Having a laugh is about all that’s left, and it’s better do it at a shaggy dog story than at rubbery wind load figures. By Paul Davis
After 2020 went viral faster than anyone thought it could, we are seeing more of the same in 2021. It’s affected everyone’s work, even comedians; for months nobody has walked into a bar.
However, the big winners of the pandemic in 2021 are not the mask manufacturers, not the vaccine suppliers, not even the premiers of the states – it’s the dogs. No longer do they have to watch forlornly as their owner drives off to work, sport or meetings yet again. Rather, lockdowns mean owners are stuck at home and the dogs are no longer socially distanced.
So, it’s a good thing that the World Health Organization announced that dogs don’t seem to spread the virus that causes Covid-19. Dogs held in quarantine can now be released. To be clear, WHO let the dogs out.
My particular lockdown has gone on so long that after clipping the dog, my wife has cut my hair with the dog clippers. With the closure of barbers, nail salons, waxing and even tanning places, it’s getting ugly out there.
Cats on the other hand are naturally distanced and don’t care. Being ignored for so long is causing their owners to hallucinate. This morning I saw a neighbour talking to her cat. It was obvious she thought her cat understood her. I came into my house, told my dog – we laughed a lot.
Chickens are one animal that are naturally socially distanced – crossing the road to do so. Meanwhile, parents have found that lockdown is like having an insane parrot glued to your shoulder. “Muuuuum, Daaaaaad, can you help me with my maths? Mum, Dad, I’m boooored”. My suggestion; make some inside jokes!
Which brings me to another joke: the way we determine wind loads for houses using AS4055 – Wind loads for housing. Let me go through the basics of how we arrive at your wind load.
For a start we measure wind speeds using wind meters which are imprecise. And because we need consistency wind is measured in wide open spaces like airports – not real-life situations such as suburbia.
So maybe we have 50 years of wind data. But houses are designed for a 1-in-500-year wind load. So, statisticians somehow extrapolate the short-term result to come up with a number. And why 500 years? That’s just part of the joke – there is no particular reason other than it’s a nice round number that’s big enough so that not too many houses blow down.
We then take the wind speed for an open area and make a rough modification for real variations of wind terrain. We then approximate the ground slope effect, pretending it’s uniform and again make a sketchy modification. And then we do it again for local shielding of the particular structure. We then shoehorn all of this information into one of six wind categories which have been semi-randomly selected by some unknown boffin in a dark university office somewhere.
If that joke isn’t enough, we pretend that the house is rectangular in plan rather than the complex shape that it invariably is. Wind loads pressure distributions on even a simple roof are highly variable and complex, but we use these approximations to work out the wind loads on actual trusses to ten decimal places!
This process results in houses in more exposed situations getting higher design wind loads than houses in less exposed situations, but the actual wind speed number’s grounding in reality is slim at best. The really comical thing: we then take these wind loads and then design our trusses to a precision of ten decimal figures!
The irony of all this is that these rubbery wind loads have a serious effect on your designs and costs. So, given these wind loads are something of a prank imposed by the code, why would so many fabricators be designing for even higher loads than the code requires? If you are designing a typical house in a typical new subdivision in the southern states, it’s almost always N1 if worked out using the code but for some reason it’s nominated as a N2 wind load. And in Brissy a house in the burbs is more likely to be N2, not N3.
I reckon it’s time for us all to stop joking around and since we have to jump through these hoops, at least take the determination of the wind category seriously.
So, there will be no more joking about; I won’t even tell you a coronavirus joke because I’d have to wait a week to see if you got it.
Paul Davis is an independent structural engineer managing his own consulting firm Project X Solutions Pty Ltd. The views in this column are Paul’s and do not reflect the opinions of TimberTrader News. Phone: 02 4576 1555, email: paul@projectxsolutions.com.au
