The Monumental Centre Wall


The wall is built of concrete blocks on deep foundations, with stone panels hung on the front face. The back, facing towards Oakington Road, has been rendered with a mortar of sand and cement. Bricklayers built the concrete block wall and the concrete bases for the dais and the sculpture stand. Then the masons moved in. They had pre-cut a set of wedge-shaped voussoirs in Portland Stone which would be assembled to form the circular opening. The geometry of these voussoirs is interesting.

Neither the Egyptians nor the Greeks ever learnt to build arches, so their temples consist of hundreds of tall pillars with flat lintels at the top. The distance apart of the pillars was limited by the length of the stone lintels they could find. The longer the lintel, the bigger the cross-section it had to be and the more difficult it was to put it in place.

The Romans however learnt how to bridge wide gaps by making a temporary wooden support, called a centring, and mortaring wedge-shaped stones on top of it. When the mortar was dry and the centring was removed, every stone wanted to fall down, but could not do so because the ones on either side wanted to fall down as well. As a result, they held each other up and the weight was carried round the arch to the ground on either side. If somone had lifted up just one stone, the line of thrust would have been broken and the bridge would have come tumbling down. The voussoir in the centre is called the keystone and is often larger than the others.

Here the masons were building not an arch but a circle. The thrust was carried right round the circle so that the weight pressed against the bottom voussoir and from there to the ground.

There are no other examples in the school of building arches with voussoirs, but at the corner of Elgin Avenue and Shirland Road are some splendid examples. The four-centre arches over the shop windows in the Welford Dairy building are exceptionally good, built by tip-top bricklayers. They are made of specially cut and rubbed V-shape bricks, with very fine mortar joints. This is the sort of building that craftsmen boast to their grandchildren about.

The geometry of the circular hole.

The circle was divided into twelve equal parts and the voussoirs shaped to fit inside a square. If the corner ones had filled the square they would have been enormous, so these voussoirs are reduced in a set of steps. The height of the steps then controlled the widths of all the courses of stone for the rest of the wall and the height of the granite panels. It is a pretty piece of geometry.

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