Shapes for Arches

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Arches have been built in many different shapes, especially in masonry. Obviously the number of possible shapes is infinite, but practical considerations, such as creating centring or shuttering, mean that the actual number in use is actually fairly restricted.

HerefordOldAS.jpg (306307 bytes)KerneCSS.jpg (290791 bytes)In some medieval bridges the arches are not only variable in size - they are not even all the same shape. The bridges at Bideford and Hereford are good examples of irregularity. The first example here has been made even more bizarre by the additions made when the road was widened. A more symmetrical variation is seen in bridges in which the arches are largest in the middle and smallest at the ends, usually in examples which have sloping roadways, as in the second example.

Seville8.jpg (96590 bytes)Seville7.jpg (71552 bytes)These Arabic style arches in Seville show variations in size and shape.

BredwardineAMirror2.jpg (382121 bytes)On the other hand, when the banks of a river are high, the arches will very likely be all the same, often to stunning effect. This picture is of course a cheat, made by copying the top half to the bottom.

Aelius.jpg (63926 bytes)Perhaps the oldest type of arch was the semicircular arch, used by the ancient Romans, with very wide piers.  

GlosNave.jpg (40227 bytes)The Romanesque arch was still in use during the 11th and 12th centuries by the Normans who came to Britain.

LoynBig.jpg (257053 bytes)BewdleyBigAA.JPG (177847 bytes)A flatter arch can be made by using a segment of a circle.

Arch3Radii.gif (3721 bytes)WideEllipseF.jpg (129358 bytes)CamSagArch.jpg (211355 bytes)The three segment arch is another way of reducing height by using three circular arcs.

EllipBrickJY.jpg (62180 bytes)Yet another flattened type of arch uses an ellipse.

OgeeArchSpA.jpg (69795 bytes)ArchOgee.gif (2747 bytes)GlosFanB.jpg (65664 bytes)WeirdArch.jpg (64242 bytes)The ogee arch is purely decorative: the reverse curvature has no structural significance and is sustainable only because the blocks are thick enough to contain a funicular. A compressed shape held in a curve by forces requires those forces to act into the convex side or out from the concave side. In arches that force is the weight of the masonry. The idea of steering forces around a curve is fallacious.

EastWindow.jpg (43587 bytes)ChesterZ.jpg (87835 bytes)Gothic arches and lancet arches also use arcs of circles: the apex is not structurally significant, because it does not correspond to a load at that place.

Arch3RadPoint.gif (3167 bytes)This list of shapes is not exhaustive - a number of other shapes exist, but no photographs are available at present. You can create shapes by modifying existing ones. The example was made from the three-segment arch above by removing a piece from the middle.

The existence of so many shapes in masonry must not lead us to think that shape has no significance. The much smaller variation in the shapes of arches built in other materials gives us a clue.  The masonry above the voussoirs exerts forces which tend to compress the voussoirs into place. In large elliptical masonry arches there is often masonry inside the bridge which carries the horizontal thrust through to the next span or to the abutments, in a way that is not mirrored by the visible voussoirs. The diagram below shows external and internal views of an idealised example.

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