One type of construction occasionally seen in historic bridges is ribbed construction. The design most commonly associated with stone arch bridges is a solid arch as wide as the bridge. At least as early as medieval times, however, in Great Britain stone arch bridges were being built as ribbed arches. In a ribbed arch bridge, the bridge is consists of a series of relatively thin arches parallel to each other. Across these ribs, large, thick flagstones are laid to span the gaps between them. In this fashion, a solid bridge that tends to put less weight on the piers is obtained.
Advantages of Ribbed Arch Construction
The primary advantage of a ribbed stone arch is that the amount of formwork/centering required is much smaller than for a more conventionally built arch. Most arches use one large and long centering wide enough for the entire arch to rest on. With a ribbed arch, however, the centering only needs to be wide enough for one of the ribs. Then, when a rib is finished, the centering can be reused for all of the ribs one by one until all the ribs are completed. Since the rest of the arch then rests on the ribs, no further centering is required. Another potential advantage of ribbed arch construction is that the precision stonecutting needed for a top-quality arch is considerably reduced. Only the ribs need to be cut precisely; hence less precision stonecutting is required.
Ribbed arches are commonly associated with reinforced concrete bridges. In the case of a ribbed concrete bridge, the amount of material required is drastically reduced from what would be required for a single, wide arch. Only two or three ribs are required for most ribbed concrete arch bridges, as the deck essentially consists of a slab bridge laid across the ribs.