Bridge Structures and The Use of Geometry in Their Design What is a bridge? A bridge is a structure providing continuous passage over a body of water, roadway, or valley. Every bridge looks different from the another. Their bridge structure is different. There are nine types of main bridges. Bridge engineers use a lot of geometry to help them design the different bridges. Without geometry, the bridges would not be safe. Bridges have been around for a long time. The earliest bridges were probably formed by laying one or more logs across a brook or by stretching ropes or cables across a narrow valle