller and bigger buildings with wood as the primary building material.
Over the past five to seven years, we’ve seen a number of these buildings constructed in other countries. But in the US, the building codes tend to be restrictive with respect to the allowable height of wood buildings. With any sort of combustible construction, we’re limited to about five to six stories. While this is a practical height limit for light timber, the practical range for heavy timber is eight stories and greater.
At this point in time, we have the structural knowledge to safely design high-rise timber buildings. We can demonstrate through engineering principles that we can reach the level of structural stability that the codes require, even in highly seismic zones like the Pacific Northwest.
The challenge is fire safety, which is perhaps the biggest restriction in the building codes in terms of that height allowance.
Is that because the fire research just hasn’t been done yet for these new wood products?
In the last five to ten years there’s been a huge push internationally to do a lot of research and fire testing to better understand the fire performance of timber building elements. This was necessitated by the desire to build taller. If an architect said, “I want to build ten stories,” the building authority might say, “OK, but we’re going to need to see research and testing to prove it can be safe.”
We’ve made a lot of progress in terms of our understanding of fire performance of timber buildings, so the argument for safety is significantly stronger than it was in the recent past.
So fire engineers support tall wood.
I can’t speak for all fire engineers, but from my perspective, if it can be done safely, then yes.
From a financial perspective, developers are starting to acknowledge the benefits of tall wood construction using prefabricated elements because it reduces construction times and costs. So that’s ultimately what’s driving the push. And I think that as governments continue to quantify sustainability costs through credits, rebates, and upholding energy and carbon standards, it makes the business case to support timber as the primary structural material even stronger. And architects are driven to wood by the sustainability benefits and increased design flexibility.
From a fire safety perspective, at this point in time we have the research to demonstrate safety in these buildings. However, I think the main challenge for approval of tall timber buildings is the current code environment. In my opinion, it’s a matter of engaging the building authorities and demonstrating a technical understanding of the real fire safety issues – potentially overcoming any myths or misconceptions of fire safety in timber buildings.
In my experience, many people think that a timber structure is basically a tinder box, that these sorts of buildings are more dangerous and have greater risks than other buildings. Our task as fire engineers is to use the information available to us to evaluate and communicate the real fire safety risks, develop appropriate fire protection strategies, and allow society to decide an acceptable level of risk for these buildings.
Fortunately, building authorities are increasingly open to this conversation.
How are fire engineers interfacing with architects and others who are eager to see these buildings increase in number?
Fire engineers often act as code consulting specialists for architects and developers. We will evaluate a building design to check that the plans are in compliance with the building and fire codes. Where there are non-compliances, such as a proposed building material or height, there are a couple options: the non-compliance can be fixed, or the fire engineer can engage the building and fire officials to discuss means by which an alternate design might be approved. This is often accomplished through demonstrating equivalence with the code design, or by providing additional fire protection measures to achieve the level of safety intended by the code.
Can you describe some of the fire safety research that’s been conducted?
One of the first major research projects was an initiative called Timber Frame 2000, out of the UK. The timber industry realized there was a desire to try to build to up to five or six stories using light timber, but the UK building codes limited timber buildings to three stories in height. This prompted a series of tests on a six-story light timber framed building. Results of the tests were used to demonstrate that light timber frame buildings could meet the functional safety requirements that would be required for non-combustible steel or concrete buildings. This motivated changes to the prescriptive code to increase the height limit to six stories for wood buildings.
Another major test was recently published by a group called FPInnovations in Canada. They tested CLT panels as walls and floors to demonstrate that the panels have a certain level of fire performance comparable to non-combustible building elements like concrete, for example.
The tests showed the walls and floors could be designed for up to three hours of fire resistance, in many cases exceeding the code requirement for structural element fire resistance ratings. As a result, the use of CLT has been adopted into the prescriptive building codes in the US and Canada. The researchers were able to present this information to the code councils and request changes in the building codes based on the demonstrated performance through fire testing.
While quite a bit of research has been done, there are still opportunities for further research. We’ve actually summarized findings and prioritized gaps in a study we recently published in collaboration with the Fire Protection Research Foundation.
The best examples of contemporary high-rise timber buildings are in Europe and Australia. Is that because the building codes there are more timber-friendly than in other parts of the world?
The building codes in Europe tend to have taller height allowances compared to the US. This makes the approvals process quite a bit easier. But the prescriptive limit isn’t a deal-breaker. The height limit in Australia is actually capped at three stories for wood buildings, and Australia recently built the tallest modern timber building in the world. The primary difference in terms of approvals, I’d say, is there’s a greater understanding of timber fire safety risks in those regions, resulting from research and testing and educating building authorities. This is something we’re currently working on in the US.
Additionally, the codes in the UK and Australia are a bit different than the US building codes. They incorporate more of a performance requirement and fewer prescriptive regulations using what are called functional objectives. These are fire safety principles that apply to all buildings regardless of building material. Agreement that the functional objectives have been satisfied allows significantly greater flexibility in the building material and design – including using wood as the primary building material and increasing a building’s height.
So the US code basically says you have to do A, B, or C, whereas the other countries allow more flexibility as long as the outcome is X?
Exactly. In theory, there’s greater flexibility in a performance-based code environment. And while the US codes allow the use of what are called alternate means to satisfy the prescriptive requirements, it’s ultimately up to the discretion of the code authorities to approve the alternate.
This limitation in the prescriptive requirement is where the fire engineer becomes valuable. We will engage the building and fire officials to discuss an alternate approach to a prescriptive requirement. Ultimately, the role of fire engineers is to utilize our knowledge of fire safety to engineer fire protection strategies and demonstrate that the level of safety required by the prescriptive code has been met using an alternative approach.
Is the general consensus among fire engineers that tall wood buildings eventually will be approved in more building codes and built more often in the US?
I’d say so. It will probably happen in Canada first; they’re interested in building the first North American high-rise timber building. The Canadian government has recently funded design competitions to promote tall wood building design.
We’ve seen a renaissance of tall timber building design with approval of tall timber buildings around the world, and I think it’s likely this trend will continue to the US in the near future.
Robert Gerard is a licensed Fire Protection Engineer in California with a Bachelor’s in Architectural Engineering from California Polytechnic University, San Luis Obispo and a Master’s in Fire Engineering from the University of Canterbury in New Zealand. Robert’s work with fire safety in timber buildings spans three continents; he is currently based in Arup’s San Francisco office.