The Forces of a Low-Rise Arch

Low-rise arches present unique stability problems related to the high horizontal thrust coupled with a very low tolerance for shifting or settlement of the abutments. That said, low-rise arches can be built to be stable structures.

In a low-rise segmental arch, the forces at play are remarkably intense. However, the line of thrust of a low-rise segmental arch tracks rather neatly with the curve of the arch, making it a solid structure. That said, the low rise means heavy horizontal thrust at the ends. The necessity of resisting this horizontal thrust is further complicated by the fact that a low-rise arch is considerably more vulnerable to even slight movements of its abutments than higher arches. It will be found that the relationship of arch rise to tolerance of abutment sliding is nonlinear, with the result that the low-rise arches are disproportionately more susceptible to shifting than their high-rise cousins.

Forces, Forces

As seen above, the difficulty with a low-rise arch is simply that it produces a heavy horizontal force and, at the same time, it can handle only a slight horizontal shifting of its supports. So how is a low-rise arch built so it can stand? If the bridge is crossing a rocky gorge, a low-rise arch can work well if the stone sides of the gorge are used; then the arch is essentially built as part of the earth itself. However, this method is rarely an option. Thus, the answer is to add enough weight to the ends of the arch that it can’t slide out. Or, perhaps a better way to look at it is that enough weight is added to the ends that the horizontal line is “steered” downwards to a more vertical direction.

Another way of viewing the issue is that the abutment is thick enough that the line of thrust from the arch can be contained well within the thickness of the abutment on its way down to the ground.

Andes Bridge — This bridge features a somewhat low-rise arch and a Roman arch. The low-rise arch does not have or need substantial abutments. Why? Because the arch is basically started directly on the bedrock of the stream (the rock is easily visible in the photo), the thrust is almost immediately transferred to nature’s solid abutments.

If the ground below the arch is solid, the bridge will be stable if the abutments are thick enough to carry this thrust down to the ground.

The Stability of the Ground

Regardless of how you look at it, the fact is that the abutments are only as solid as the ground they are on. A slight settlement can be fatal to the stability of the arch. Thus, we come back to solid rock as the best foundation under the abutment. Soil foundations of any kind allow some settlement, especially under the enormous weight of a stone bridge. Furthermore, soil is far less capable of resisting horizontal pressure than vertical pressure, so a segmental arch set in soil will have to have heavy abutments to ensure most of the thrust is vertical. Unfortunately, the heavier the abutment, the worse settlement problems become. All this translates to that low-rise arches are far more suitable to areas where rocky ground is easily found. And yet, where solid rock is available to brace the ends of the arch into, a low-rise segmental arch can be a very strong, stable structure.