The stone arch bridges of Cowley County, Kansas, have intrigued many over the years with their simple gracefulness, their apparently gravity-defying construction, and their massive scale, frequently involving the use of individual stones that are, well, huge.
How were these bridges erected, especially considering that they were built before modern construction techniques and equipment? And what keeps them standing, anyway?
How Arches Work
To begin with, a brief survey of how arches work is in order here.
How does an arch fight gravity? By using gravity! Each individual block in the arch is a wedge. As we go around the arch, each wedge rests on the next. When all the wedges are in place, none of them can move because of their neighbors. After all, for one block to go down, the other ones would have to move up and out. Gravity pushing down equally on all the blocks makes them stable.
Given the weight and the massive amount of friction of the stones, the arch can be an entirely free-standing structure. Add the amount of fill material on top required to make the roadway, and the arch is even stronger, the extra weight pushing the stones even more firmly into place.
The stones can be any size and thickness, provided that they are wedges—even if the angle is microscopic or if the correct angle is largely made up of smaller stone chips and wedges—and provided that they follow the line of the arch. What does that mean? Well, in a Roman arch (half of a circle) the radius is drawn from the center. Every joint (where a stone meets a stone) should be in line with the radius; if the line formed by the joint were continued straight, it should land in the center of the arch for a Roman arch or to wherever the radius lines start from—below the start of the arch for a flatter segmental arch.
A Roman arch in Otter Creek Bridge, Greenwood County, Kansas. The arch starts flat on the pier. 
A segmental arch in a bypassed road bridge in Elk County. The arch starts at an angle from the abutment.
In practice, there is quite a bit of leeway in how close to radial the joints are. In Ireland, for instance, there are numerous examples of early stone arch bridges with joints that are anything but radial. These are not only still standing, but are still being used hundreds of years later. Arches are surprisingly forgiving.
To sum up how an arch works, it is the individual stones that make the arch stand, and, at the same time, the line of the arch that makes the individual stones stay in place.
Formwork
The fact that all the stones are required for the arch to stay put and that the arch shape is required for all the stones to stay in place brings up a rather important fact in arch construction: A temporary form is required for the arch to stand while the two halves of the arch are being built up toward the center.
There are several ways to do this. There is the crude-but-effective method of piling soil up in a rounded heap and building the arch on this. When completed, the soil is dug out, and the arch stands. Wooden frameworks were also common and were used in Cowley, as evidenced by newspaper photographs. Cowley was well known for its timbered waterways, which is undoubtedly why timber was frequently used. As the form work is temporary, the wood does not have time to decay, eliminating the problem Cowley had with “permanent” wooden bridges.
Placing the Stones
The way the stones (which are certainly rather large) were placed in the Cowley stone bridges is also rather amazing: man-and-horse power.
A mason’s derrick, essentially a hand-powered crane, was erected. It was geared down such that a person could lift even the largest stones. The stones could be turned by hand while suspended in the air and, when in position, lowered into place.
The stones were hauled to the site by horses and sledges. One stone was the typical maximum load!
By using methods as simple as these, the farmers of years gone by were able to build these enduring structures, some of which are still in use today.
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