Curved members or structures , using material in compression , the centre part rising above the level of the ends. They may be pinned at one end, at both ends, or at both ends and in the centre. The pinning of an arch rib ensures that there is no bending effect at that point, and permits movement to accommodate alterations of dimensions due to temperature changes.
The chief feature of an arch bridge, apart from it being a handsome structure, is that at the abutments there are horizontal forces as well as vertical ones, so that the resultant reactions at the abutments are inclined to the vertical. The invention of the arch was a wonderful forward step in structural design.