As the eye of Tropical Cyclone Yasi passed over the Mission Beach region of Queensland, it was estimated wind gusts reached 245 kilometres per hour. Buildings in the surrounding areas were hit by gusts reaching 90 per cent of their design wind speed.

Wind forces were intense enough that door and window hardware failed. In a case in the town of Cardwell, after doors and windows failed on a house, the windward openings generated such pressure within that the roof literally popped off, much if it flying 100m downwind and causing serious damage to a newly-constructed neighbouring house.

Such damage caused by the cyclone prompted the Australian Building Codes Board (ABCB) to commission the Cyclone Testing Station at James Cook University (JCU) to undertake a field study in the area.

A flat roofed house in Cardwell which experienced winds near 90 per cent of the design wind speed. (Photos courtesy of Cyclone Testing Station, JCU)

According to the report, the damage in the Cardwell case happened despite that house incorporating numerous anti-cyclone damage measures. For example, its 11m wall, which faced the ocean, consisted of louvered windows protected by steel mesh and a single door entrance. The aluminium roof cladding was screwed with cyclone washers to 75x50 mm timber battens spaced 1000 mm apart and fixed to 125x50 mm timber rafters by two 75 mm nails.

The fact the windows and door failed in this example clashes with the robustness provisions of AS/NZS 1170.0 which require that “the repair of a structure be a function of the extent of damage to it”.

In other words, it is not reason able to expect to replace your entire roof after a window has failed. Let alone the neighbour’s house.

Hence JCU is recommending a detailed study be undertaken into revising AS/NZS 1170.2 so that low-rise buildings in vulnerable regions are only designed for wind pressures obtained using potential dominant openings.

But overall, the JCU report was quite positive. Queensland’s Deputy Premier and Minister for Local Government Paul Lucas even issued a press release in response saying that current building standards for structures in cyclone zones are “up to scratch and working well”.

The inspectors found less than three per cent of all post-1980s houses in the worst-affected areas experienced signif icant roof damage. This compared to significant roof damage to more than 12 per cent of the pre-1980s housing inspected — which were obviously built to less stringent requirements.

The team was satisfied that contemporary housing had performed well where builders had diligently applied the Codes and Standards.

This was unsurprising, given that wind loads experienced in the effected areas were less than what may have been expected from a Category 5 storm — and were less than the design wind speed of buildings in the region.

The researchers said the study reinforced the need to design the whole low-rise building envelope - including doors and windows, roller doors, cladding, eaves lining and skylights - to resist the expected ultimate limit states wind forces.

They emphasised the role of dominant openings in determining the inter nal pressures in buildings, and suggest ed any investigation into the requirements of AS/NZS 1170.2 include determining internal pressures in tropical cyclone-prone areas. As examples showed, while a house may have had metal screens over the windows, it would still be exposed to inter nal pressures from dominant openings because of failure in door furniture.

In fact, a chief reason that many older houses sustained only minor or negligible structural damage was that the envelope of the house remained intact, resulting in low internal pres sures. This effect was complemented by combinations where considerable venting occurred, for example through vented eaves.

The window and door damage observed was often due to frame separation from the building, and the fail ure of furniture used to secure the door or window, allowing it to open during the high winds.

A recurring theme: when large opening were created, it often led to other failures.

A number of cases were observed where door and window furniture failure meant that a previously closed door or window became an opening. This furniture included latches, bolts and hinges: items not traditionally thought of as structural elements, but crucial to the satisfactory performance of the building envelope.

A building in which windows 7.7m wide were blown in. The loss of windows allowed the wind to internally pressurise the units causing loss of the roof cladding and failure of the first internal beam connection to the wall.

The investigation also found window furniture failure associated with locks and movement of windows within frames. And there were a number of instances observed where sliding glass door panels had blown out of their tracks.

Damage to window and doors during cyclones can create a dominant opening and allow large internal pressures that may lead to the failure of the roof sys tem. But even where windows failure did not cause damage to the roof system, the JCU team observed evidence of sub stantial water ingress through these openings.

Areas for improvement

The report suggested a number of areas where improvements could be made in building design.

As part of the building envelope, doors and windows are recognised as important elements for separating inter nal and external pressures in buildings. The JCU teams say these elements should be designed to resist the differen tial pressures that may exist across them. This includes not only the glazing, but also the frame and any furniture that secures opening panels.

With Cyclone Yasi, most failures of structures were put down to non-compli ance with current Codes and Standards but there were a few areas where updat ed industry information was deemed vital. In terms of windows and doors, this included:

• Connection of window frames to the supporting members to transmit wind forces to the rest of the structure

• Selecting appropriate door and window furniture to transmit wind loads without allowing the door or window to open

• Selection of sufficiently durable materials for use in near coastal environment

• Detailing windows to resist the wind pressures

• Selection of the right connections for use with the given wind classification

• Installation of sheeting and tiles fasteners in accordance with the manufacturer’s recommendations

• Determining the wind classification of sites.

A useful resource on the topic was released in mid-2010 by the Australian Windows Association’s (AWA). Its win dow fixing guide effectively addresses the issue of adequately fixing windows — and many sliding doors — to a structure. And this information is available on their website.

Of course the technical report also contains a number of recommendations beyond the openings in building struc tures, and these can be viewed on the university’s website at www.jcu.edu.au/cts.

Development Manager at the Cyclone Testing Station, Graeme Stark, points out for example that “The bigger prob lem from our point of view was the roller doors.”

The report details several issues relat ing to the role of garage doors and open ings in determining the internal pres sures in buildings. It is also suggested the development of new construction requirements for buildings within a storm surge zone, as well as measures to ensure education and the ongoing main tenance of homes.

Building Codes Queensland is currently working with the ABCB as they review existing cyclone standards and zones.