Solid backing for arches provides several structural benefits to stone arch bridges. Solid backing can be considered, in effect, something of a continuation of the spandrel walls throughout the width of the bridge, thereby significantly evening out the rigidity behind the arch, and reducing loads on spandrel walls (see part 1 of this series). Furthermore, solid backing actually provides a load-bearing capability of its own to the bridge, on top of that which the arch provides.
Methods of Bridging With Stone
Besides the arch, there are two other means of bridging gaps with stone. These methods are very excellently summed up in the April 6, 1906, edition of The Augusta Daily Gazette. Pay special attention to the “arch culvert” picture.
The arch culvert shown above is, of course, not a “true” arch, but rather a corbelled arch. It works because the successively stepped out stones transfer the force down to the abutments. Note the red lines added to the corbelled arch photo on the right. These lines show roughly the effective “curve” of the corbelled arch. Notice, too, that the top stone in this “arch” is also acting as a slab across the two corbelled bases. One final thing to note is that corbelling can also be successfully implemented as a free-standing architectural feature that juts out beyond the base that it rests on.
The Hidden Arch
How does the corbelled arch relate to the typical stone arch bridge? The answer is subtle, but becomes obvious upon detailed examination.
In the photo above, the red lines added show, roughly, the joint between the arch and spandrel walls. And, while not perfect, note how this results in a corbelled wall very similar to the corbelled arch culvert diagram above. What are we to take away from this? The spandrel walls act like a crude corbelled “arch” that rests just outside the actual “true” arch of the stone bridge. Furthermore, if we build a solid backing behind the arch, which is something of a continuation of the spandrel walls themselves, we have in effect an arch within the arch. Thus, we can greatly increase the load-carrying capacity of a stone bridge with solid backing!
Even a soil fill will tend to distribute the force around the arch. This is why, ironically, a stone bridge can be strengthened by increasing the amount of fill over it (see Improving a Stone Arch Bridge’s Serviceability by Strengthening: Part 2). However, as should be obvious, loose soil cannot be made to bridge a gap, while a corbelled arch can. Thus, solid backing behind the arch can act as a corbelled arch, adding to the load-bearing capacity of a “true” arch bridge. This is true even if the backing does not continue to the top of the arch; it still steps some of the force off away from the sides of the arch. Note, however, that the solid backing becomes less effective on the higher portions of the arch. Here the two halves of the arch curve inwards towards each other so that the backing can no longer “corbel” the forces down effectively; in effect the backing would become more like stones sitting on the arch, as opposed to being a step down to the next corbelled stone below and so on. For this same reason, solid backing would be less effective for low-rise segmental arches.
Solid backing is an important structural part of a stone arch bridge. It helps evenly stiffen the arch and can even carry some of the weight of loads in its own right. The main possible concern with solid backing is in the rare case where the backing is so solidly enmeshed with the arch proper that the arch ceases to function like it is intended. In this scenario, it is possible for the top arc of the arch to act somewhat independently of the bottom half of the arch. This scenario aside, solid backing is a highly effective means for greatly improving the strength and durability of a stone arch bridge.