Over the course of the last 10 years I seem to oscillate between learning and teaching. I don’t seem to have yet had too lengthy a period where I was content with my own level of knowledge, and keeping it to myself.
It began in the first couple of weeks of University. I had been accustomed to thinking of myself as the second sister of academia. Boys would call on me, but only to get close to my more attractive and looser elder sister. But fortunately, when I went away to university, I left her behind. My first lesson was given to a fellow first year student about static equilibrium. It was for a physics course that I was exempt from due to pretty reasonably bursary grades. After some reasonably frustrating discussion, I drew him a couple of diagrams and built a little model that demonstrated the principal being mathematically interpreted*.
Building some kind of simplifying model is pretty much the basic and intrinsic skill of an engineer. A process aided by computers: but many is the time when some simple little model can be built of your complex problem that will make it all clear. I encourage our technicians to build little models, and I have developed a large repertoire of analogies that make most of what they deal with professionally seem as familiar as their childhood playgrounds.
The thorn in my side, however, is the concept of ductility. It is a poorly understood word, usually synonymous in the minds of the masses with some kind of elasticity or flexibility. Probe your own mind, and see what associations are conjured. Some kind of malleability? To an engineer, ductility is how far something can deform after it first begins yielding. Up to the point where it begins yielding it is “elastic”. Afterwards it is “plastic”.
Ductility is mostly of interest when we think about earthquakes, and then only because earthquakes are a transient load on our buildings. Ductility means that when something starts to break, it continues to break for a long time. In the case of earthquakes the general hope is that the shaking finishes before your building is quite finished breaking. This is represented by a high ductility.
You can imagine the extreme of low-ductility. Something like a plastic ruler. You can bend it a certain amount, but as soon as it starts to break, it shatters. It goes from having 100% of its strength to 0% with almost no warning. You could compare it to something like press-stick, which will start to yield, then stretch a long way, then snap. Any more accurate example skips the familiar world, however, and goes into the specific world of engineered structures: because ductility is a highly unnatural concept.
Some kinds of ductility are easier to imagine than others. The reinforcing in a concrete beam slowly stretching is easy. You can imagine an elastic band (though in real life, elastic is a highly brittle material) stretching, but not breaking. But some forms of ductility are hard to imagine, like a plywood sheet. There’s no part of it you can really imagine stretching much at all, and yet it has a ductility of 3 (meaning the “maximum deflection” is 3 times the “yield deflection.” In fact, the ductility comes not from the sheet at all, but the nails connecting it to everything else.
There are materials which have zero ductility: any deflection implies it’s broken. Brick is the leading material of this type in construction in New Zealand. Actually, the concrete part of concrete beams is also brittle: the steel does all the movement while the concrete part is destroyed. But perhaps that’s another post for later on.
So naturally, when you have a brick structure subjected to an earthquake, you must ensure that there is no relative movement between two different bits of any continuous bit of brick. A wall must deflect as a rigid body. Which means it must be strong enough to resist all the inertia forces generated by the shaking. And since brick is rather weighty, but not very strong, you develop problems. These problems are solved by adding other materials that have a better strength to weigh ratio.
The other materials that you add to your brick building have a much greater ductility. This is typically how they survive earthquakes: because they flex, the plastic deformations (damage to their materials) absorbs a tremendous amount of energy. But almost all of the common ways of strengthening a building also have some small amount of deflection in the elastic range. This means that you have two parts of the structure with very different behaviours. The brick is very stiff, but not very strong. The steel portal frame or whatever is not as stiff as the brick, but is much much stronger.
Load naturally goes to the stiff part first, which means that in order to develop the strength of the support structure, the brick must already have suffered some damage.
This is a very difficult contradiction to sort out! It is difficult to even really imagine. It is also very difficult to explain to a client who owns a brick building. In essence, what will happen is that the brick will be damaged, but the strengthening work you do will hold up everything else. You can’t stop damage! But, the very point, in the mind of a client, is that you are strengthening the building so that it won’t be damaged. Otherwise, what, they ask, is the point? But everything gets damaged in an earthquake: it is designed to get damaged safely. Which raises the question: why not simply accept that brick will be demolished, and separate the strengthening works to ensure that this doesn’t matter?
Alas, my whole profession has no terribly compelling answer to this. But I hope that in asking this question, I have illuminated a tiny part of why you shouldn’t build or clad your house with brick. Brick is evil.
* Just for your interest, he completed a B. Sc in physics in 3 years then won a scholarship into a post-grad programme in New Mexico, where he still resides in my mind, although his PhD should be well and truly finished if he ended up getting there. I got him through Stage I physics, and so naturally I claim credit for everything.