There are a variety of arch shapes and sizes to choose from when building a stone arch bridge. Thus, the question of what design is best is a natural concern. As it happens, there are no hard-and-fast rules here, just some considerations that will make the build easier.
What Are We Spanning?
The shape of the arch will primarily hinge on what is being spanned. A gorge can readily accommodate an arch with a high rise, while a swamp can prove rather challenging to span, at least not without a noticeable “hump” from the bridge. A good rule of thumb for stone arch bridge building is this: If multiple locations to choose from are available, the deeper and narrower the stream, the easier it is to bridge with a stone arch.
Narrowing it Down
The design can be narrowed down. The presence of bedrock for a foundation allows for a much greater variety in design. If no bedrock is available, it is usually safest to build a Roman arch or even a Gothic arch. These higher-rise arches have better immunity to settlement. A segmental arch may certainly be used, but anything flatter than a 130-degree segment of a circle will be rather particular about the solidity of its foundations. Even if bedrock is available, arches flatter than a 90-degree segment of a circle are generally best avoided, as the considerable outward thrust of the arch makes these arches difficult to build and they are just generally weaker. Arches flatter than a 90-degree segment of a circle are best saved for locations where the ends can be firmly fixed into bedrock, and a long span at low rise is of utmost importance. No matter what is being designed, ensuring a good foundation is the most important criterion.
Multiple Spans
Multiple spans result in a pier in the waterway, which is a definite weakness. However, using multiple short spans may be preferable to one longer span with a large “hump” in the road, though the hump can be mitigated by making the approaches longer.
And, the fact is, especially when crossing a swampy, low area, multiple spans can be a very efficient way to bridge a broad gap. A long approach and a relatively small arch can be used, but sometimes it is actually more efficient to go with more arch or arches and less approach, even if the extra waterway is unneeded. Furthermore, if a hard, solid foundation is unobtainable, it is best to opt for shorter spans, even if this requires more arches to be built, as the lesser weight on the individual piers and abutments will help mitigate the danger of settlement.
Conclusion
To conclude, the designer must choose from a variety of options:
- How much waterway is actually needed?
- How much (if any) of a “hump” in the road is tolerable?
- What sort of foundation is available?
- How much material/time/expense can be put into a project?
Once you’ve chosen your criteria, it is best to stick with the original plan, rather than second-guessing. Furthermore, if the final product simply is not adequate, there are usually ways to fix the problems. Too high of a “hump” can be rectified by adding longer approaches and/or more fill at the ends of the bridge. An additional arch can be added to a bridge to rectify grossly insufficient waterway. Again, we emphasize the fact that the most important consideration is the solidity of the foundation.
Designing a stone arch bridge is not a massive challenge, but, with practice, is easy. You will find that this type of construction can be remarkably forgiving.
Stone Arch Bridges 