Arch Building Simplified: Walter Sharp’s Secret to Building Stone Arch Bridges

Detail of Walz Ford Bridge Arch

Perhaps one of the most well-known individual stone arch bridge builders is Walter Sharp. While primarily building in Kansas, he also built several stone arch bridges in other places, such as New Mexico. Walter Sharp seemingly appeared out of nowhere early on in Kansas’s years of stone arch bridge building. He quickly rose to prominence, as his highly competitive pricing allowed him to win numerous contracts. And, unlike bridges built by other builders, Walter Sharp’s bridges rarely failed — surviving time and floods much better than wooden or even iron truss bridges did.

While in Kansas Walter Sharp was already well known in Marion, Chase, Greenwood, and Butler counties, it was in Cowley County where he really rose to fame. Cowley County needed permanent bridges at affordable costs, and Walter Sharp provided them. The Cowley commissioners were apparently already aware of the permanence of stone arch bridges, but did not know of anyone who could build them at a cost they could afford. Stone arch bridges could be fantastically expensive (as an example, the famous Clements Bridge in Chase County, Kansas, was triple the cost of the largest stone arch bridge in the state — the Dunkard Mill Bridge, which Walter Sharp built.) However, whereas some stone arch bridges were quite expensive, Walter Sharp’s pricing proved to be but slightly more expensive than the cost of a steel bridge, and his stone bridges lasted much longer with much less maintenance than a steel bridge could.

Floral Bridge Plaque
Walter Sharp was responsible for most (though not all) of the famous Cowley County stone arch bridges, including this one over Timber Creek near Floral. And it would appear that other builders of Cowley’s bridges more or less adapted Sharp’s methods.

 

The Secret to Walter Sharp’s Success: Local Labor

One factor that played into Walter Sharp’s competitive bridge prices was the fact that he used local labor. Rather than hire a team of professional masons, Walter Sharp hired local people — usually farmers — to build his bridges. This practice kept his costs down and made his bridges rather popular projects locally. These structures were built with a degree of pride, often by the very people who would use them the most.

To be sure, these local farmers often had little stone masonry training, but they were shown what to do, and with what success needs only to be determined by looking at Walter Sharp’s bridges. Granted, the spandrel walls of the bridges were pretty simple to build; the stones were usually quarried out of hillsides with feathers-and-wedges. Since the stones were limestone, they could be cut very simply into massive, brick-like blocks in this fashion. These large blocks of stone were easily stacked, being laid into a bed of mortar with the aid of a hand operated crane. Building walls with stones shaped like oversized bricks was all simple enough, but building the arches was another story.

Walter Sharp’s Simplified Arch Building

To build an arch, wedge-shaped stones are required. Wedge shaped stones, however, are not readily obtained like large brick blocks are. While, due to the nature of limestone, rectangular stones are readily manufactured, making a stone the right shape for an arch usually requires time and effort with a chisel.

Walter Sharp’s work-around to this was to make the angles with mortar. This meant that rectangular stones could be used to make the arches. While Walter Sharp had the faces of his arches cut fairly accurately, using mortar on all of the other angles meant that the amount of stone cutting required besides the basic quarrying was low.

Basically, Walter Sharp would place the arch stones on the centering, using a small amount of mortar to fill in the gaps at the bottom of the stones, and when all of the stones were in place, his workmen would pour in a mortar-based grout, thereby filling in all of the voids between the arch stones to make the necessary angles.

Walter Sharp stated that he had a groove cut at the ends of the arch stones that fit into boards placed on the arch forms. These boards held the stones in their appropriate positions until the arch was grouted. This meant that the time and effort required to build the bridge was low, and, by cutting the joints in the the arch faces precisely, Walter Sharp kept the arch less vulnerable to waterborne debris than it would be otherwise.

One interesting fact to note is that a close look at Walter Sharp’s bridges show the outside faces of the arches are usually quite rough (see the picture of the Floral Bridge above), revealing he did not spend time to smooth the faces. On the other hand, some stone bridges built by other builders (such as C. C. Jamison’s Rock Creek Overflow bridge in Butler County, Kansas or the Thompson Bridge in Cowley County, Kansas) show the tell-tale marks of the toothed chisels used to smooth them off.

Detail of Walz Ford Bridge Arch
A close look at a small, partially exposed arch built by Walter Sharp reveals large gaps between the stones. Originally filled with mortar, as the mortar leached out the mortar was slowly replaced with stone fragments, gravel, and soil from above. Note how the joints tend to be tighter toward the end of the arch and get looser towards the middle of the arch. As Walter Sharp himself said in regard to his arch building: “Some of the openings are large enough to put your arm in, but all are filled with a grouting of one-two-four mixture.

 

Adapting Walter Sharp’s Methods to DIY Projects

For the DIY enthusiast, there is, perhaps, some potential here, at least for building small bridges. The main problem with Walter Sharp’s methods is that the mortar leaches out over the years, tending to weaken the arch. It appears that the mortar used back in those days was non-standardized, with some batches being quite durable and others amazingly weak.

Besides the obvious expedient of using better quality mortar, a simple solution to this mortar loss problem (and one which, incidentally, appears to have been employed by another Cowley County bridge builder – Jerry Hammond) is to add stone fragments with the mortar. This makes the mortar more like concrete in form. Even if the mortar does leach out, the bridge can still stand, the stone fragments making the angles. In fact, this solution provides a much stronger result than using mortar alone, for the stone fragments are bound into the mortar making a concrete.

An alternative would be to actually use concrete. The thing about concrete is that there is some concern about its effect on stones during freeze and thaw cycles. If you live where it does not go below freezing much if at all during winter (like Florida, for example) you probably do not need to worry at all about the effect of concrete on stone.

Modified Sharp Arch Detail
Detail of the arch of a mostly dry-laid footbridge. The arch is a “modified Sharp arch,” i.e. it uses concrete to help make the angles for the interior portions of the arch. The faces and lower portions of the arch were heavily shimmed with stone fragments, and the arch stones themselves were chosen for reasonably good fits. The concrete was then poured into the arch joints and tamped with a shovel. The concrete mix used was a fast-setting blend. The stones were neither cleaned nor moistened first, which prevented the concrete from “taking” to the stones very readily. The idea was to create custom-shaped well-fitting concrete shims between the arch stones to help form the arch quickly. Unfortunately, the batch of concrete was a little thick with large gravel. Runnier batches with fine gravel would be better for this purpose, for the concrete did not quite extend all the way through the arch, though fortunately the stones still fit well enough that this was not a serious issue. The bridge has been a success, and to date this bridge shows no signs of trouble related to this method of creating the arch.

Incidentally, one accepted form of simplified stone wall is the slipform wall where large rocks are poured with concrete into a form. When the concrete sets up, the form is removed revealing a type of stone wall. It would seem that this method of construction is not terribly harder on stones than pouring a runny batch of concrete into the angles of an arch would be.