Arches are in incredibly useful structures and often appear in building technology as way to hold material above a space. They occur in nature and we build them. But what are they exactly?  Engineers describe them as a pure compression form.  This is because the forces that maintain the arch shape tend to compress the materials that make the arch. 

A masonry arch includes: 1. Keystone 2. Voussoir 3. Extrados 4. Impost 5. Intrados 6. Rise 7. Clear span 8. Abutment. The arrows indicate the direction of compression forces.  image: wikipedia

Arches can span a large area by resolving forces into compressive stresses and, in turn eliminating tensile stresses. This is sometimes referred to as arch action. 


Pont du Gard, a Roman aqueduct in Vers-Pont-du-Gard, Gard, France. Consists of a several arches on top of each other.  image: wikipedia/Emanuele 

As the forces in the arch are carried to the ground, the arch will push outward at the base, called thrust. As the rise, or height of the arch decreases, the outward thrust increases. 

In order to maintain arch action and prevent the arch from collapsing, the thrust needs to be restrained, either with internal ties, or external bracing, such as abutments.