Arc Bridge
The first major long - lasting bridges were made by romans more than two thousand years ago when they pioneered the masonry arch bridge. Arches work by transferring the load through the arch to the supporting foundation via the abutments. As the load tries to straighten out the arch, the outward movement is resisted by the abutments and the downward force is transferred to the foundation.
A significant advantage of the masonry arch bridge is that the length of the span is not limited by the size of the individual components, as was the case with early beam bridges. The romans produced a semi circular arch that spanned 50 meters, a considerable span even by today's standards. By using stone the romans avoided many of the shortcomings of Tiber. it had vastly superior weather resistance and wearing characteristics. it had excellent compressive strength and was fire resistant. Even the fact that it was a heavy material was an advantage in holding the arch together. Many roman built arch bridges still stand today, testimony to the durability of the material and the skill of the bridge builders.
The main component of the roman arch are voussoirs, tapered blocks of stone or brick masonry.
The main component of the roman arch are voussoirs, tapered blocks of stone or brick masonry.
The voussoirs were laid on top of each other to form the arch with the weight of each block bearing down on the previous block. A form of mortar was used to hold the blocks together, although this was not necessary for a well constructed arch bridge as the weight of the bridge pushed the block together. In all arch bridges the components of the arch are in compression.
In an arch bridge, the longer the span the higher the arch, witch presents a problem for traffic. the early solution to this problem was to make a number of smaller arches. However, this created other problems. Because the arch was made from masonry, the piers supporting the downward and outward forces had to be very large. the piers of a high roman style arch were usually about one-third the sixe of the span and restricted the smooth flow of water bellow the bridge.
The designers of the original London Bridge, built across the Thames river in 1176, still had not overcome this problem. during times of high tidal flow there was a 1.5 m difference in water level on either side of the London bridge due to the number and size of the piers.
Another draw back of the early arch bridge was that it couldn't be built out from two sides the way a cantilever bridge could. the arch needed to be fully supported during construction until it was ready to take its own weight. the romans would construct a coffer dam to divert part of the river to allow the arch and its piers to be constructed one arch at a time.
little changed in arch design until the latter stages of the European renaissance in the 15th century. During the industrial revolution in the 18th century, techniques were developed that allowed the arch to be much flatter. An example is the Perronet arch, which uses slender piers and low arches. this enabled greater bridge spans without and increase in height. Understanding the importance of building the piers on a foundation of solid rock and a greater Knowledge of the outward forces produced by the arch enabled the piers to be reduced considerably in size to about one tenth of the span size.
In an arch bridge, the longer the span the higher the arch, witch presents a problem for traffic. the early solution to this problem was to make a number of smaller arches. However, this created other problems. Because the arch was made from masonry, the piers supporting the downward and outward forces had to be very large. the piers of a high roman style arch were usually about one-third the sixe of the span and restricted the smooth flow of water bellow the bridge.
The designers of the original London Bridge, built across the Thames river in 1176, still had not overcome this problem. during times of high tidal flow there was a 1.5 m difference in water level on either side of the London bridge due to the number and size of the piers.
Another draw back of the early arch bridge was that it couldn't be built out from two sides the way a cantilever bridge could. the arch needed to be fully supported during construction until it was ready to take its own weight. the romans would construct a coffer dam to divert part of the river to allow the arch and its piers to be constructed one arch at a time.
little changed in arch design until the latter stages of the European renaissance in the 15th century. During the industrial revolution in the 18th century, techniques were developed that allowed the arch to be much flatter. An example is the Perronet arch, which uses slender piers and low arches. this enabled greater bridge spans without and increase in height. Understanding the importance of building the piers on a foundation of solid rock and a greater Knowledge of the outward forces produced by the arch enabled the piers to be reduced considerably in size to about one tenth of the span size.
the late 18th century saw a significant development in bridge building. the first all metal bridge was built over the river severn at coalbrookdale in Engleand. this bridge had a 33 m cast iron span and was based on an arch design. cast iron is an alloy of iron and carbon.
Cast iron was quickly superseded by wrought iron and later by steel. Wrought iron has three times the tensile strength of cast iron. this material development led many bridge builder away from the traditional arch towards other forms of bridge design.
The arch bridges that were built during the second half of the 19th century were often constructed in a traditional manner but by using concrete instead of stone. A small shallow arched bridge was built in 1869 in France. this pedestrian bridge has a span of only 13 m but is considered to be the first bridge to use reinforced concrete.
the arch is always in compression, whether you are using masonry materials, cast iron, more modern steel trusses or contemporary prestressed reinforced concrete to make the arch. This is also true whether the roadways hangs bellow the arch, as with the Sydney harbor bridge.
The arch bridges that were built during the second half of the 19th century were often constructed in a traditional manner but by using concrete instead of stone. A small shallow arched bridge was built in 1869 in France. this pedestrian bridge has a span of only 13 m but is considered to be the first bridge to use reinforced concrete.
the arch is always in compression, whether you are using masonry materials, cast iron, more modern steel trusses or contemporary prestressed reinforced concrete to make the arch. This is also true whether the roadways hangs bellow the arch, as with the Sydney harbor bridge.