Cyclone-rated glazing: No room for compromise
Standards Australia has just upgraded the performance requirements for cyclone-rated glazing. If your building is in a cyclone-affected region, now is the time to assess the structure’s realistic capacity to withstand a super storm, warns JOHN POWER.
When Cyclone Yasi struck the Queensland coast in mid-September 2012, the Category 5 storm reminded all Australians that we inhabit a vulnerable and violent part of the world. Severe weather conditions have always been front of mind in the development of our building codes and construction standards, and technology has largely kept pace with our understanding of the threats of cyclones and similar catastrophes.
However, despite massive technical improvements in cyclone-rated glass products in recent years – improvements that have gone hand in hand with more stringent Australian Standards performance benchmarks – many older buildings in northern Australia continue to feature antiquated glass and structural fittings. More worryingly, the temptation to prioritise price over quality and function, along with the absence of a national product certification scheme, means even new buildings are being fitted with glass products that may fail to comply with specified requirements – a hazard to property owners and public safety, as well as a drain on the public purse.
Non-compliant products in cyclone conditions may undermine the integrity of an entire building, threatening the safety of occupants and the public, and leading to massive expense to property owners, their insurance companies, and an unnecessary burden on emergency services. And, the use of compliant products that adhere to minimum standards of quality and performance extends to installation practices. A poorly installed window featuring weak seals, for instance, is just as dangerous as a window containing improper glass.
We will examine further compliance and certification issues later in this article, but first let’s consider the main selection guidelines relating to cyclone-related glass across different parts of Australia.
UPGRADES TO AS1170
In 2011, Australian Standard AS1170.2:2011 Structural Design Actions Part 2: Wind Actions was released, representing the latest upgrade of the standard. This document, according to industry expert and building physicist, Ian Bennie from Ian Bennie and Associates, contains guidelines based on the division of Australia into different regions according to their susceptibility to cyclonic activity.
“Australia is broken into four main regions, Regions A and B being non-cyclonic and applicable to most of southern Australia (in other words, all of Australia below 25 degrees latitude and all inland areas more than 100 kilometres from the coast),” Bennie explains. “Above 25 degrees latitude in coastal areas, the cyclonic areas are broken into Regions C and D, with D being the most severe in north-western Western Australia from Carnarvon to Port Hedland.”
The location of a new building in a cyclone-affected zone (Region C or D) and its intrinsic design characteristics, orientation, shielding from natural or manmade features and other physical attributes all figure in calculations used to specify the most appropriate type of cyclone-rated glass for a specific application. There is no single type of cyclone glass; rather, there is a range of different thicknesses and strengths fabricated according to the abovementioned criteria. Most cyclone-resistant glass consists of high-strength laminates with thick cushioning interlayers and, at present, thicknesses are in the order of 15 to 18 millimetres.
It is up to the building designer/engineer to specify the appropriate configuration of cyclone-rated glass depending on wind load calculations.
“Specification is somewhat of a grey area,” Bennie concedes. “If a window can be broken by cyclone debris, then the windows and the structure need to be designed to a higher load because wind can enter the building and pressurise it through the opening created by the impact; meanwhile, wind blowing past the structure creates suction on the outside.
“If you can breach the window, then you pressurise inside the structure at the same time that you have suction loads from the wind on the outside, thereby increasing the pressure that the windows and cladding need to be able to sustain. AS1170 only says that where windblown debris impact is specified, then it sets down what should be done where specified (in other words, it comes down to the specifier).”
HIGHER IMPACT TEST SPEEDS
The new version of AS1170 includes much tougher testing requirements for cyclone-rated glass in both Regions C and D. One of the most significant amendments relates to the speeds of objects used in impact tests to establish performance characteristics of different types of cyclone-rated glass. Tests involve two types of object: a four-kilogram piece of hardwood (100 by 50 millimetre) and an eight-millimetre steel ball bearing.
“Before March 2011, the wind code included a cyclone debris impact requirement with a much lower impact speed than currently exists,” Bennie states. “Before, in any cyclonic area, the impact speed that was required to be tested to was 55 kilometres per hour. Then, in 2011, AS1170 included significantly more severe requirements for impact resistance. In Region C, which goes from Bundaberg through northern Queensland, the Northern Territory and down to Port Hedland in Western Australia, the impact speed was increased from 55 kilometres per hour to approximately 120 kilometres per hour; and in Region D, from Port Hedland to Carnarvon, the speed was increased from 55 kilometres per hour to about 160 kilometres per hour.
“The important thing to note is that the impact speed has increased dramatically since the 2003 wind code. The other thing that’s very important,” Bennie continues, “is that in Region C in Queensland there are special Design Guidelines for Queensland Public Cyclone Shelters for buildings that need to function as emergency public cyclone shelters. If a building is to be used as a public cyclone shelter, then it has even more onerous test requirements than those set down in AS1170.”
To date, five shelters have been completed in Queensland and another nine are in development. Most of these structures are schools or recreation and leisure facilities.
R&D ALIVE AND WELL
In light of the recent changes to AS1170, there is a clear necessity for manufacturers to make sure their product ranges satisfy up-to-date performance requirements. Lynton Wombwell, technical services manager at Viridian, says research is complex and needs to take account of not just glass products, but also framing and installation systems.
“We’ve been going through a developmental process undertaking various tests in relation to cyclone-resistant glass products,” he says. “There is a relationship between the performance of the frame and the glass. If the glass can stop a projectile, but the frame fails to hold the glass, then the system doesn’t work. So, the glass and the frame need to work together to achieve the desired outcome.”
Meanwhile, research is ongoing in relation to the performance of glazing. Bennie adds that in most cases the selection of cyclone-rated glass need not require trade-offs in terms of energy efficiency, colour and tinting options and other performance-based characteristics: “Virtually any kind of cyclone-rated glass could include low-E and tint options to give the versatility that non-cyclone-rated solutions have today.”
CERTIFICATION PROCESS LACKING
Bennie advises that while there is a certification process for safety glass, none currently exists for cyclone impact-resistant glass. With no independent, third-party certification schemes in Australia relating to cyclone-rated glass, how can there be peace of mind that products will perform to specification?
“It is self-evident that high quality, fit-for-purpose glass products in cyclone-affected areas are vital for the safety and protection of people and property,” the Australian Glass and Glazing Association’s (AGGA’s) executive director, Nigel Carpenter comments. “There is absolutely nothing to gain and everything to lose by taking shortcuts with product quality or installation workmanship. We recommend the use of reputable and reliable AGGA-accredited member companies to ensure products supplied and installed are fully compliant.”