Arcuate and trabeate


Many children, perhaps most, are fascinated by this once it is brought to their attention with illustrations: they may start observing buildings—and perhaps even trees. See the Images menu for a preliminary selection. The following is a summary.

In this country, ‘Norman architecture’ refers to the semicircular arches found in the early parts of English churches—or in most parts, like Tewkesbury Abbey. The European word for it is Romanesque, and it was indeed the Romans who first used it all over Western Europe. It was not their invention: they learnt it from the Etruscans.

All architecture is concerned with enclosing space: chambers, doorways, bridges: and it can be divided into two main structural types, trabeate (trabs, trabis f. – a beam) and arcuate (arcusūs m. – an arch or bow).

A trabeate structure consists of props supporting a horizontal lintel. The lintel may be a timber, a stone block or a steel girder. It rests on walls or columns, and its weight bears vertically downwards with what engineers call dead weight. Examples of trabeate structures are all Greek temples, Stonehenge, and most private houses.

The limitation of this structure is the space it can span before the lintel snaps, which it will do if it bears weight. Before the steel girder, this restricted trabeate architecture, and large rooms had to have supporting uprights at intervals. The nearest equivalent to a girder of length was the specially cultivated oak tree, grown by the skill of the forester.

The Romans made use of arcuate structures, finding that an arch can cover a far greater area, because most of the weight from above is directed sideways.

But the sideways thrust has to be counteracted by side supports. These may take the form of buttresses, or a length of wall, to take the sideways weight. The Romans realised that a good method is to place another arch beside the first one, cancelling the thrust of both. Hence came structures like aqueducts, which require only one sideways support at each end, or buildings like the Colosseum, whose arches cancel out the sideways stress of all the others.

The dome is an arcuate structure, and the same principles apply in three dimensions.

Examples of arcuate structures are Roman bridges and aqueducts, amphitheatres, St Paul’s Cathedral, St Peter’s in Rome, Hagia Sophia in Istanbul, igloos, tunnels, and some famous railway stations.

Useful and colourful link: Construction Techniques#458A965

See also illustrations in the Images menu above.

Additional points:

  • The need for so many columns in a Greek temple, and the way this was turned into their defining feature;
  • the skill of the forester in growing tall oak trees, and the time needed to do it;
  • the durability of oak wood (that is, from Quercus robur)—more resistant to fire than a steel girder;
  • the beautification of otherwise ugly buttresses: see Canterbury Cathedral;
  • Sir Giles Gilbert Scott’s words: “Don’t look at my arches, look at my spaces.”

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