The State of Queensland

As Cyclone Alfred bears down on Brisbane, our capital city is woefully unprepared for the wind and rain accompanying a cyclone, so I feel compelled to write about the state of Queensland and what’s been happening.

These are some of the things that have been bothering me, and if you are in the construction industry or you live in a house, a unit, or anything other than a tent, they should bother you, too.

Cyclone Alfred

Let me start with why Brisbane is unprepared for a cyclone.

For the whole of my 30 years of practice as a structural engineer in Queensland, it has always been claimed that cyclones don’t go so far south as Brisbane. There is this magical, mystery line somewhere near Bundaberg that strong winds and deep low pressure gradients are forbidden to cross.

Beyond that line, high winds might come and go from time to time, but the winds are never so fierce as to cause debris to fly through the air – or so goes the claim.

You see, it’s the debris that can bring houses undone.

As bits of houses start flying around in high winds, these objects become missiles capable of penetrating (ie breaking) the fragile glass panes with which we clad our houses.

With a broken window, or windswept doors or some little opening in a wall, comes pressure. Not pressure as you know it at work – but wind pressure. As the wind whistles past your house, some of it enters these openings and tries to blow your house up like a balloon.

This internal pressure pushing up and out on all of the walls and ceilings is combined with the wind that whistles PAST your home which tries to lift the roof off (like the wind over an aeroplane wing).

Now, the problem is, we don’t even need there to be a broken window or door for the combinations of internal pressures and external suction to be a problem.

Security screens and fly screens sometimes get left open during a wind storm. These open doors and windows can become what engineers call ‘dominant openings’ – when the ratio of openings on one wall is much larger than the openings on all other walls combined.

That’s when the internal pressures become pronounced and houses, even in ‘con-cyclonic’ areas, are affected by the full force of wind on a building.

So whether you accidentally have an opening on one wall or it is caused by impact damage, a dominant opening – especially on the windward wall – is bad news.

In Brisbane, Gold Coast, Sunshine Coast and everywhere south of Bundaberg, your engineer and/or designer and/or builder were not required to take this dominant opening into account when they designed your house!

Are Brisbane homes cyclone proof?

Well, yes and no.

Most recently built homes have been built pretty well. They have straps or brackets connecting the roof trusses to the tops of the walls and straps or rods connecting the top of the wall to the floor frame or slab.

It’s that continuity of vertical load path that engineers look for when confirming that a home is cyclone proof.

The other consideration is sideways bracing walls – typically panels of plywood spaced out through the house underneath the plasterboard walls.

With the vertical uplift loads from the roof and the sideways loads accounted for, your home on the whole should be ‘cyclone proof’.

The last remaining details are the details. Accounting for high pressure zones in corners of roofs and walls, connecting upper walls to lower walls at stair voids, bracing ends of houses where the designer has tried to capture expansive views by having only glass on one side or ancillary structures like window hoods and patio roofs.

I think that newer Brisbane homes will generally survive TC Alfred with minor damage to these structures, or a little bit of roof torn off at most.

Then we turn our attention to the older housing stock. The houses that Brisbane Council said are part of our heritage and should be retained.

For more than 30 years they have sat sturdily on site throughout Brisbane with barely a breeze to test their resistance. Those homes are at risk of more extensive roof loss or wall collapse.

You see, the guys that put those homes together never considered that high winds could damage a house. Their primary goal was to ensure the roof could self-support and the walls could hold the roof UP.

Some of those old builders are still working today and their methods haven’t changed – despite years old Australian standards telling us to design for at least some uplift throughout Brisbane.

Their legacy and inability to transition to a world where roofs need to be held down will be their downfall. The old semi-retired engineers that failed to hold them to account will be their greased rope.

This cyclone won’t put every old house to the test – but some will be tested to be sure, and some will find themselves short.

Are Brisbane homes cyclone proof? I guess we’re about to find out!

What Can YOU Do?

1. During a cyclone, keep all your windows and doors closed. If you need to open something, do it on the leeward side – the side wind is blowing away from.
2. Remember that the wind direction during a cyclone changes as the eye of the cyclone moves. If you do open a door or window, the opening should always be on the leeward side, and that wind direction will change.
3. If you are leaving your home during a cyclone, close all the doors and windows – even those openings with security screens on them.
4. If you don’t trust your home can withstand the forces from a cyclone, prepare and leave early before the wind picks up. Close and lock all the doors and windows.
5. If a window on the windward wall breaks during a cyclone, try to create an opening on the opposite wall of at least equal size. The wind will whip through the building, but that is better than the wind taking your roof with it.

Years Later

Years later, after he cyclone becomes a distant memory, re-read this article. If you are an engineer or a builder or a certifier or a homeowner or anyone involved in construction in a non-cyclone area, always remember to take a few extra precautions to protect the home you are working on to give it extra dexterity in the event of a high wind.

While you could always try blaming the Australian standard, or the builder or the engineer or the certifier or the building designer if you lose your roof, wouldn’t it just be better to not lose your roof at all?

• Engage an engineer with experience in designing for cyclone wind loads.
• Don’t listen to those old, semi-retired builders and engineers that say you don’t have to design homes for wind loads.
• Take some extra precautions to keep your property safe.
• Even with security screens, don’t leave doors and windows open when you go away.

I’m Matt Cornell. Best of luck with #TCAlfred and future wind events.