The stones of an arch do not necessarily have to fit in place perfectly for the arch to be strong. The key requirement for an arch is that the effective arch thickness is sufficient. What is effective arch thickness? As the name implies, it is how much of the arch is actually transferring forces.
Effective Arch Thickness: An Example
If you have ever built a drystack DIY arch with nice flat slabs of rock without spending much time cutting them, odds are you will sooner or later notice that little gaps appear on the inside surface of the arch where it rests against the centering. These gaps will often not be noticeable until the centering is removed. They are caused by the fact that no flat slab of rock is truly flat, unless it is smoothed. For whatever reason, our observation is that natural slabs of limestone have an annoying subtle curl at the end, which leaves gaps in the arch when laid that are often invisible until the centering is removed. As it happens, however, these gaps do not necessarily mean that the arch is weak, even though a bottom couple of inches of the arch stones do not make physical contact with each other. The arch still succeeds because there is enough contact between stones that the defective bottom part of the arch does not cause problems.
This is where effective arch thickness comes in. If my arch is a foot thick, but the bottom three inches of most stones are not making contact, my effective arch thickness becomes nine inches; nine inches of the arch is actually carrying the load and the bottom three inches are not doing anything.
Effective Arch Thickness: Building Strong DIY Arches With Minimal Stone Cutting
This concept of effective arch thickness is actually a lifesaver for those of us who are not professional stonecutters. It means that we can build an arch thicker than it needs to be, knowing that there will be some gaps in the arch, and there will still be enough effective thickness in the arch for the bridge to stand. Of course such gaps should be mitigated as much as possible, but gaps in the arch do not automatically spell disaster. How much thicker an arch needs to be to compensate for imperfections varies, but in general multiplying the required arch thickness by 1.25 seems to be a very safe bet based on our experience. In general, you cannot build an arch that is too thick, though practically speaking the thicker the arch the more material required and the higher the “hump” of the structure becomes.
Effective Arch Thickness: Load Ratings
One other use of effective arch thickness is when determining the load rating of an old, deteriorated or roughly-built stone bridge. Periodically a stone arch bridge with ragged inside edges on the arch will be found. When calculating the load rating for such a bridge, the rough parts of the bridge are factored out, leaving the effective arch thickness, which then is used to determine how much load the bridge can safely carry.
Stone Arch Bridges 