Backing: A Continuation of the Abutment

Tagged:
Design, Forces, Quality of Construction

Backing is used to provide resistance to the thrust of the arch of a stone arch bridge. In this post, we investigate good practice and using the backing, and demonstrate how it helps to significantly strengthen the arch.

We have often mentioned solid backing for arch bridges, and have pointed out how it can provide great increases in bridge strength. One aspect of solid backing worth bringing up, however, is the way in which it acts as a continuation of the abutment. Or, as another way of putting it, the solid backing acts as an abutment for upper portions of the arch.

The Thrust Problem

Why do the upper portions of an arch need an abutment? Simply because some arches, particularly basket-handle arches and Roman arches may have a difficult time keeping the thrust line within the arch’s own thickness. The reason for this, of course, is that these shapes do not track well with the natural line of thrust of the arch. Where the thrust escapes the backside of the arch, that section of the arch will behave as a segmental arch and as such will have a sliding motion that is necessary to resist.

Removing the backing on one side of a Roman arch made of pavers. The arch was loaded with two large lead weights on top. As the backing was removed the arch loosened and began to sag and slide slightly as the joints opened and the pavers slipped. This may be hard to see in the second photo, as the opening of joints and sliding was very slight until all the backing was removed. Removing the last of the backing allowed the bottom arch stone to slide out, *despite the fact that the arch was a Roman arch*, and the arch collapsed. The last photo shows the arch mid-collapse. It is worth pointing out that, while soil in a stone bridge will substantially improve the durability as compared to a free-standing arch like that pictured, soil is soft enough it can and will allow gradual shifts and failures of the arch, whereas a true solid backing will act like a solid abutment and hold the arch firm.

A Consideration

One important consideration, however, to keep in mind is that the backing should not be bonded too tightly with the arch. Otherwise, a segmental arch will form with the upper part of the arch interacting with the backing, largely independently of the rest of the arch below. In this scenario, the arch bridge will ultimately break up over time with changing loads, as the arch is divided against itself, and, with the upper part of the arch acting independently of the lower part, it cannot stand long term, as the arch loses the cohesiveness so crucial for long-term survival.

Backing as a Means of Improving Load Handling

The best use for backing is increasing the durability of the arch under massive loads. The arch should be built self-sustaining; in other words the arch will stand on its own without relying on the backing. The backing, however, greatly improves the rigidness of the arch, giving a solid barrier against the arch forming hinges and failing under massive loads. And, at the same time, the backing needs to be independent of the arch enough that the arch can still interact with itself as a whole, responding to loads and flexing slightly, but being hindered from flexing catastrophically thanks to the backing. In the final tally, the arch itself should be designed to handle the loads imposed, with the backing primarily being used to provide a comfortable buffer of safety and long-term durability.