Arch Bridge
Part of Bridges
Constructing masonry or timber arch bridges — humanity’s most proven long-span crossing technique.
Why This Matters
The arch bridge is arguably civilization’s greatest structural invention. Roman arch bridges built two thousand years ago still carry traffic today. The arch converts bending forces — which crack and split most materials — into pure compression, which stone and brick handle beautifully. An arch bridge built from local stone with basic tools can last centuries with minimal maintenance, crossing spans that would require large timber beams or steel in any other structural form.
In a post-collapse rebuilding context, arch bridges are the path to permanent infrastructure. Timber beam bridges work and are faster to build, but they rot and must be replaced every generation. Masonry arches are a permanent investment. Communities that build masonry arches create infrastructure that serves their grandchildren and great-grandchildren.
The key challenge with arch construction is that the arch requires support — called centering — during construction. Once the keystone is placed and the arch is complete, it becomes self-supporting. Before that moment, it cannot stand alone. This requires temporary timber falsework that must be designed and built before a single arch stone is placed.
How an Arch Works
An arch works by redirecting the downward force of its own weight and any loads placed on it into diagonal compression forces that flow down the arch ribs to the abutments. At no point in a properly proportioned arch is the stone in tension — it is always being squeezed together. This is why arches can be built from materials with low tensile strength like brick, rubble stone, and unreinforced concrete.
The shape of the arch determines how well it handles different load patterns. A semicircular arch (half a circle) generates high horizontal thrust but handles uniform loads well and is simple to lay out with a compass. A segmental arch (a circular arc less than a semicircle) is flatter, generating less vertical clearance but also somewhat less horizontal thrust. A pointed arch (two arcs meeting at a peak) is structurally efficient and generates less outward thrust than a semicircular arch of the same span — this is why Gothic cathedrals used pointed arches to reduce the buttress size needed.
For most bridge applications in a rebuilding context, a semicircular or segmental arch is easiest to construct and easiest to calculate. The rise should be at least one-quarter of the span for structural adequacy.
Laying Out the Arch
The arch is defined by its span (the horizontal distance between abutment faces) and its rise (the vertical height from the spring line — where the arch begins — to the crown).
To lay out a semicircular arch: the radius equals half the span. The center point is at mid-span at the spring line elevation. With a string compass of that radius, you can mark the curve exactly on your centering timber.
For a segmental arch, choose a rise that is roughly 1/4 to 1/3 of the span. The radius of curvature is calculated as: R = (S²/8r) + (r/2), where S is the span and r is the rise. This can be worked out on paper or with a drawn geometric construction.
The voussoirs — the wedge-shaped stones that form the arch — must have their faces cut radially: each face points toward the center of the circle. The joint lines between voussoirs, if extended, would all meet at the center point. A voussoir with non-radial joints creates stress concentrations and is prone to slipping.
Building the Centering
The centering is the temporary timber framework that supports the arch voussoirs while they are being placed. It must support the full weight of all the arch stones until the keystone is placed and the arch becomes self-supporting.
Centering is built from timber ribs curved to match the intrados (inner face) of the arch. For small arches under 3 m span, simple curved ribs of sawn timber are sufficient. For larger arches, built-up laminated ribs or king-post truss ribs are used.
The centering must:
- Match the arch curve accurately — errors propagate into the finished arch
- Be strong enough to carry the weight of all voussoirs simultaneously
- Have a way to be lowered (struck) after the arch is complete without damaging the new masonry
- Be stable against sideways movement
The traditional method of striking centering uses sand-filled tubes or timber wedges beneath the centering supports. When the arch is complete, the sand is bled out or the wedges are knocked back, gently lowering the centering away from the arch. Never try to pull centering out suddenly — the shock can crack new mortar joints.
Placing Voussoirs
Voussoirs should be placed symmetrically from both sides toward the crown. Never build one side higher than the other — unequal loading tilts the centering and deforms the arch before it is complete.
Each voussoir must bear solidly on the centering and on its neighbors. Mortar joints should be consistent in thickness — traditionally 10–15 mm. Thin joints are stronger; thick joints compensate for inaccurate cutting but introduce more compressible material into what should be a rigid compression ring.
The keystone — the central stone at the crown — is the last stone placed. Its placement locks the arch together. The keystone must be driven firmly into position; if it is too loose, the arch will settle unevenly. If it requires significant force to place, check that the centering has not bowed inward under load.
After the keystone is placed, allow the mortar to cure for at least 7 days (14 in cold weather) before striking the centering. The first loading of the arch will cause slight elastic compression of mortar joints — this is normal. However, if visible cracking occurs at the crown on the intrados (inner face) or at the springings on the extrados (outer face), the arch is being overloaded or has a geometry problem.
Spandrel Fill and Deck
After the arch ring is complete, the triangular spaces between the arch and the road surface level (the spandrels) must be filled. Options include:
Solid fill: Compacted rubble and earth. Traditional, simple, and adds stabilizing dead weight. Use well-compacted granular fill, not loose soil that can compress or wash out. Place fill in layers, compacting each layer before adding the next.
Spandrel walls with hollow fill: Build low masonry walls on top of the arch extrados, parallel to the road, and fill between them with compacted gravel. This reduces the total weight compared to solid fill.
Open spandrel: Leave the spandrel open with only the arch ring visible. This requires the road surface to be carried on the arch ring directly or on small cross-beams. Structurally the lightest option but architecturally complex.
Lay the road surface — stone paving, compacted gravel, or timber planking — on top of the fill. Ensure adequate cross-fall for drainage. Standing water on a bridge deck accelerates deterioration.
Spanning Multiple Openings
Long crossings can be managed with multiple arch spans on intermediate piers. Each pier must be designed to handle the thrust from both arches it supports. If one arch is unloaded (e.g., during flood passage), the other arch’s thrust is no longer balanced — the pier must resist this unbalanced thrust. Make piers substantial and well-founded.
As a rule of thumb, pier width should be at least 1/5 of the span. A wider pier is always safer but increases the obstruction to water flow, which can worsen flooding. Balance hydraulic requirements against structural ones.