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With structural steel pushing the bounds of what’s possible in construction, it’s more important than ever that we integrate effective structural fire engineering principles into building design and build. Dr Linus Lim makes the case for a holistic approach to ensuring the integrity of steel structures.

“When constructing any building it’s important to have the structural fire engineers involved early on in a project,” says Dr Linus Lim, Technical Director at Holmes Fire. “This will result in optimal designs for fire and life safety as consideration can be given to both the micro detail and the bigger picture.

“Our experience is that builders are starting to understand the significant benefits and cost savings that can be attained by getting us involved as part of their design team sooner rather than later”.

Linus’ approach to structural fire engineering is a holistic one that takes in a balanced, whole-building approach to attain inherent fire resistance of the structural system, rather than relying entirely on passive fire protection of the structural elements.

“We are building structures that are bigger and more complex than before,” Linus says. “Our fire testing systems and methods of analysis need to keep up in order to provide the appropriate level of safety for these more complex structures.

“We’re seeing situations in which one tenant can take up three, four or five floors, so we’re seeing the likes of open circulation stairs and atriums inside offices that connect those floors. It can give a feeling of open air and connection; however, in the event of a fire, not only is there an issue of smoke spread but also fire spread across multiple floors simultaneously, which is normally not considered in codes in regard to structural fire performance.”

Linus says there are ways to combat smoke spread in such situations, such as the use of smoke and fire curtains, but our assumptions regarding structural fire design don’t currently cater for a fire across multiple floors.

“With open-plan offices, the chance a fire could spread around the whole floorplate is increased. The behaviour of such a fire is one that the modern fire engineer faces now that we didn’t face 20 years ago.

Performance-based fire engineering

Structural fire engineers increasingly find themselves needing to implement cost-effective solutions to design challenges. In that context Linus recently conducted an Australian Steel Institute (ASI) seminar on the benefits associated with performance-based structural fire engineering and how contemporary methodologies can be balanced with longstanding prescriptive fire protection methods.

Fundamental to the work of a structural fire engineer, he says, is understanding how the demands of a fire on a structure will affect its design. This involves understanding what the design fire for a structure is.

“For example, if we want to have open-plan floor spaces – or use more plastics in our furniture or have a paperless office – it will affect the fire design, which is basically the size of a fire and how it can spread.”.

Linus says the Building Code exists to provide certain requirements and guidelines in relation to building design: ensuring occupant safety and safe egress, the need for a structure to remain resilient enough to allow fire brigade intervention and preventing building collapse onto its neighbours.

“Either you achieve those objectives by following prescriptive Code requirements, which says you shall achieve a certain fire rating. Or you can do it by taking a performance-based fire engineering angle, which allows you to take a first-principles approach and analyse your structure as needed.”

He says the demands of construction in the 21st century mean that we need to start to look at more advanced methods of analyses and more realistic design fires. “If we take the objectives of the Code as a basis, then we can come up with performance-based solutions that meet the demands of the structure itself.”

Steel in construction

In addition to examining human behaviour, occupant evacuation and the dynamics of fire spread within buildings, Linus’ work requires him to analyse the relative benefits of different building materials, all of which have pros and cons associated with their use. He contends that the use of structural steel compares favourably to building materials such as concrete and timber.

“Concrete may be the most common building material used in Australia and it has good inherent fire resistance because it’s non-combustible” Linus says. “The sheer mass of concrete means it protects the structural reinforcing inside the building and keeps it cool.

“Timber’s primary advantage is that it’s much lighter than either steel or concrete and it offers fantastic aesthetics. But the risk with using timber is that it’s obviously combustible, so we need to ask how much it will contribute to the fire load.”

Linus says steel’s advantages as a structural material lie in its high strength and stiffness ratio relative to its weight, which means it lends itself to large spans and lighter sections. He says fireproofing is applied to steel so that it can withstand temperatures of around 1000 degrees. Steel’s strength and rigidity is known to drop off at temperatures beyond 400 or 500 degrees.

“Historically it was assumed that if a multi-storey steel structure was built and a couple of elements weren’t fireproofed, the building could suffer a catastrophic collapse in the event of a severe fire,” Linus says. “However, research in the last 30 years in the UK shows that if you build a building with good inherent fire resistance into the structural system, there is no need to fireproof all of the steel used in the building. Steel buildings can be designed for very good fire performance if certain load resisting mechanisms can be mobilised.” 

Another benefit offered by steel, Linus says, is that it will yield gradually to heat, so you don’t get the kind of sudden catastrophic failures you can sometimes see when concrete is exposed to heat.

“What you’ll see sometimes is a phenomenon called explosive spalling, which is essentially when surface concrete starts to pop off and expose the reinforcing steel within.

“In a nutshell, while there is no single perfect structural material, steel can be designed to perform very well,” he says.

What does the future hold?

Fire safety engineering, as a field of study and a practice, has come a long way in the last three decades, Linus says. Not only are there more specialists now compared to those in traditionally popular fields such as seismic engineering, the dynamic nature of the construction industry makes fire safety engineering an endlessly rewarding endeavour.

It’s clear Linus sees research as fundamental to meeting many of the challenges that lie ahead – and he says the steel industry has an essential role to play.

“There’s been a huge amount of research led by the steel industry in Europe, UK, New Zealand and Australia. This has involved physical fire testing and also advanced computer modelling to predict how structures behave in a fire event. I’d also like to see more fire testing to understand the effects of modern furnishings and appliances we put inside our rooms and to understand the fire loads we’re dealing with at the moment,” he says.

At the same time, Linus says we need to exercise care about how we interpret test data and how different building products are used at scale. The use of products not suitable for their intended application is a current hot topic, with Linus saying products might perform adequately in a small sample test, but that the results can be very different when applied to a much taller building.

“Because the design and use of buildings are different decade on decade, we need to support more research and testing so our practices can be verified constantly. In that regard, the steel industry has an important role to play.”

Liberty OneSteel’s role in fire testing

Liberty OneSteel undertook extensive fire testing research in the 1980s and 1990s when the company operated under the BHP banner. The following videos of tests from that era show outcomes that resulted in changes to the Australian Building Code for carparks and offices that are still in place today.

Open-deck and closed carpark test
 
These tests revealed that it isn’t necessary to fire protect steelwork in open-deck and closed carparks (with a functioning sprinkler system). The tests provided the basis for modifications to the relevant Australian building regulations.

High-rise office building test

This test showed that unprotected steel in combination with other components of a fire safety system can deliver a high level of fire safety. It was later supported by tests conducted in the UK that revealed no need to fireproof all the steel in an office building.