When you’re near the bottom of the food chain, you are always going to being picked on. Billions of krill are killed by whales every year. Do people feel sympathy for the krill? No, they love those brutal, murderous whales for their splashyness. Are people shocked by the bamboo deforestation caused by pandas? No, they love pandas for their cutesy eye markings and roley-poley antics.
It’s the same for workers. At the fringes of my childhood-memory are the men who used to come and empty the dunny of “night-soil” and carry the can out to a waiting truck at the front. Did anyone ever think about their job security when they introduced flush toilets? And the guys fixing the potholes in the road are always getting the blame for the potholes. Do you see how we pick on the underdogs? They are doing the fixing – we are the ones making the holes.
Whole businesses at the bottom of the food chain get the rough end of the pineapple. In fact, pineapple growers are one of them. Picked on by globalisation, all the pineapple farms near to my parents’ house have been bulldozed. We don’t eat fresh pineapple anymore and all our pizza pineapple comes from somewhere near a sewage farm in some poor south American country. Pineapples and their growers have been picked on!
So let’s face it – the truss and frame industry is at the bottom of the building food chain. We are contractors to contractors and have no control. We have to design structures where the architect has not thought one iota about how the timber structure might work. We have to brace houses with no walls and take the blame for a sagging roof when the electrician cuts through a girder. The first to get blamed and the last to get paid. The builder’s patsy.
And we all know that when a cornice cracks that the truss manufacturer is automatically blamed. It couldn’t possibly be the plasterer’s poor work, the carpenter’s leaning trusses, the concrete slab with a big hump, foundation settlement… or, in the case of this article, poorly designed steel lintels.
It may seem strange writing an article about steel in a timber magazine, but there is a particular kind of failure of steel to perform that I have come across over the years that keeps causing grief to truss fabricators.
You could imagine that if you loaded a wall frame onto a steel I-beam uniformly along its full length but off-centre that it would want to twist about its longitudinal axis. Its deformed shape is like a mildly twisted piece of liquorice. That extra twist creates additional vertical deflection at that edge of the flange of the beam. And in severe cases the beam’s overall deflection increases also.
It turns out that steel I-beams are particularly flexible in this form of longitudinal twisting and it doesn’t take much to make them deform significantly. The accidental saviour in many cases is all the other elements that are connected to the beam – the timber framing, supported trusses, plasterwork, window frames etc that stop the thing from twisting.
So, if you do ever have a “truss” problem that may be related to a twisting steel I-beam look closely at not only the overall deflection of the beam along its centreline but also whether there is extra deflection at its flange tips.
But there is an even more problematic case. I-beams are lovely symmetric sections and gut feel tells you that if they’re loaded on the centreline they will not twist. Steel channels are different. They have no proper centreline because the web is on the face. Where do you load a channel to stop it twisting: the mid-line, the face or elsewhere?
It turns out that the location to prevent twisting, known as the shear centre, is outside the beam! So, for example, for a 200PFC the shear centre is about 25mm outside the vertical face. So, a channel loaded centrally on its top flange will still twist in a mode like the diagram on this page.
This deflection, and the extra stresses induced, may not be computed by your software packages. And in fact many engineers seem to have no idea that this can be an issue. In particular engineers are sometimes guilty of designing dual function beam/brick lintels that underperform.
Once again, the situation can be accidentally rescued due to the restraining effect of other cladding and structural elements. So, turned around, the most vulnerable beams seem to be lintels set well down, under jack studs and away from the restraining effects of the top plate. So next time you have a “truss” deflection issue that is not the trusses, have a very close look at any steel beams that are supporting it.
Despite being at the bottom of the feeding chain, don’t for a minute think truss and frame fabrication is the worst job imaginable. Fancy a hangman as a job? My family’s ancestors include the last person in England to hang a child. And just two generations back the surname is “Fuller”. Fulling is a manufacturing process to cleanse and thicken woollen cloth. It was a job so bad that it was left to slaves in Roman times. That’s because it involved wading in urine – a powerful cleaning fluid. Compared to that, the truss and frame industry is bliss!
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: pauldavis@timbernews.com
