When mankind became a creature of settled habits, he set about making sure that he could cross the local rivers safely by laying down flat stones in the local ford for secure footing.
If a particular crossing was used frequently enough to justify the effort, he would build a bridge making use of indigenous materials such as bamboo, wood or stone. During the next 7000 years of bridge building, only the most sophisticated civilizations thought of using man made materials such as Nebuchadnezzar’s (reigned 605-562 BC) use of fired bricks in his bridge across the Euphrates or the Romans' use of pozzuolana cement.
Wooden bridges are easily built or replaced. They do require constant maintenance as they are vulnerable to fire, dry rot and flood. A few of them were very ambitious structures. London bridge was built and rebuilt of wood from Roman times right up until King John (1167-1216) rebuilt it in stone. Caesar and Charlemagne each built famous wooden bridges over the Rhine but did so 800 years apart.
The earliest stone bridges were the “clapper” bridges built of gigantic slabs of stone raised above water level by resting them on top of large boulders serving as piers. A clapper bridge is heavy enough to survive in place during most floods. To keep the rushing water from eddying around the boulders and undermining them, elliptical stone shear waters were placed to divert the current around and away from the foundation rocks and to deflect flood debris safely up and over the bridge. Clapper bridges are easily rebuilt by reassembling the scattered stones after a disastrous flood.
Given the materials at hand which could not span a great distance, early bridges of any length required piers in midstream. Piers cause problems for bridge builders and for the boats using the waterway beneath the bridge. The pier's foundation is difficult to lay down securely under moving water. The piers are vulnerable to being undermined by the swirls and eddies created by their interruption of the current. Sometimes the riverbed soils cannot provide the stability needed to support the bridge whose weight will be concentrated at the location of the piers.
For the river traffic, piers are a hazard to navigation situated right in mid-channel. The shear-waters protecting the pier foundations increase the turbulence of the water for passing boats, especially in tidal creeks and rivers. Shipping must also have good clearance beneath the bridge at all water levels. It has been the goal of bridge builders through the ages to place the decking well above flood level and to have as few piers as possible .
To minimize mid-stream bridge piers, one must maximize the span of the bridge between its supports. That span is limited by the tensile strength of the material being used to build the bridge. (In other words, what is the distance that the loaded bridge deck can it be unsupported before it begins to sag or fail?) Wood and stone have relatively poor tensile strength. Over the ages, mankind invented various ways of increasing the tensile strength of the few natural materials he had at hand in order to lengthen the span of his bridges.
The Romans and the Chinese invented slightly different versions of the stone arch and various cultures devised the cantilever, both of which redistribute a good deal of the load at the center of the span onto the supporting piers. Rope or hand wrought iron chains were used in suspension bridges.
Wooden truss construction, which is based on the principle that the sides of a triangle are mutually self-supporting, was recommended by Leonardo Da Vinci (1452-1519) who noted below a sketch of one “This bridge is unbreakable.”
In Europe by the end of the 17th century engineers felt that they had maximized the potential for greater spans using these traditional materials. A new material was needed for a new age. During the next century, British iron masters gradually developed a new process for refining iron much more economically and on a large scale. The resulting product had greater tensile strength than the 'old' iron, and had infinite versatility. The first substantial all iron bridge was opened in the Severn Gorge at Coalbrookdale in 1797.