The history of bridges is fascinating. The oldest and simplest type is the post and beam which the Trojans used over 3,000 years ago. The Roman Empire contributed the arch, cofferdams (encircled watertight walls from which water is removed and bridge foundations dug inside), concrete, and the concept of public works which enables an empire to flourish. The Dark Ages that followed collapse of the Roman Empire was a confusing time of battles, famines, and epidemics. The great bridges of the Romans seemed superhuman to those living in a simple feudal system. Myths sprang up about evil spirits in the streams and bridges. The Roman Catholic Church became the best bridge builders in Europe. The religious orders had the education and travel to research bridges so every bridge had a chapel and an alms (donation) box (for those evil spirits).
In the twelfth century the first London bridge, across the unruly tidal Thames River, was begun with twenty cofferdams to build the piers. There were nineteen pointed arches for a distance of 937 feet. Piers blocked two-thirds of the river which twice daily rose up to 15 feet with the tide. Water rushed through the remaining one third and a dangerous sport evolved called "shooting the bridge" (riding a boat in the fast current between the piers was extremely dangerous). Dozens of tall buildings with shops and homes perched on the bridge. It was a small village on a bridge and the best address in town. By the late 1700's the bridge had to be rebuilt and two more since then.
In the middle ages, bridges that could be used for invasion were fortified heavily with drawbridges, iron gates and high walls. Through the centuries invasions were always a looming danger. More bridges have been destroyed by war than by wear.
By the late 1400's the Dark Ages awoke to the Renaissance in Europe. Stability, wealth and great artists combined to create great bridges that were not only useful but beautiful. Powerful city states competed for architectural beauty. In Venice, 1587, the best known architect, Palladio designed a monster, heavy, showy and grand bridge. It was not good engineering, however, because it restricted canal traffic with its three arches and tradesmen had to carry their heavy sacks up long stairs to get cross. Da Ponte solved these problems by building the Rialto Bridge, the most famous and beloved covered bridge in Europe. It is a single shallow arch bridge, still strong and beautiful. Because of the soft soil on the sides, he built cofferdams on either side of the Grand Canal with special terraces with a ramp that met the diagonal thrust of the bridge. Engineers were just beginning to understand that directional forces, not just weight, held structures together. Leonardo Da Vinci and Galileo developed theories that helped architects make structures of strong, lightweight material.
The Age of Reason arrived in the 1700's. Complex forces, tension (stretching) and compression (squeezing), inside structures were explored. Triangles were engineered into trusses. Long, strong beams were designed with the least weight. A huge wooden train trestle built in the 1800's spanning a deep canyon was very strong with its layers of trusses. In 1813, Lewis Werwag, a master of wood construction, created a huge and beautiful bridge that was rot proof, light and strong. He knew how to eliminate sources of decay and his bridge could resist hurricane winds. It was a monument to reason and grace and its ingenious engineer.
In the late 1700's came the Industrial Revolution, quickly transforming the rural economy into industrial. The development of metal, first iron, then wrought iron and then steel transformed bridges. Steel could be mass produced in large quantities. The first iron bridge, the Coalbrookdale Bridge, built in England in 1779 used cast iron. In 1826, he built another high suspension bridge of wrought iron spanning 1000 feet across an important harbor. Cast iron is cast from a molten state into molds then cooled and hardened. It has low tensile strength. Wrought iron, three times the strength of cast iron in tension, was made into 3.5 inch bars to form the chains of the suspension bridge, the Menai Straight Bridge. In 1850, rectangular hollow tubes of wrought iron were used to make beams on the Britannia Tubular Bridge which were strong but used less material.
In 1878 in Scotland, the longest train bridge at the time was almost two miles long. Considered one of the seven wonders of the modern world, the Firth of Tay Bridge, an iron girder truss bridge, collapsed in a storm and a train with everyone aboard went down with it. The design had not allowed for wind forces and a gale of almost 80 mph hit the bridge. In addition, parts were uneven, misshapen and loose or weak. This disaster resulted in new solutions and designs. Four years later, the Firth of Forth Bridge, in 1890, was built as an immense, massive cantilever bridge with new material, steel, stronger than iron was used. It was the greatest railroad bridge ever built. In 1990, a commission declared that it would last even another hundred years. Engineers now knew how responsible they had to be about inspection of materials, building methods and maintenance.
Drawbridges were designed where very high bridges were impractical. In the late 1800's, came the practical availability of steel and huge, watertight iron caissons. Caissons are chambers that create an air lock so men can dig the bridge foundation under the water. Men had to climb down iron tubes and entered the air lock of high pressure atmosphere. The difference in air pressure made them dangerous and cost many lives. These caissons did allow the Brooklyn Bridge, spanning almost 1600 feet, and the Eads Bridge, the first to crossover the Mississippi River at St. Louis, Missouri, to be constructed in 1874. The Brooklyn Bridge has more than 1000 suspended cables hanging from the four main cables. The deck is stiffened by trusses. Diagonal bracing increased the stiffness. P.T. Barnum took twenty-one elephants across the Brooklyn Bridge to declare it stable. These bridges were built during the Gilded Age when ornamentation was considered beautiful. These bridges declared the beauty in engineering that worked.
In 1925, the George Washington Bridge was erected. It was two times the length of any previous suspension bridge. It had 600 foot towers, a record height. In 1937, the Golden Gate Bridge, spanning the almost 4200 feet between towers, crossed the San Francisco Bay. It was the longest suspension bridge for the next 28 years until the Verrazano-Narrows Bridge. Its deck is 220 feet above the water. The towers rose to 736 feet tall. Nothing surpassed this achievement for many years. Joseph Strauss had never built a suspension bridge. Only six miles away is the San Andreas Fault, so because of new technology, it is being reinforced to withstand an 8.9 earthquake. The Verrazano Narrow Bridge, one of the most expensive structures ever made, had cables alone that were worth more than the Golden Gate Bridge.
In the early 1900's, reinforced concrete impressively heavy and durable was used. Stressed concrete created much stiffer beam (girders). This miraculous construction material created two kinds of bridges - brilliant and boring. New materials, strong, rust resistant cables, new connectors and new designs for stressed concrete beams created new astonishing bridges like the cable stayed bridges.
Students can research and imagine what kind of bridge they could cross twenty years from now. Imagine the impossible because the impossible sometimes leads to the possible.