Stone bridges are, historically, built to a rather narrow width. While suitable for the horse-drawn traffic they were often designed for, narrow stone bridges can pose a bit of a problem in modern vehicle applications. Rather than demolish an existing stone bridge, widening the bridge can prove a helpful alternative. Widening a stone arch bridge not only preserves the existing, historic structure, it is typically more cost-effective as well.
When is a Bridge Too Narrow?
How narrow is too narrow for a stone bridge is a judgment call for the local traffic authorities.
In general, most bridges are too narrow by modern standards. In practice, this narrow width does not pose too much of a problem on lightly traveled rural roads.
Sometimes, a stone bridge is so narrow it cannot accommodate any traffic unless somehow altered.
Rare is the stone bridge wide enough to handle a modern two-lane blacktop.
In the final tally the vast majority of remaining American stone arch bridges are narrow by modern standards but wide enough to remain functional for the roads they carry.
Widening a Stone Bridge: Increasing Useful Deck Width
By nature, stone arch bridges tend to be somewhat wasteful of their width. The roadway is fill material, held in by the spandrel walls. The spandrel walls, in turn, must be reasonably wide to hold in this loose material. The end result is that several feet of the bridge’s width actually consists of the tops of the spandrel walls.
The spandrel walls usually extend above the fill providing a curbing. This opens up some ideas for widening.
A theoretical “quick fix” would consist of either removing the tops of the spandrel walls or building up the fill to be level with the spandrel walls. While this would increase the bridge width, it also would make the crossing much more perilous. Furthermore, the fill is relatively unstable; the odds of it always remaining precisely level with the tops of the spandrel walls are very improbable.
An alternative solution is to level off the top of the bridge with a concrete slab. The concrete slab is poured on top of the whole bridge, covering both the fill and spandrel walls.
If no curbing is provided, this is rather a road hazard and best reserved for very small spans. Curbing can be added, but will quickly eat up the available width again.
Widening a Stone Bridge: Building an Adjoining Bridge
Historically, a stone bridge was widened by building another stone arch bridge directly against the existing bridge. The two structures are almost totally independent of each other; in fact, sometimes the two bridges didn’t even use the same type or span of arch.
A more common example of this is in Europe where a bridge with Gothic, pointed arches is widened with a bridge using Roman arches.
These days, stone is rarely used to widen a bridge. However, adjoining concrete arch or slab bridges can be added, or, as in the case of the Thompson Bridge in Cowley County, Kansas, an extension with I beams supporting a corrugated metal deck can be used to widen small spans.
Widening a Stone Bridge: Cantilevered Slab
A very popular, very successful method of widening a stone arch bridge is to pour a reinforced cantilevered concrete slab on top.
The slab covers the fill and spandrel walls, but then extends out further, and can be easily terminated with some sort of railing or curbing. The ends of the slab usually do not need to stick out very far, so the structure tends to be stable. The ends of the slab are usually supported by triangular supports that go off from the ends of the slab at a diagonal and rest firmly against the stone bridge. The end result is a wide, stable, safe stone arch bridge.
As an added benefit, the concrete slab tends to improve the load-handling ability of the stone bridge.
This is an excellent solution for keeping an existing stone arch bridge on the road system.