November 3, 2020

The Cyclone Season Is upon Us Again!

November 1 marks the official start of the Australian tropical cyclone season, and although Mother Nature is wreaking havoc in Vietnam and The Philippines, the Bureau of Meteorology (BoM) currently advise there are no indications of tropical cyclone activity developing in the Australian region in the next few weeks.

So far so good, but this has not been the case for Philippines which has been hit by the strongest tropical storm (ex-typhoon – Goni) so far this year. With landfall winds of 225 kph and gusts of 280 kph, the storm is estimated to have damaged 90% of the buildings in Virac, the town capital of Catanduanes province.

Cyclonic wind design- Virac

Flooded town of Virac

What Is a Tropical Cyclone?

A tropical cyclone is a rotating, organized system of clouds and thunderstorms that originates over tropical or subtropical waters.

In our Southern Hemisphere, tropical cyclones rotate clockwise while in the Northern Hemisphere they rotate counterclockwise. Tropical storms and cyclones are classified as follows:

    1. Tropical Depression: Sustains winds of 60 kph or less.
    2. Tropical Storm: Sustains winds of 61 to 120 kph.
    3. Cyclones/Typhoons: A tropical cyclone sustains maximum winds of 120 kph or higher.
    4. Severe Cyclone: They have winds of 180 kph (96 knots) or higher, corresponding to a Category 3, 4 or 5 on the Saffir-Simpson Hurricane Wind Scale.

Last season 2019-20 (November to October), 10 tropical cyclones were recorded around Australia, five of those becoming severe (Category 3+) with:

      • 4 systems in WA
      • 2 in the NT
      • 4 in QLD

This season, the BoM predicts an average to slightly-above-average number of tropical cyclones. On average, there are 9 to 11 tropical cyclones each season in the Australian region, four of which typically cross the coast.

La Niña conditions in the tropical Pacific Ocean and average to warmer-than-average ocean temperatures to the north of Australia have influenced this season’s tropical cyclone outlook. In La Niña years, the first cyclone to develop across the Australian region typically occurs earlier than normal, around the middle of December. During average years, the date of the first tropical cyclone to make landfall over Australia is typically in early January.

You only need one cyclone to significantly affect communities and infrastructure. Will Your Assets Be Ready?

Sadly, we see a lot of damage to houses and domestic structures from wind gusts around 100 kph. That’s really not that much. It’ll blow your hat off sure, but it’s not meant to blow your roof off!

In most of Australia, we design for at least 176 kph gusts (at 10 m in open terrain). We shouldn’t be seeing roof sheeting blown next door after a “strong wind” in Perth.

Cyclone Outlook

A building destroyed by strong wind

The difference in FORCE between a 100 kph wind gust and a 176 kph wind gust is HUGE! Actually, the factor is 3.1. So when things blow over at 100 kph the force is only one-third of the design force required by our codes.

Poor design, poor construction and inferior (non-compliant) building materials all contribute to this problem that insurance companies seem happy to pay out on.

Who Should Be Responsible for the Effects of the Damage?

In my opinion, structural damage should be avoided through proper design and construction in accordance with the National Construction Code (and that’s not that hard), and if it’s not, then the owner should be responsible for the effects of damage, and not the insurer!

Structures need to be properly designed for all expected loads and load combinations in accordance with our standards and regulations. For us as structural engineers, cyclones (and the remnant strong winds in non-cyclonic regions) are just another load case.

What about those structures that have been designed for a lower wind region, and then re-purposed in a higher wind region? It’s encumbent on the owner to ensure that the structures meet current standards and withstand the environmental loads applicable to the region.

Cyclone Map

Cyclone Outlook Map


Ensure your structures are safe and compliant – especially those “temporary ones” that have been sitting there, unanchored, for 20 years – by having them assessed by a structural engineer.

We perform structural audits on existing structures, particularly those on mine sites.

Don’t wait until it’s too late. Cyclone George resulted in two fatalities and 22 notified injuries at the camp.

Have a Project that You Want to Discuss?

Book a meeting with us and we will assist you with the structural design and analysis of your structure to help you produce a structure that is fit-for-purpose and capable of resisting all applied loads without failure during its intended life.

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