A crucial part of arch building is the temporary formwork used to support the arch during construction, otherwise known as the centering. The centering is necessary as the arch cannot support itself until it is completely finished.
Requirements of the Centering: Strength
In fact, this centering is as critical as the arch itself; the arch cannot be made without it, and not only must this form be exactly the shape the arch is to be, it must be able to support the entire weight of the stonework, which weight is enormous. As a historical example of the last point, when building a large stone arch bridge over Hickory Creek in Butler County, Kansas, C. C. Jamison had the expensive misfortune of complete failure of his centering, with the resultant collapse of the uncompleted arch, forcing him to start over. C. C. Jamison did eventually succeed with this bridge, as anyone driving over this beautiful stone arch bridge today can testify. The strength of the centering is crucial.
Even if the centering does not fail, it can distort under the weight of the arch, causing the resultant arch to be weak. For larger arches, some form of truss-work is used to increase the strength of the centering. Ironically, the centering is something of a bridge in its own right — not only must it be strong enough to support several tons of masonry, but it also must be placed firmly enough that it won’t be washed out during any high water events which occur during construction.
Requirements of the Centering: Accuracy of the Arc
The arch can only be as true as the formwork itself is, and it is crucial to cut the wood to the correct shape. An easy error to make when building the centering is to build it out of a series of straight lines rather than a true arc. And, as mentioned above, the centering must be strong enough to maintain its shape when fully loaded.
Material Choices for Centering
Although wood has always been a common choice of material to build the centering with, wood is by no means the only option available here. These days, rebar can be used as an arch form. Rebar makes a very simply created arc when the ends are pushed towards each other, though technically, unless forced into a different shape with bracing or skilled bending, the result will be more of a parabolic shape rather than, say, a true segmental arc. A highly economical centering is a pile of soil, formed into the desired shape. The arch can then be built on top of the soil. The primary concern here is getting the soil into the correct shape, though with some skill this can be done. Another problem with this alternative centering is that soil tends to plug up streams very effectively; either the stream will have to be rerouted, the bridge will have to be built during a dry period of time, or the soil will have to be built on a platform of some sort that is raised above the waterline. Perhaps, alternatively, a modern culvert can be added through the dirt pile to allow some drainage.
Requirements of the Centering: Provision for Removal
After the arch is completed, the centering is ready to be removed. Unfortunately, unless some planning was done ahead of time, removing the centering can become a challenge due to the enormous weight pressing onto it from the arch. If a pair of wedges, one atop the other, are used, with the tapered ends towards each other (resulting in a flat surface again, provided the two wedges are identical), when the arch is completed, these wedges can be worked out and the centering dropped down. Once the centering is clear of the arch, and is no longer holding any weight, it can be slid out or broken up. The key is not to damage the arch during the removal of the centering. In the case of a soil centering, removal is easy — the soil is dug out from under the arch.
Frankly, there is no hard-and-fast rule on how the centering is to be created; imagination is the limit here, and all that is required is that the centering is strong enough to hold the weight of the arch without distorting or collapsing, that the centering is the correct shape, and that provision is made for successfully removing it when the arch is completed. You’d be surprised – sometimes, in desperation, people have burned the centering out from under the arch after being unable to remove it in a more conventional manner.