A rather intriguing element in stone arch bridge design occasionally seen is methods used to lighten the structure. There are several reasons for lightening a stone bridge; economy is an obvious one, but frequently the structure is lightened to compensate for potential structural problems.
Reducing Weight on the Foundations
A stone bridge is inherently an enormously heavy structure. A bridge in which the arch alone weighs around 100 tons or better is by no means uncommon. For a bridge founded on solid rock, this is rarely a problem. However, when shallow foundations on soil are used, this weight can cause settlement problems. Ribbed construction is one way to reduce weight on the foundations.
Another method to lighten up the bridge is to insert cavities in the piers. The Westminster Bridge, which was a stone bridge in England built over the river Thames and completed in 1750, showed signs of settlement during construction. Charles Labelye, its builder, added arches into the piers themselves in order to reduce the amount of material used. These arches in the piers are invariably located in the spandrel walls above the piers, but often are blocked up on the outside, presumably for aesthetics. The Romans also put arches in the spandrel walls directly over the piers. These arches were left open, and were probably intended for improved flood discharge as opposed to lightening the bridge.
Lightening Arch Load
For a long arch span, the weight of the fill over the arch can cause problems. A common solution is to add small arches above the main arch in order to create hollows and hence less load. Another possibility is to use a light-weight fill material. In the Pontypridd Bridge in Wales, lightweight charcoal was used as fill, supplementing the hollows added above the arch for the same reason. The Pontypridd Bridge is a classic structure from an engineering standpoint, for, though the arch ring is quite thin, relatively speaking (2.5′ thick for a 145′ span), the hollows in the spandrels and the lightweight fill allow the thrust of the arch to remain essentially in the very center of the thin ring, making it stable.