Ex Die in DIEM

by duncan mcnicholl

Prestressed

If you want to build a bridge of any significant size these days, you’re going to be using at least some prestressed concrete to do so. Concrete’s great stuff for big buildings, you see: strong, relatively light, and very very easy to work with. You can simply pour it into an appropriately shaped mould and produce whatever shape you want, ready to be slotted into place in your structure. You can even cast it in place, building temporary walls around the volume you want your edifice to occupy and then filling it up, letting it set and voilĂ : a big old chunk of building.

The problem with using straight-up concrete is that it’s frustratingly asymmetrical: it is great when you press in on it, but tends to crumble if you pull it apart. In slightly more scientific language, it is much stronger in compression than tension. That’s where the prestressed part comes in: if you want your concrete part to be strong when it has an adverse load on it, sometimes you have to make sure that there’s a load on it before you even finish making it.

There are a couple of ways of forming prestressed concrete, but the coolest is to make a mould and then stretch steel cables inside it, with the stretched ends poking through holes so that you can maintain the tension while the concrete hardens. Once you’re all set (pun completely intended), you break down the mould and cut the steel cables flush with the new concrete surface. Removing the tension on the cables makes them contract, putting a compressive load on the concrete. Remember, concrete does well under compression, so what we have now is a component that’s already filled with stresses, but importantly with stresses it can deal with comfortably.

When you subject prestressed concrete to tensile loads in the stressed directions, the overall load remains compressive, and so the concrete doesn’t fail in use. It’s a cool technological magic trick, and it means your bridge will be both beautiful and deeply functional, as long as no one removes the steel.

I don’t think concrete is the only thing that does better with one kind of load than another: I do too. If you need my help, I will always do my best to give it to you, but if I need my help, I often find that I don’t know what to do. I’m quite good at getting myself stuck in the mental equivalent of a tailspin, going round and round and round with no way to manoeuvre myself back onto an even keel, and despite my experiences I’m often at a loss for what to do.

So I think what I’m trying to say is that the best preparation for surprises that I’m ill-equipped to deal with is to prestress my mind. If I can find people who will rely on me at least some of the time, and seek out opportunities to help them, I think that that will help me stay anchored and steady when the rough winds come.