Sawmilling South Africa’s (SSA’s) Talking Timber webinar series aims to overcome misconceptions about wood as a structural material by informing architects, engineers, insurance companies, regulatory authorities, academics, and developers about engineered timber products and innovative mass timber building systems.

MODERN TIMBER BUILDING SYSTEMS REQUIRE A FIRE SAFETY MINDSET

Climate change mitigation is fuelling the global move by architects and structural engineers favouring timber over conventional building materials. However, the controversial issue of the safety of timber structures is always top of mind.

Timber and Fire was the hot topic of the fourth webinar in the Talking Timber events. Over 95 people attended the online sponsored by SSA, the Institute for Timber Construction SA (ITC-SA) and the SA Wood Preservers Association (Sawpa).

In her closing remarks, Tafadzwa Nyanzunda of the Department of Trade, Industry and Competition (the dtic) said the webinars form part of the Forestry Sector Master Plan strategy of promoting the use of timber in construction.

Prof Brand Wessels facilitated the discussion between Prof Richard Walls, head of the Fire Engineering Institute at the University of Stellenbosch, and Dirk Streicher, the founder of Ignis Testing Centre.

The Fire Engineering Institute is Africa’s only fire engineering and structural design research institute. At the same time, the Ignis Testing Centre is Africa’s only fire testing centre.

Walls and Streicher said fire safety engineering design is the starting point when designing timber buildings. “It is not an add-on at the end,” they emphasised.

Walls warned that many structural engineers don’t realise that when they sign off a building under the Code of Practice for Designing Buildings, SANS 10160-1, the “accidental design situation” clause includes material failure and exposure to fire.

The presenters discussed the fire rating of materials and how they are derived. Streicher has conducted over 225 tests on various materials, including doors and panels, to see whether they have a sufficient fire rating and satisfy the time given for evacuating a burning area.

Materials

Construction materials provide different protection during a fire:

• Steelwork provides passive protection. For example, intumescent paint on steel swells and presents a char layer. It protects the steel, and the extra time is measured to give a fire rating. However, sometimes the paint can cost more than the steel at higher fire ratings, such as 120min.
• Concrete expands and can fail. New, high-performance concretes are less fire resistant.
• Timber provides passive and sacrificial cover.

The failure of a material is measured against three elements known as REI:

• The stability, strength, and structural resistance of the material. In other words, the load-carrying capacity and deflection
• Integrity: The passage of smoke and flame through the material must be prevented
• How much heat passes through the material? The average temperature rise of the upper surface (unexposed side) may not exceed 140°C, and the peak temperature rise may not exceed 180°C.

What makes mass timber safe?

“The fire chief in a South African city will have a heart attack if you apply to build a 10-storey wood building. But mass timber is very popular in the northern hemisphere, Australia, and New Zealand. Stringent fire-safety engineering is applied when designing multi-storey timber buildings, some over 18 floors high, said Walls.

Mass timber elements are designed so that a sufficient cross-section of wood remains to sustain the design loads for the required duration of fire exposure.

“Mass timber is a unique building material. It achieves structural performance and passive fire-resistance objectives for larger and taller wood buildings while offering enhanced aesthetic value and environmental responsibility,” he said.

When timber is exposed to flames, it dehydrates, starts flaming (pyrolysis), and forms a char layer. The char layer acts as an insulator and protects the core of the wood section.

“We rely on char oxidation to protect our structures. Structural engineers must design for charring and what is known as the ‘worst short-lived event in 50 years’. You need to know how fast the charred front moves to determine the fire rating of a timber species,” Walls explained.

In a standard building, a 60min fire rating is based on the fuel load. But all the elements contribute to the fuel load when it is a timber building. The heat release rate and fire intensity increase with exposed timber. Some international building codes limit exposed wood areas and require that a percentage of beams be clad to reduce the heat release rate.

There are two factors to consider when designing with cross-laminated or mass timber: the adhesive and the connectors. Wessels said some tested adhesives like melamine urea-formaldehyde (MUF) perform better than polyurethane (PUR). It is the subject of ongoing post-graduate research.

A roof structure without metal connectors provides about one-hour resistance. Still, when plates or gang nails melt, you lose all structure integrity in five minutes. We need to do a lot of research into gang plates and the best way to connect timber members.

“Think about it,” remarked Walls. “You can design the timber to work, but the real danger lies in the steel connectors that melt at high temperatures. Buildings collapse because the steel connectors are always the first thing that fails. Hence the use of intumescent coatings that protect them.”

Streicher pointed out that intumescent coatings can be applied to timber, but they have not been tested enough. “It takes time to activate. If it stays bonded, it will provide additional protection. Still, the chemistry of the burning timber may influence the intumescence,” he said.

“The data pool we currently have is small, and more would be very helpful for professionals working in the timber industry,” Walls commented.

“Fire is one of the biggest challenges we face for large-scale adoption of mass timber in the South African built environment. We must not stop searching. We need more testing and thinking about timber and fire in structures,” the presenters concluded.

Written By: Joy Crane
Source: WoodBiz Africa