By Mark Frauenfelder at 11:11 am Thu, May 24, 2012
[Video Link] There are probably easier ways to cause a column to buckle, but this way looks like fun.
This is why rebar was invented.
I wonder why the demolition contractors were wearing swimsuits.
It was a hot day.
cheers from Brazil. We all work like that here. kidding
Didn’t have enough vertical load.
Ewwww, bricks in the swimming pool.
Yeah, if you ever need to move something heavy, chain it to something even *harder* to move and apply force perpendicular to the chain. The tension on that rope (and pulling on the column) is many times the weight it’s bearing.
I find myself curious why it was being recorded. Having a bit of a plant stuck in the frame gives it a creepy stalker vibe.
Seems plausible to me that the tightrope walk would be considered video-worthy.
Parents are going to be pissed when they come home…
No, Kelly LeBrock is going to fix it in time!
How do you say “Ooooo, you’re sooo busted!” in Portuguese?
I’m guessing that most folks (specifically the folks in the video) think that the lateral force on the line would just be half their weight, or at most all of their weight. But as I remember from my old climbing days, the load is only half if the angle between the anchor points is like 20 degrees, and anything above that is adding more weight to each line, up to 120 degrees where rather than splitting the load each anchor point actually bears the full weight of the load. Above 120 degrees, as we see from this video, the weight is actually multiplied – it could be five times the weight or more depending on how slack the line is, and if there is any bouncing, etc. And then of course there’s the problem that the column was designed to support a vertical load not a horizontal one. In other words, this guy thinks that he’s 200 lbs and the column can support that much easily, but actually he’s pulling sideways on the column with half a car’s worth of weight.
Lots of people don’t pay attention and/or forget their physics class when Resultant (or Net) Forces was covered. Me, I paid attention during class. This way, I didn’t have to study for the exams.
Anyone remember the Pythagorean Triangles? The forces on the line are calculated just like the sides of a right-angle triangle.
If the line is bent 180 degrees (that is, straight up and down around the weight, then the force on the line is exactly the weight of the object.
Think of the upside-down triangle that the line and weight forms: the base is size zero, the height is the weight of the object suspended. The force on the line is the square root of (the square of the height plus the square of half the base) – just like right angle triangles. In other words, the force on the line is the weight of the object, or more accurately, each half of the line supports half the weight of the object.
Stretch that line taut enough to keep it almost p;erfectly straight no matter how small a weight is in the middle, you need an near-infinite amount of force.
So, let’s assume the kid on the line weighs 50kgs, the line is ten meters long, and at the midpoint, the kid on the line pulls is down half a meter.
So, the height (half a meter) is equal to 50kgs. Half the length (5 meters). Plugging these into formula, the resultant force on the line is the square root of the sum of the squares of 0.5m and 5m, or the square root of 0.25 plus 25, or 5.025.
Since the half-meter deflection, at the point where the kid is, is caused by his 50kg weight, then the force on the line is, or rather that force that is pulling on the brick column is…(cross multiplications, anyone?)…251kg!
Likewise for loading the column: the force required to keep the column unbent, especially given a very conservative 250kg of pull (the material does have some force-absorbing flex) would be greater than the crushing resistance of the bricks.
It’s a nit for non-scientific people, but kg measure mass, not weight/force. If you said pounds, above, you’re right.
Most people fixate on the conversion factors, and forget that US/Imperial is a force-distance-time system where mass is measured in grains… whereas Metric is a mass-distance-time system where force is measured in newtons.
That’s the big deal with metric… whole classes of engineering calculations are easier because you don’t have to divide by 32.2 ft/s^2 to get to mass.
I started drawing a poor-quality gif of the force triangle in the case you describe. Where theta is the angle off level, the tension in the rope is an inverse of sin(theta) and for small theta the tension actually goes to infinity.
Yeah, I should’ve reworded the sentences or used the correct units. My bad.
penguinchris: Years ago, I had a conversation with a technician/repairman (ok, ok, my boss), who, faced with a wobbly chain drive mechanism, proposed to increase the chain tension.
I explained, twice, that the force required would break the chain. He didn’t understand what I saying.
I ended up fixing the problem by straightening the rail/channel that the chain was travelling in, not by increasing the tension.
You’re all way over-thinking it. Physics was the last thing on the minds of these guys. I understand the physics too but I could see myself making the same mistake. Well, assuming of course that I was drunk and wanted to do whatever it was he was doing, which admittedly would probably never happen :)
If they put any thought into it at all, I think they probably didn’t assume it was just a pile of bricks without any additional support. It’s very intuitive that a pile of something no matter how artfully arranged is easily toppled and I’m guessing they knew that. But most column supports are not built that way these days – I might have thought the bricks were a facade around a rebar’d concrete column or even just a metal pole, for example.
Okay, I need to give the hanging of my hammock a little more thought…
To fix this, you could put a large steel pipe running along the backside of the brick column, and tie the line around the column and the pipe together. If the pipe is sufficiently rigid, it will spread the load along the entire brick column vertically instead of focusing on a breaking point.
You still need the loads and weights to work out, and a horizontal anchor to the top of the now-rigid brick column would still be a really good idea.
Build it that way, and I can break it.
I’ve absolutely no doubt. So could I, but I’m losing weight. ;)
Without an horizontal anchor the column would just snap off at the base.
hooray for north american building codes
Ummm..last time I checked, Brazil is in South America
That was the point.
Is anybody else willing to get nit-picking with me about the word buckling? Since the column failed from a lateral load, then this is actually a failure in flexure. There is such a thing as a beam-column that supports compressive loads and flexure. But nit-pickedly I must protest that it did not buckle.
First thing I thought when I saw the video.
Column broke due to overloading by nits.
This why we have “onerous” building codes. Dumb acts are frequent & pervasive & if something can break relatively easily someone will do something to break it. Not figure out how to break it, just break it by using it in a way that seems perfectly normal. How many American cities are largely brick due to being rebuilt after a catastrophic fire? Philly & Chicago come to mind.
And conversely, you find a distinct lack of brick buildings in California because of experience from earthquakes.
So those rope bridges in Indiana Jones, Xena, Hercules, and pretty much every other adventure movie shouldn’t actually work? Since the ropes are usually just tied to 2 tree stumps on either end?
Tree stump and brick columns have nothing in common structurally.
And the rope bridges usually droop quite a bit, thus lowering (heh…) the force the exert on the anchors.
/Of course, all those movies and shows are factual, as far as the science goes.
Brings to mind an incident some years ago when a brick wall collapsed at a suburban swimming pool in Melbourne. A bench seat had been dynabolted to the wall (probably after the initial construction) and a bunch of school children sat down on the bench. IIRC there were some deaths because the kids were sitting right under the wall as it fell.
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