As I mentioned in an earlier post, one of the buildings near my office is currently being redeveloped. By redeveloped, I mean a mix of rebuilding, extending and renovation. In the last few weeks the building has been stripped back to its structural shell – pretty much all things that aren’t necessary to hold the place up have been removed. It’s being extended at the front, and will have a lovely glass frontage when finished and generally be much more pleasant to look at than when in its former state.
It’s been interesting to see just how little is required to hold up a four-storey building. The main concrete supports really don’t look much at all, and the that support each floor span an unnervingly long distance. One must assume that the original architects knew what they were doing and that a medium-sized earthquake isn’t going to send the thing falling.
The key is what is inside the concrete. On its own concrete is great at bearing compressive loads. The ‘Engineering Toolbox’ website gives me a compressive strength of 20-40 MPa. (One MPa is a million newtons force per metre square, or about 100 tonnes of weight per metre square – that is about 10 kg on a square centimetre) But begin to stretch it (e.g. by trying to bend it) and it will fail. The tensile strength is only about a tenth of its compressive strength. A steel rod on the other hand does the opposite. It’s fantastic when stretched (a tensile strength of several hundred MPa), but a thin rod of it will buckle easily in compression. Mix the two together, and you get the best of both. A reinforced concrete beam is a great example – as it curves under loading, the inside of the curve will be in compression (and the concrete can take that) but the outside in tension (and the steel can take that). One simply hopes that there is enough steel to do the job.
It’s a cheap, effective way of building, which is why there is so much of the stuff about. Hide it behind something else, and it needn’t look ugly, as this building shouldn’t when finished.
So the next question is, when will they do my building?