Building long-span arch bridges has always been a challenge with which many builders have been fascinated. It is all fine and well to use multiple relatively small arches and piers to create a long bridge, but the challenge of setting a record span, understandably, seems to have had its appeal to some builders.
There are certainly advantages of a single long span versus multiple spans and piers. For one thing, piers represent waterway constrictions, and can cause problems with debris. Furthermore, piers are much more easily undermined than abutments, simply because abutments are at the edges of the stream and piers are smack in the middle.
So the question becomes, what is difficult about building long-span arches? Aren’t they just like any other arch, only bigger?
Long-Span Arches: Increased Span Means Increased Weight
It is true that a small arch is not much different than a big arch. The challenge with big arches results from the magnification of the forces encountered with small arches. These same forces on a long-span arch become huge enough to create problems.
One way to look at it is to consider the weight of a small arch versus a big arch. The longer the span of the arch, the more it weighs. The more the arch weighs, the more sturdy the arch’s construction must be. Voids in the arch and imperfections in the fitting of the stones in a 5-foot span arch may not cause problems. If the arch is 100 feet in span, all of a sudden those same voids become serious. Voids in the arch mean tensile stress. In the case of the small arch, these stresses are probably not large enough to cause trouble. For a big arch, however, the enormous weight of the structure can cause the stones to break and fill in these gaps. The arch stones breaking, obviously, can be quite a serious problem.
Eventually, a practical limit is reached when it comes to span length. One of the biggest problems with super-long-span arches is that the enormous weight of a huge arch can cause fracture of the stones in it, no matter how perfectly fitted they are. One can try to increase the thickness of the arch ring to compensate, but as soon as you do this, the weight of the arch is increased proportionately as well, which means the problem is not mitigated as much as one may hope.
Basically, the weight of the arch itself is what makes long arch spans so challenging.
The Implications of Increased Arch Weight
When building a stone arch bridge with a large span, the abutments must be strong enough to handle this huge weight. Settling may become a serious issue if the bridge is not firmly set on bedrock. For low-rise segmental arches, it must also be remembered that the horizontal thrust of the arch is significant, so the longer the span, the more weight the abutments must have to keep things stable.
Another practical problem with long-span arch bridges is the formwork required to support the arch. If the arch is huge, so must the centering be. Also, of course, the falsework must be strong enough to actually support the weight of the uncompleted arch, a weight that is quite enormous.
One of the most interesting challenges with long-span arch bridges is one that, while present in all arch bridges, becomes particularly significant with long spans: the weight on the haunches of the arch.
The Imbalance of Weight in a Traditional Arch Bridge
If you look closely at a stone arch bridge, you will see that there is relatively little material above the crown of the arch. Yet, on the sides (haunches) of the arch, the amount of fill is enormous. The reason for this is logical enough; the roadbed must be reasonably flat. Since the haunches of the arch are much lower down than the crown of the arch, it follows that more material must be added to create a level road. This disproportionate loading of the haunches of the arch as compared to the arch’s crown means that, in extreme cases, the sides of the arch can buckle and the crown of the arch pop up, causing collapse.
While a thicker arch can help prevent this problem, there are other innovative solutions as well. The story of Pontypridd Bridge in Wales is a classic example. Its history illustrates well the advantages and challenges of single long-span arches, and a tricky solution to the haunch weight problem.
The Pontypridd Bridge: How William Edwards Overcame the Long Span Challenge
The original Pontypridd Bridge was a simple triple-arch structure. The builder, William Edwards, was given the job of building the bridge and repairing it as needed with funds from his own pocket for seven years after completion. Unfortunately, the bridge was undermined and collapsed before the seven-year period was up. This meant that Edwards had to build a new bridge at his own expense!
Edwards decided to build one large arch to replace the three small arches, eliminating the vulnerable piers in the river. The first attempt at this massive arch, nearly 150 feet in span, failed because the centering was unable to support the weight of the stonework being laid on it.
A second attempt used a thinner arch ring, presumably to reduce the weight on the formwork. This attempt lasted but a short time before the sides of the arch buckled inward and the crown came out of the top of the arch, causing collapse of the structure. It appeared that the arch ring was too thin to support the bridge’s own weight.
Edwards’s third attempt at the long-span replacement bridge involved an ingenious solution to the excess weight of the fill on the haunches: Edwards built three “bull’s-eye” arches into the spandrels at the sides of the bridge. These smaller, cylindrical arches meant that there were sizable hollows in the fill on the haunches, reducing the weight enough to make the bridge stable. This design succeeded; the structure is still there to this day.
The Pontypridd Bridge, with its ingenious method of reducing weight on the haunches of the arch, received much recognition among builders of the time, for it showed a clever way to build massive arch bridges with long spans.
The so-called “open-spandrel” arch is an idea similar to what Edwards did on Pontypridd Bridge. In an open-spandrel bridge there is relatively little weight on the haunches of the arch. Where this weight is applied on the arch can be easily controlled. Open-spandrel arch bridges are usually found as concrete arch bridges, but they can be built out of stone.