How does an arch bridge transfer loads to its foundations?

An arch bridge carries loads primarily through compression, which directs forces into the foundations as both vertical and horizontal components.12

The mechanics of this transfer involve the following:

• Compression and Thrust: Downward pressure on the arch forces the wedge-shaped pieces, known as voussoirs, together. This action converts the load into a diagonal force called thrust, which pushes outward against the supports.1
• Foundation Requirements: Because of this horizontal thrust, foundations must be engineered to prevent both vertical settling and horizontal sliding.12
• Buttressing: To maintain stability, the vertical supports or abutments must be massive enough to buttress this thrust and conduct it safely into the ground.1
• System Stability: In multi-arch structures, the thrust of one arch can be counteracted by the thrust of its neighbors, provided the arches at the ends of the row are properly buttressed.1

Modern materials like steel or concrete can reduce horizontal thrust, sometimes utilizing ties between the arch ends to further manage these forces.1