Bridge

Introduction

Even the most primitive humans have created bridges from material lying around in their surroundings. Hunters and gatherers always shared a common problem; streams, rivers and trenches needed to be crossed. A fallen tree can be dragged into position to serve as a plank. Forest tendrils may be intertwined as an elementary suspension bridge. Or rafts can be tied together in a pontoon (Cridlebaugh, 2002). That was the invention of the first bridge. The innovation of this basic concept came in during the Roman Empire. Romans would create a structure that could withstand great pressure and weight. This is the Roman arch - an invaluable principle which was the trailblazer for bridges in the future

There are three basic designs for bridges that we analyzed. The beam bridge is a good design when trying to span a short gap that is also not very high. It is supported on either end by land or tall columns. This bridge is normally quite heavy. The truss bridge is similar to a beam bridge except much stronger. These bridges use a pattern of geometrical shapes (triangles) called trusses. These trusses are very rigid (they do not move when pushed) and, thus, make the bridge very strong. (Llewynn, 2005). Truss bridges are often used for railway bridges as they must be able to support the great mass and the vibrations of the trains. The arch bridge is strong because no one point along the arch supports more mass than any other point - thus, the mass is spread out very evenly (the force spread out both downwards & outwards from the center). (Cridlebaugh, 2002).

Precision/Design Philosophy

The design of our bridge was based on our background research, competition guidelines and observations. First we decided that the most logical bridge to be built was either a truss or beam bridge because an arch seemed to be irrational. Both a truss and beam bridge were built and broken to decide which design would be used for the final bridge. However, we chose to combine both of these bridges. The beam bridge design aspect was modified using the haunches because one method of increasing a load capacity while minimizing its width is to add haunches at the supported ends. Haunches are the part of the bridge that are at the sides of the beam and angle down toward the legs. We also wanted to keep the bridge at minimum width because a beam bridge is most effect with short spans. The truss design of the bridge was the central triangular truss used to support the roadway. The truss is used to spread out the weight applied to the roadway. The bridge was kept to the minimum height to apply less pressure on the legs of the bridge. The shorter the legs the more pressure is channeled to the surface as to which it is rested on. Also due to prior knowledge the majority of the weight applied to the bridge is held by the glue. Therefore, we focused on plastering the joints, haunches and center of the bridge with as much zap glue as possible without breaking any of the weight restrictions.

Appendix A