Final answer:
The force that has the most impact on arch bridges is called thrust. An arch bridge pushes outward at the base due to this force, and it needs to be restrained to prevent collapse.
Step-by-step explanation:
The force that has the most impact on arch bridges is called thrust. An arch bridge is a pure compression form, and as the forces in the arch are carried to the ground, the arch will push outward at the base. This outward thrust needs to be restrained to maintain arch action and prevent the arch from collapsing, either with internal ties or external bracing such as abutments.