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April 20, 2021

Why Cyclone Seroja Wreaked Havoc Around Kalbarri and the Midwest

Cyclone Seroja

Introduction to Wind Engineering

During a windstorm or wind gust, damage to the main structural frame of a building rarely occurs. The total wind load normally acts on components and cladding, creating load paths through the various components and back to the supporting structural members (i.e., beams, joists, purlins, girts, studs, etc.).

The ability of a building to resist this wind load is based on the weakest link in this load path, which is usually the supporting components and cladding.

For this reason, designing and ensuring that the building envelope is firmly sealed is one of the most effective ways of mitigating the destruction that can be caused on your entire structure by severe winds.

Understanding Wind Regions In Australia.

Potential wind speed is just one of many factors are taken into account when designing any structure around the world. In Australia, structural standard AS/NZS 1170.2 sets out the procedures for determining wind speeds and resulting wind actions to be used in the structural design.

The Standard covers structures within the following criteria:

  • Buildings less than or equal to 200 m high.
  • Structures with roof spans less than 100 m.
  • Structures other than offshore structures, bridges and transmission towers.

To calculate the gust wind speeds appropriate in which a structure is to be constructed, design engineers follow the regional wind speed table and map as provided in the code.

windregions-1 (1)

Australia’s wind region map

Design wind speed is calculated using specific design criteria:

  • Wind region
  • Importance Level
  • Terrain Category
  • Height of structure
  • Topography
  • Wind direction

Why Cyclone Seroja Wreaked Havoc Around Kalbarri and the Midwest

The small towns of Kalbarri and Northampton fall under wind region B which is classified as a non-cyclonic region.

Wind Speed table

Australia’s regions wind speed table

A regional wind speed of 57 m/s (205 kph) is applicable to Region B. The regional wind speed is a 3-second wind gust measured at 10 m in terrain category 2 (open terrain, grassland with few, well scattered obstructions such as trees and shrubs).

Wind speed is less at lower heights and in suburban areas.

The design wind speed in the Kalbarri township for normal buildings at 3 m height would be 52 m/s (187 kph). This is based on an annual probability of exceedance of 1:500.

Strong wind gusts of up to 170 km/h were recorded as tropical cyclone Seroja swept through the region.

Okay, this doesn’t answer the question. Why were so many buildings damaged or destroyed. It’s claimed that 32 buildings were destroyed, 23 of them homes, with 800 buildings damaged.

The trail of destruction left behind by TC Seroja

It always amazes me – we design structures, members and connections to withstand calculated forces and bending moments, and yet wind storms continue to take down walls and hoardings, signs, transmission towers and poles, and roofs where the maximum recorded wind gust is within the design values. And we claim on insurance and move on. The Aussie spirit to pitch in and rebuild the towns is amazing and heartwarming, but is anyone investigating why so much damage has resulted?

Does the responsibility for design not lie with the engineers and builders responsible for the design and construction of buildings? Does the responsibility to maintain structures and buildings during their design life remain with the owners? Yes, and yes.

So perhaps the reason why so much damage occurred was because structures had not been designed for current wind speeds – quite possible as many are old and the wind code upped the numbers after Cyclone Vance ripped through the Pilbara in 1999.

Or perhaps maintenance was lacking, and corrosion-affected screws and bolts were less strong than when the engineer selected them.

Or maybe, the cyclone wasn’t just a regular cyclone, and the wind actions in region B weren’t just the cyclone remnants the code writers expected them to be…

How Cyclone Seroja Was out Of the Ordinary

Seroja was the first storm of that intensity to hit the Kalbarri and Geraldton regions in more than 50 years.

Scientists estimate that cyclones of this intensity have only travelled this far south 26 times in the past 5,000 years. However, Seroja curved south when it interacted with a different tropical system earlier in the week.

This clash caused a rare phenomenon known as the Fujiwhara Effect. This effect caused the two cyclone systems that interacted (TC Seroja & TC Odette) to rotate around one another and launch Seroja towards the west.

Odette-Seroja_fujiwhara_effect

TC Odette & TC Seroja interaction to create the “Fujiwhara Effect”

Seroja intensified due to warmer-than-normal sea surface temperatures influenced by La Niña conditions. Strong winds then helped to propel TC Seroja away from the coast, allowing the cyclone to sustain relatively high intensity.

Mitigating Risks - Designing For Worst Cases Scenarios.

Back in 2018 when Hurricane Michael struck the coast of Florida, one structure remained structurally unscathed as the hurricane was leaving its trail of destruction.

The famous house that survived Hurricane Michael back in 2018

Unlike other structures, this building was not designed according design wind speeds codes applicable for that region, but rather designed to withstand wind speeds 4x greater than those stipulated in the code.

Engineering is about managing risk!

Sure we have codes to design with, but do you know that codes provide minimum requirements? There are instances where the codes are good enough, and others where a project-specific set of design criteria is necessary.

The most paramount engineering aspect to consider during the design stage of a structure is to assess and understand the risks that might occur during the structure’s design life and designing it in regard to those risks.

Conclusion

Our team uses codes that are based on loads that have a 5% chance of being exceeded during the design life. And the standard design life is 50 years.

Not all structures have clearly defined loads or a design life of 50 years.

Getting clear on the function and purpose of your structure, its design life, and your expectations is really important for a project’s success.

That’s how we start all our projects. Visit our Services Page for information about the structural services that we offer.

Nonetheless, structures should also be properly inspected to ensure maintenance is carried out and that any structural degradation that might occur over time is remediated.

We also perform structural audits on existing structures, particularly those on mine sites. Check out our Structural Inspection page for more details.

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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