How Arch Bridges Are Built

DIY Arch Bridge Under Construction

Possibly one of the more mystifying elements of stone arch bridges is the manner by which they are constructed. While it may be easy enough to see that the arch stands because of every stone pushing against its neighbor, the obvious question is, “How do the stones stay in place before they are all stacked up?” The short answer is they don’t. Or, rather, they wouldn’t … on their own. A temporary formwork of some type must be built before the stones are in place. The arch depends on all of the individual stones staying in place — and all of the individual stones depend on the arch as a whole to stand. Therefore, before the arch is finished, the stones must rest on something or they will splash into the river.

The Process in a Nutshell

Before the arch can be built some type of foundation must be laid. Typically the foundations are quite shallow. In the case of the Cowley, Kansas, bridges, the bedrock at the bottom of the stream seems to have been a popular choice.

Once the foundations are laid, the arch can be built. Typically, a wooden formwork is put in and the stones are laid on top of it.

DIY Arch Bridge Under Construction
A small stone arch footbridge under construction on a plywood form.

The formwork must remain in place until all of the stones are in place and the arch is “closed.” However, in some cases it may be imprudent to remove the formwork even then. If the arch is low and flat, it has a tendency to slide out at the bottom, requiring a substantial amount of weight on the ends to hold it in place. Once this is attended to, in most cases the formwork can then be pulled out, although it still can be left in until the bridge is finished, if desired.

Once the arch is complete it is usually desirable to have at least a somewhat level approach to it. The typical solution is to build up a pair of parallel walls called “spandrels” to hold in some type of fill material, thereby creating a passable bridge. The fill can be any number of things; dirt and gravel are popular choices. The fill is put in between the two spandrel walls until the desired road grade is reached. Sometimes, depending on the rise of the arch, it is desirable to have a ramp up to the top of the bridge. This is done with the aid of approaches, which are usually just a long continuation of the spandrel walls.

Polecat Creek Bridge
The roadbed of a stone bridge with a rather pronounced hump on the arch — Polecat Creek Bridge, Butler County, KS

At this point, the bridge is pretty much complete, although some finishing touches such as roadbed material and curbing or a guard rail are usually added.