The problem with brick

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.

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11 Responses to The problem with brick

  1. exiledinpn says:

    Brick is evil.

    It may well be from an engineering point of view. But think of the other benefits: once your earthquake is over and those bricks are lying in shattered lumps all over the street, people can pick them up, stick half of one in a sock, and go looting. They’re also quite handy as an improvised missile weapon, or for building a barricade in a hurry. So when the zombies attack, people who built their buildings out of brick will be sitting pretty (I understand it is also effectively wolf-proof).

    What? You don’t plan all your buildings with a zombie apocalypse in mind?

  2. evie_fae says:

    I’m sure you’ll be delighted to know that that all made perfect sense, and I know regard the brick portions of my workplace with great suspicion. >_>

  3. queneva says:

    What’s your take on concrete block construction? Same as brick?

    What about aerated autoclaved concrete, available in block and panel formats in NZ as Hebel? I’ve looked at it in the past and been curious about it, but I’m not sure whether the aeration means it would squish a bit before it failed in an earthquake.

    • mashugenah says:

      Concrete block is functionally similar to ordinary concrete: it has reinforcing with all the usual detailing. The main three differences in performance is that the actual material used for the blocks is about half the strength of ordinary concrete, so you simply can’t be as ambitious with it. The second major difference is that the internal spacers, mortar joints, etc, introduce a lot of limitations in the size of reinforcing and the specific properties of the concrete you fill them with (specifically: it must be very liquid, so that it flows everywhere in the wall/beam/whatever.) Thirdly, you are limited in member depth and width to the size of a block module.

      The nett effect of this is that concrete block is labour-reduced concrete, with a trade off in scale of application. The tallest block wall you can build is about 3 or 4m, whereas in concrete you can go much higher with only a slight amount of ingenuity.

      Hebel… is not a good structural product. It only has about a fifth of the strength of ordinary concrete, or in that order. So what it’s mostly used for is in-fill panelling in more conventional concrete-frame construction and simply as a cladding.

      As a cladding only, it seems to represent little problem, and isn’t even excessively heavy. I probably wouldn’t use it because it is an uncommon product with the attendant concerns of not being properly explored by long-term use.

      As an in-fill panel, I am a bit more concerned about it. In an ordinary concrete frame the strength of the legs of the frame is in to parts. We talk about “bending strength”, which is when one face of a concrete column is in compression and the steel on the opposite face is in tension. This is the primary load-resisting mechanism. The second mechanism is in shear, which is harder to explain, but is basically the joint between the column and the slab trying to slide over one another with additional resistance from the steel. The problem with in-fill panels is that they tend to reduce the bending demand, but concentrate the shear demand. Bending is typically a ductile failure, while shear tends to be a brittle (non-ductile) failure.

      In the one Hebel house I did, this shear concentration was manageable… but in general it is not the ideal way to arrange things.

      So, I would avoid Hebel generally. πŸ™‚ But it is a legitimate product (not like brick. πŸ™‚ )

      • queneva says:

        Hmmm… I hadn’t really considered the effect that too-rigid in-fill would have on structural members. Interesting…

        Okay, next round of construction approaches:

        Steel framing. Hardly used in NZ domestic building, but I’m not sure whether that’s just because timber-framing is our default. Seems to have potential for interesting cantilevers and design stuff. I’m not sure how easy it is to extend and modify. I guess I’d prefer to have good access to the structural members so I could check them easily for corrosion and such. And I’m realising that for all I used to know about steel qua steel, I know very little about the types of steel actually used in construction.

        Rammed earth. Hard to generalise, because it can mean so many different things and be used in different ways, but what are the considerations and the things to watch for in design?

      • mashugenah says:

        Steel framing for residential is mostly a cost thing I think. Steel is a lot harder to work with, and there isn’t the infrastructure set up to pre-frame walls the way there is for timber. However, steel is commonly used to augment timber construction. So you can have large cantilevers, etc, etc, with a steel base supporting the timber.

        How buildings learn may have given you the impression that buildings are easily modified, but this isn’t really even true in timber. If, say, you take out a “non-load bearing partition wall”, you need to replace the lateral stiffness of the wall by re-lining other walls, and potentially fix up ceiling runner splices (and so on.) Steel is broadly the same, except that when you take out timber you have the option of putting in a smaller steel arrangement, but there is nothing commercially available with a better weight/strength ratio for you to replace the steel with. Cantilevering titanium? Not likely in any domestic budget.

        Generally, if steel is embedded inside an enclosed structure, it’s longevity is similar to, or better than, the surrounding structure. The reality is that once you’ve clad your building, almost everything will be hidden and impossible to inspect.

        Rammed earth.

        The basic problem with any soil structure (i.e. made from soil) is that soil cannot be quickly compressed, and so shrinks with time (or, in the case of overconsolidated clays, expands with time). Which is why all major road-works take so bloody long: you have to just put massive weights on the ground and let them compress it slowly.

        Essentially, if you can do anything else, be it piling or whatever, you should do that instead of trying to rely on the strength of artificial fills.

  4. Also, in red, it’s ugly.

    How about Oamaru stone?

    • mashugenah says:

      Stone has most of the problems associated with Brick, only it’s usually irregular which makes the bond between wythes and courses even less stable. Also, the difference between the mortar and material strength is much greater. I’ve never actually done a design in Oamaru stone, so I haven’t thought through all the issues in detail.

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