Brief description

An Arch Bridge is a bridge with abutments at each end shaped as a curved arch. It works by transferring the weight of the bridge and its loads partially into a horizontal thrust restrained by the abutments at either side. It is one of the oldest and strongest types of bridges, capable of standing for thousands of years.

Use / Function

• Transportation: To allow passage over obstacles like rivers, valleys, or roads.
• Durability: Stone arch bridges are incredibly durable and low maintenance compared to wood or iron bridges.
• Load Bearing: Excellent for carrying heavy loads due to its compressive strength.
• Aesthetics: Often considered aesthetically pleasing due to the natural curve.

Operating principle

The arch bridge works primarily by compression.

1. Load Transfer: The weight of the bridge and the traffic on it pushes downwards.
2. Compression Path: The arch shape directs this force outwards and downwards along the curve of the arch.
3. Abutments: The force arrives at the supports (abutments) at the ends of the bridge. These abutments push back against the arch, preventing the ends from spreading apart.
4. Keystone: The central stone at the top of the arch locks the voussoirs (wedge-shaped stones) together, ensuring the structure remains stable under compression.

Unlike beam bridges which experience tension on the bottom, a well-designed masonry arch is almost entirely in compression, which suits materials like Stone and Concrete perfectly.

How to create it

1. Foundations: Excavate and build solid foundations for the abutments on both sides of the gap. These must be immovable to resist the outward thrust.
2. Abutments: Construct the massive supports at the ends.
3. Centering: Build a temporary wooden framework (centering) that mimics the shape of the intended arch. This supports the weight of the stones until the arch is complete.
4. Arch Ring: Lay the wedge-shaped stones (voussoirs) or bricks on the centering, working from both sides towards the center simultaneously to keep the load balanced.
5. Keystone: Place the final stone (keystone) at the top center. This key piece locks the arch into compression.
6. Spandrels & Deck: Fill the space above the arch (spandrels) with rubble or masonry to create a flat surface, then pave the deck.
7. Removal: Carefully lower and remove the wooden centering. The arch settles slightly and supports itself.

Materials needed

• Essential:
  • Stone or Brick: Strong in compression, shaped into wedges.
  • Mortar: To bind the masonry and distribute pressure evenly (though dry stone is possible).
  • Wood: Absolutely critical for the temporary centering structure.
  • Concrete: For modern arch bridges.
  • Steel: For modern through-arch bridges or reinforcement.
• Tools: Chisel, Hammer, Level, hoists or cranes.

Variants and improvements

• Roman Arch: Semicircular arches. Strong but require tall abutments relative to the span.
• Segmental Arch: An arch that is less than a semicircle (flatter). Allows for longer spans with lower rise, but exerts more horizontal thrust.
• Pointed Arch: Gothic style, directs forces more vertically, reducing side thrust.
• Multiple Arches: A series of arches supported by intermediate piers (viaducts).
• Through Arch Bridge: The deck passes through the arch (often steel), with the deck hanging from the arch above.

Limits and risks

• Span Limit: Stone arches have a limit to how wide they can span before their own weight causes them to collapse.
• Abutment Failure: If the ground moves or the abutments slide, the arch loses compression and collapses instantly.
• Construction Danger: The structure is unstable until the keystone is placed. Failure of the centering during construction is catastrophic.
• Rigidity: Masonry arches do not handle earthquakes or ground settling well; they can crack and fail.