DIY Arch Building

How Arch Bridges are Built

How Arch Bridges are Built

While building the arch of a stone arch bridge is the most challenging part the project, it actually is not as difficult as it looks. There are several simple tricks that make the construction easier.

Thin Stones Mean Easier Arc Creation

As mentioned in our article on turning the arch, the use of thin stones allows for easy creation of the curve of the arch. The thinner the stones, the closer they are to flat slabs and the less of an angle they need to have. Fairly flat slabs are much more easily obtained than wedge-shaped ones. As long as the tapered end of the slabs are placed downwards, they will still act like wedges. If the flat slabs are a tad less angled than desired, stone fragments in the joints can be used to shim it up. And, of course, if the slab is a tad wobbly, stone fragments can be used to help fix that, too.

Neer Brige
The Neer Bridge in Cowley County, Kansas, uses thin stones in the arch ring. This means that these individual slabs are nearly flat, which in turn means that the arch angles were much more easily obtained. Interestingly, Walter Sharp, who built this bridge, normally used relatively thick, block-shaped stones, and relied on mortar to help make the angles of the interior joints of the arch. By the time the Neer Bridge was built (1916), Walter Sharp was experimenting with various methods of improving stone arch bridge construction, while keeping the cost low. Perhaps the use of thin stones was one such experiment, for it would mitigate his reliance on mortar to make the angles while still keeping the labor required in shaping the arch stones low. Alternatively, the thin slabs of the Neer Bridge may have reflected what was available at Sharp’s quarrying site.

 

The Bottom Gaps in the Arch are Significant

It is obvious that the stones should be placed on the arch centering such that their tapered end is downward. However, another key point to remember in arch building is to make sure that the bottom joints of the arch are more or less closed. The idea is that, even if there is a noticeable gap between a pair of arch stones, this gap can be filled in with chips and pieces of stones if the bottom of the gap is fairly closed. If, due to the shapes of the stones, the gap abruptly widens at the bottom, these stones will be very difficult to wedge in.

Stone Arch Bridges: Methods of "Turning the Arch"
On this small arch, the tighter joints at the bottom of the stones allow for the gaps at the top of the arch stones to be neatly filled with rock pieces.

Flush and Flat Arch Faces Make for Easier Construction

The faces of the arch should be fairly flat and flush. In high-velocity streams this can prevent damage to the arch, but the main reason for keeping the faces of the arch flat and flush in a DIY project is that if the arch is irregularly shaped, the spandrel walls can be rather challenging to place on the arch. The arch stones are best cut  before being placed into the arch, as, otherwise, they can actually be knocked out of the arch again when trying to trim them.

Thick Arches are Forgiving

Just because your arch stones do not fit perfectly doesn’t mean the arch is weak. Arches are surprisingly forgiving. Furthermore, the thicker the arch is, the more room for error you have. Making the arch ring thicker than strictly necessary ensures that the structure will be strong, even if the joints are not perfectly tight.

The Life of Stone Arch Bridges
As this arch on a road bridge reminds us, the arch joints do not necessarily have to be perfectly tight for the arch to carry a large amount of weight. Of course, the tighter the joints the stronger the bridge. That said, arches are surprisingly forgiving.