Lateral stability is more complicated than you might expect for free roofs. By Paul Davis
Over many years I have investigated damaged structures – something around 2000 of them. I’ve seen all sorts of damage – ranging from the dramatic to the mundane: the devastation caused from massive chemical explosions, bus impacts, collapsed retaining walls, category 5 cyclones, earthquakes, foundation movement, roof collapses, tree impacts, right through to leaking taps. The structures involved have been houses (lots of), commercial buildings, industrial buildings, retaining walls, roads, dams, brothels, bars, boats, and bridges.
Every job is different, but after a while you begin to see patterns. So, for goodness’ sake people, make sure your showers are waterproof, that you don’t leave heaters on when you depart the house, and that you are concentrating when reversing your car!
One minor pattern that I have picked up on is lateral movements (failures) or open walled structures – or what in an engineering sense you would call free roofs. These structures can, for instance, be entry roofs, alfresco roofs, carports or free-standing roofs such as picnic shelters, gazebos and cabanas.
So, this is in fact the third in a series of Truss Talks about free roofs. The first article was about wind pressures for free roof trusses. The second article was about fixings and tie downs. This article is again about something that is relevant for a normal building – wall bracing. But with a free roof on some sort of posts the solutions are now radically different.
Let’s imagine a not-so-standard engineering problem; an elephant standing on a toothpick that is miraculously strong enough to hold the beast’s weight.
The toothpick will still ‘fail’ because it will rotate at its base and the elephant will slide sideways. This thought experiment teaches us three things: firstly, elephants shouldn’t stand on toothpicks; secondly, no matter how strong the elephant, the toothpick will still rotate and; thirdly, no matter how strong the toothpick, the elephant will inevitably slide sideways.
Let’s now imagine a free-standing carport sitting on timber or steel posts with simple bolted connections top and bottom. Similar principles apply; no matter how strong the roof, the posts will still rotate and no matter how strong the post the roof will still slide sideways. And this is exactly the sort of construction problem pattern that I have picked up on when looking at all those damaged structures.
The same potential mode of failure can happen with a house – the roof slides sideways, and that is why we design and provide bracing walls. A bracing wall provides stability in its long direction because it is fixed to the slab or timber floor. So, what we need for the elephant is a really long toothpick that is embedded in the ground and what we need for the free roof is a post that is fixed against rotation about its top or at its base.
Your standard steel post base or a standard steel 8mm base plate with two bolts doesn’t provide this fixity. That 90×90 post (or whatever is used) is in effect a very long lever and it will just bend the post base– easy-as. We need to get more serious – either use a heavy engineered post base or cast the post into the footing.
The other option is a portal frame of some sort and that may well be the subject of next month’s Truss Talk.
Even a brick pier doesn’t give that lateral stability. The weight helps a bit but, again, the lateral wind load from the roof structure is applied with such large leverage that it easily rotates each pier about their base. Those brick piers need to be reinforced with starter bars running out of the footings into a reinforced concrete grout core if it is to be stable. Last month’s article had a photo of my carport and that is exactly how that was built.
If your free roof projects out of a building (say an alfresco) then it gets some lateral stability via the house walls. But with the projection beyond a certain point, it’s possible that the outer edge can slew sideways and in effect it twists the projecting roof off the house.
So, if you are responsible for the design of wall bracing for a structure then you really should consider lateral stability for any free roof portion. More often than not these challenges are not solvable using the standard methodologies in AS1684 and your software. So, we get to a position of needing a bespoke engineering design. And that gets us to the point that this all should be considered early on when you quote a job – who is going to own and solve the problem of lateral stability of the free roof? And, more particularly, who is going to pay for that solution!
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
