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Cyclone, Typoon and Strong Wind

posted Sep 9, 2018, 8:17 AM by jeffery jim

I believe that most of you have witnessed the wrath of mother nature raging through Japan from shared videos in social media. So what have you learned from such tragedy? *The above photo was taken very close to Jesselton Point, Kota Kinabalu, Sabah.

We saw how light gauge steel roofing blown away like tissues and how cladding including bracing which are very fragile blown apart by typhoon of such magnitude.

How safe is our roofing if designed in accordance to Malaysia Standard or British Standard? There are seven scale or category of cyclone where it started with tropical depression, tropical storms and then category 1 to category 5. Our roofs are designed to resist tropical storm with speed between 74 to 95 mile per hour or 34 to 63 knots. We will not stand the chance if we face category 3?

Not really true. It depends on the geometry of the roof profile you selected and the positioning of battens and rafters. This mean, the span between batten and rafters play important roles to avoid such catastrophic event. Typical Base Metal Thickness ranges from 0.25mm to 0.50mm (exclusive of coating thickness).

Take Kota Kinabalu for instance, let say a Category 3 cyclone reached the shoreline with an estimated wind speed of 50m/s with factors such as Importance factor, I; Terrain/height multiplier, Mzcat; Shielding, MS; and Hill Shape, Mh taken into consideration plus air pressure around 945mb, the dynamic wind load pressure would be around 4.5kN/m². Since we are concerned with leeward instead of windward, the pressure would be around 3.6N/m² (0.8 of the projected wind load pressure, windward is 1.2).

When loaded this into finite element analysis software, at loading rate of 5.0kN/m² (assuming category 4 cyclone), any light gauge roofing sheet with span below 1.5m can withstand such treacherous exposure with minimal deflection is bolting is done accordingly. For 2.0m span, i recommend that anything more than 0.35mm is quite safe. Tips: Ratio of safety for 1m in comparison to 2m span is around 10 to 15 fold.

Another significant consideration is the tear-off shear not perpendicular to the wind direction instead in certain axis or planar which i will not discuss at this moment but it formed part of BS5950.

Sadly, I was made to understand many roofing manufacturers nationwide have yet to get SIRIM accreditation for this since most of them are not able to design and test in accordance to ASTM E2322-03.

So this advisory is just a brief tip on selecting the right thickness for your roofing sheet and the importance of batten and roof rafters span distance in securing your house from being destroyed by cyclone. Remember, 0.35mm minimal nominal thickness for base metal thickness of a roof sheet and a maximum of 1.5m span between rafter and batten is key to safe roofing installation. For that reason, most of the roofs where inhabitants stayed safe from this Jebi cyclone remained intact due to robust design.

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