The History of Bridges

[Music] not long after its dedication in july 1940 the tacoma narrows bridge picked up a nickname galloping gertie strong winds caused it to sway up and down and sideways instead of being frightened away people came from everywhere to ride the roller coaster bridge [Music] two [Applause] [Music] [Applause] however just four months after dedication galloping gertie went down [Music] where gertie went down another bridge was built [Music] completed in 1950 it has stood the test of time and it's doing what all bridges are supposed to do it's getting people and their belongings to the other side [Music] oh [Music] oh [Music] [Applause] be [Music] nature provided the first bridges fallen logs [Music] stones hanging vines but these natural bridges were not always in the right place so people cut down trees wove vines together or placed stepping stones in convenient locations [Music] the invention of the wheel made it necessary to build longer and stronger bridges the early romans built bridges to move armies and establish trade throughout their empire and their improvements greatly advance the techniques of bridge building [Music] during the centuries that followed bridges were built throughout the kingdoms of europe some of these magnificent bridges are still in use today in america during the 1800s covered bridges were a familiar sight in town and countryside covered bridges were not only picturesque they were practical the roof protected the floor from water which could rot the wood the roof however could not protect them from their biggest enemy fire [Music] up until the 1850s most bridges were made of wood stone or rope but with the coming of the railroad bridges had to be made stronger and longer at first iron bridges seemed to be the answer but iron had a tendency to crack in america during the 1870s an average of 40 bridges collapsed each year from iron came steel although it was made from iron steel was much stronger the first steel bridge in the world crossed the mississippi river at st louis completed in 1874 and named after its builder james eads people said it couldn't be built said it wouldn't last but the eids bridge still crosses the mississippi at st louis the brooklyn bridge a steel bridge that took nearly 15 years to build completed in 1883 it was hailed as a tremendous engineering feat a marvel of its time as design and engineering improved the size of steel bridges increased in the 1930s two very big steel bridges were built across san francisco bay the oakland bay bridge built in two sections with an island in between [Music] the golden gate bridge its graceful lines striking color and dramatic setting have made it one of the most photographed bridges in the world completed in 1957 the mackinaw bridge connects upper michigan with lower michigan affectionately called big mac it is one of the longest suspension bridges in the world one of the highest bridges in the world spans the royal gorge of the arkansas river [Music] wood stone rope iron and steel in the early 1900s another type of building material was introduced concrete reinforced with steel rods although not popular at first it is now one of the most widely used building materials before there were automobiles before there were paved roads most bridges were built for trains in the early 1900s it took automobiles at least two months to drive from new york to san francisco but as cars increased in number so did roads and highways and more roads and highways meant more bridges in the 1950s the four-lane interstate system created a real boom in bridge building to a lesser degree it is still going on [Music] today [Music] [Applause] [Music] bridge building has a long history yet from past to present nearly all bridges have been built using three basic designs beam arch and suspension like a fallen log beam bridges rest on supports at each end to make beam bridges stronger triangles are added a triangle shape is stronger and more stable than either a square or a rectangle beam bridges that have one or more triangles are called truss bridges [Music] beam bridges can be lengthened by placing supports underneath these supports are called piers [Music] if these piers are braced with trusses it's called a tressel bridge the cantilever is another variation of the beam bridge construction on a cantilever starts at the piers and then proceeds outward in both directions the arch another basic design is very strong and can withstand heavy loads in nature there are natural arches sculptured by wind and sand [Music] stone arch bridges are built with wedge-shaped stones these stones are placed on top of each other until the last one the keystone is set at the top of the arch the first arch bridges had small openings later on the arches were stretched the openings were made wider and the columns thinner this allowed water to flow more freely and permitted larger boats to pass underneath arch bridges can be lengthened by increasing the number of arches they can be made of wood steel or reinforced concrete the third basic design is the suspension bridge the suspension bridge is similar to a spider's web like the threads in a spider web suspension bridges are held together by strong cables these cables are stretched over tall towers between the towers they drape down toward the bridge deck steel suspenders attach the cables to the deck the cables must also be anchored securely at each end of the bridge [Music] a cross section shows that this cable is not one but thousands of small steel wires if placed end to end the steel wires in the cables of the golden gate bridge would circle the earth three times suspension bridges are often used to span deep or wide obstacles on some bridges the towers are nearly a mile apart to keep it from sagging or swaying steel trusses are placed on both sides of the bridge deck sometimes cables are used to help anchor the bridge deck from below the deck on the tacoma narrows bridge was stiffened with solid plates when these solid plates were hit by strong crosswinds it created a roller coaster effect that led to its collapse [Music] the tacoma narrows disaster taught bridge designers to eliminate solid surfaces that could catch strong crosswinds [Music] beam arch suspension and pontoon although not as common pontoon or floating bridges are another basic design pontoon bridges are often used in wartime for quick and temporary river crossings in seattle washington however there are two permanent floating bridges that span lake washington when building bridges over fast moving water concrete piers must be set deep into the river bottom first concrete boxes called caissons are placed on the river bottom these caissons have tubes through which mud and sand are removed [Music] as mud and sand are removed the caissons sink deeper and deeper into the river bottom [Music] as the caisson sinks new sections are placed on top until the first or lowest caisson rests on solid ground after the caissons are filled with concrete they become the piers that will support the bridge a finished bridge of course is never really finished bridges require regular maintenance this includes repairs and painting [Music] [Applause] hey as bridges become old or obsolete they are sometimes replaced by new ones sometimes they are abandoned sometimes destroyed in rare instances old bridges are removed piece by piece and reassembled in a different location this bridge once crossed the thames river in london england removed and rebuilt a london bridge now crosses the colorado river at lake havasu arizona where bridges now stand the chances are good that ferry boats once crossed the river nearby there are still many of them in use today crossing the mississippi crossing its tributaries [Music] crossing san francisco bay the washington state ferries are moving bridges they connect the city of seattle with islands in puget sound the olympic peninsula and victoria canada unlike stationary bridges ferry boats are not potential barriers to river traffic and they help reduce traffic congestion on bridges bridges that cross heavily used waterways must be built high enough for all ships to clear or they must be built to move [Music] so [Music] so [Applause] so [Music] bridges are also used to transport water chemicals [Music] they support light rail systems and along with being useful they can add charm beauty and tranquility to a picturesque [Music] setting bridges have been the focal point of songs movies literature bridge building isn't limited to professionals at a weekend retreat boy scouts put up temporary bridges bridges of all types it's a learning experience and fun for participants [Music] [Applause] um [Music] structures a traveling exhibit sponsored by the franklin institute of science provides a hands-on approach to the scientific principles involved in bridge construction [Applause] bridges are everywhere everywhere you go there are bridges [Applause] bridges are so common that we often take them for granted they get our attention only when they become an inconvenience bridges are all around you beam arch suspension big and small plain and pretty and they're doing what bridges are supposed to do they're getting people and their belongings to the other side [Music] [Applause] [Music] [Applause] [Music] ah [Applause] [Music] [Applause] [Music] [Laughter] [Music] [Music] you

Channel: Steve Struwing

Views: 3,209

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Id: eHqubqJ7c-U

Length: 22min 5sec (1325 seconds)

Published: Tue Apr 07 2020