Beam is probably the oldest and most primitive form of bridge. A felled tree across a river span would have been one of the first occurrences of this bridge type. We do not know the exact way that bridge construction evolved in the early times and, while remnants of early bridges exist today, it has not been possible to define the exact pathway. In any case, history has taught us that there were probably parallel evolutions with different end results depending on the location and terrain.
Evolution of the beam involved placing a number of trees alongside each other to increase carrying capacity. Cutting the trees to form planks and reduce weight was a natural progression, together with forming multiple spans where a river was wide but not too deep. The alternative material used was stone and ancient bridges of stone gave rise to the name ‘clapper bridges’. These were constructed of flagstones spanning from one large stone to another which could stretch for miles depending on the ground. As materials evolved this concept of multiple spans continued to be utilised and, in addition, as the strength of the new materials improved so also did distance that the beams were spanning.
While the beam is the simplest form of bridge construction it has its limitations. This manifests itself in two ways. The first problem is that while it is feasible to build a beam bridge over a wide area by building multiple spans of beams the terrain has to be suitable to permit all the necessary columns to be installed. This is not always possible in deep valleys or rivers. The second problem is that as a span gets longer the beam increases in depth and weight to cater not just for the increased loading but also to increase stiffness and reduce flexibility. The longest span for beams is approximately 50 metres.
The span of beam bridges is determined by the capacity of the material to withstand tension, compression and torsion forces. The development of concrete introduced a material with excellent compression capabilities but very weak in tension. The introduction of steel brought very good tension qualities and some compression. These materials had a profound effect on the construction of bridges, particularly when both were combined into reinforced concrete. The introduction of pre-stressing and post tensioning further enhanced the carrying properties of concrete beams. Beams today are used in the short to medium span range of bridges and with the development of composite materials a new form of beam building is emerging.