continued to be built by many civil engineers, including Thomas Telford, Isambard Kingdom Brunel and John Rennie. A key pioneer was Jean-Rodolphe Perronet, who used much narrower piers, revised calculation methods and exceptionally low span-to-rise ratios. Different materials, such as cast iron, steel and concrete have been increasingly used in the construction of arch bridges. Simple compression arch bridges Advantages of simple materials Stone, brick and other such materials are strong in compression and somewhat so in shear, but cannot resist much force in tension. As a result, masonry arch bridges are designed to be constantly under compression, so far as is possible. Each arch is constructed over a temporary falsework frame, known as a centring. In the first compression arch bridges, a keystone in the middle of the bridge bore the weight of the rest of the bridge. The more weight that was put onto the bridge, the stronger its structure became. Masonry arch bridges use a quantity of fill material (typically compacted rubble) above the arch in order to increase this dead-weight on the bridge and prevent tension from occurring in the arch ring as loads move across the bridge. Other materials that were used to build this type of bridge were brick and unreinforced concrete. When masonry (cut stone) is used the angles of the faces are cut to minimize shear forces. Where random masonry (uncut and unprepared stones) is used they are mortared together and the mortar is allowed to set before the falsework is removed. Traditional masonry arches are generally durable, and somewhat resistant to settlement or undermining. However, relative to modern alternatives, such bridges are very heavy, requiring extensive foundations. They are also expensive to build wherever labor costs are high. Construction sequence Where the arches are founded in a watercourse bed (on piers or banks) the water is diverted so the gravel can first be excavated and replaced with a good footing (of strong material). From these, the foundation piers are erected/raised to the height of the intended base of the arches, a point known as the springing. Falsework centering (in British English: arch frame) is fabricated, typically from timbers and boards. Since each arch of a multi-arch bridge will impose a thrust upon its neighbors, it is necessary either that all arches of the bridge be raised at the same time, or that very wide piers be used. The thrust from the