In the aftermath of the bushfires in Victoria, and with the subsequent updating of regulations affecting what building materials can be used in fire-prone zones, Ken Watson reports on why steel is now assuming greater utility. And he provides a brief on how the National Association of Steel-framed Housing (NASH) is testing buildings and products for bushfire resistance.
Following the devastating Victorian Black Saturday bushfires, today more than ever designers need proven solutions for their clients when building or rebuilding in areas vulnerable to high levels of bushfire attack. Being durable, non-combustible, widely available and easy to work with, steel framing and cladding products already play a major part in the construction of both residential and non-residential buildings to resist bushfire attack.
Conventional steel-framed and steel roof construction is known to perform well under ember attack, providing a straightforward and affordable construction solution based on familiar and readily available materials and construction methods.
The next step is proving its performance under radiant heat and flame contact.
The amended Australian Standard 3959-2009 provides deemed-to-satisfy solutions for various levels of bushfire attack, ranging from BAL-12.5 (Bushfire Attack Level) through to BAL-FZ. At the highest level of attack, BAL-FZ, there is currently some uncertainty as to what solutions are acceptable, and how these solutions should be evaluated.
Seeking a realistic simulation of ‘high end’ bushfire attack, the National Association of Steel-framed Housing (NASH) engaged the CSIRO to develop a real bushfire exposure curve and to conduct full scale fire testing using the Bushfire Flame Front Simulator at the New South Wales Rural Fire Service Eurobodalla Training Centre near Mogo, NSW.
The Mogo facility was designed to assess the resistance of bushfire fighting vehicles which may be caught directly in the path of a high-intensity bushfire. It is the only facility in the world that can model the immersion of a full scale vehicle or structure in a high-intensity bushfire flame front in open air conditions.
The testing was undertaken in April 2010, with the steel-framed and steel clad test building fixed with instruments throughout to monitor temperature and radiation levels.
The building was engulfed in flames for more than two minutes during the immersion phase, with the air temperature near the exposed face reaching around 1100°C. The fire virtually enveloped the building, with similar surface damage more evident at the back corners compared with the fully exposed front face.
The standard corrugated steel roof system with foil-backed insulation blanket, steel battens and steel trusses performed extremely well, with some refinements needed to be made to eaves and gable areas.
The open floor system also performed well, but the fire loading on the posts was severe. The tests showed they will need to be heavier or protected from the fire with insulation or shielding using well-understood fire protection principles.
While surviving the fire, the wall system will be further developed to reduce the rate of heat flow through the wall at the highest levels of exposure. Insulation products intended primarily for energy efficiency broke down during the test.
Nevertheless the interior surfaces, mainly plasterboard with timber skirting and architraves, showed minimal effects and were basically intact after the test.
The magnitude and duration of fire exposure during the full scale test has yielded considerable information on the performance of various common building elements and assemblies. And to assist designers and specifiers, NASH is now preparing a manual containing solutions for construction in bushfire areas.
Ken Watson is executive director of the National Association of Steel-framed Housing. NASH was formed in 1982, initially to represent the interests of the fabricators of steel framing systems for housing. Since then, its scope has expanded to include all forms of low-rise steel framed construction.
