As previously stated, early bridges were constructed from commonly found materials like stone (concrete), weed and vines.
Wood. As a bridge building material, wood is strong in relation to its weight. Under stress, the wood type and grain determine its strength. Since wood will bend and split easily, its strength is unreliable. When a plank's width is doubled, its carrying capacity is doubled. When the same plank's height is doubled, its carrying capacity is quadrupled.
Iron. As a result of the Industrial Revolution, methods were developed to mass-produce iron in large quantities. Iron is about four times as strong as stone and thirty times stronger than wood. There are two ways to make iron: Wrought and cast. Wrought iron is stronger under tension than cast iron.
Concrete. Concrete is made from a combination if sand, pebbles, artificial aggregates and cement. Concrete is very strong under compression and is a natural building material for arches, which are also strong under compression. The introduction and development of concrete gave bridge building a much-needed boost.
Steel. Steel, in relation to its weight is the strongest of all bridge building materials. It is 20% stronger than wrought iron. Steel is resilient, more flexible than iron and can be cut. This flexibility of steel makes it ideal for suspension bridges where the cables can shift 2 to 3 feet.
