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Bridges can be basic structures or massive, awe-inspiring pieces of art – or everything in between. A bridge serves a single purpose as long as it transports us across a gap that would otherwise be difficult (or even impossible) to cross. Bridges have played a significant part in developing our earliest civilizations, the diffusion of knowledge, local and global trade, and the emergence of transportation during the previous several thousand years.
Initially made out of most simple materials and designs, bridges soon evolved and enabled the carrying of wide deckings and spanning of large distances over rivers, gorges, inaccessible terrain, enormously elevated surfaces and pre-built city infrastructures. Starting with the 13th century BC Greek Bronze Age, arched stone bridges quickly spread worldwide, eventually leading to the rise of steel, iron and other materials in bridges that can span kilometres.
Bridges can vary significantly in appearance, carrying capacity, structural features, movable parts, construction materials, and more to serve varied tasks, transport different types of weight, and traverse terrains of varying sizes and complexities.
Types of Bridges
The different types of bridges are –
Arch Bridge – More than 1,000 stone arch bridges were erected by the Romans, some of which are still standing today, such as the Pont-Saint-Martin bridge in Italy’s Aosta Valley (built in the first century BCE). The design of this bridge stretches back over 3,000 years. Modern arch bridges are now also made of concrete.
The dead load of a bridge is the weight of the bridge itself, plus the weight of whatever it is carrying (the live load). An arch bridge is held aloft by the forces of load and gravity, which would otherwise send a bridge tumbling downhill.An arch bridge works by conveying the downward pressure of gravity inward to the centre of the structure — toward a central stone called the keystone — rather than straight down. This principle is called compression, and it enables the arch below to support the surface, or deck, above it. Temperature fluctuations can destabilize fixed arch bridges, so the arch design is sometimes modified with hinges at each base and even the centre of the span. This allows longer arch bridges to adjust to material expansion and contraction when temperatures fluctuate dramatically.
Beam Bridge – One of the most basic types of the bridge is a beam bridge. A simple wood bridge is an excellent example, which you can come upon when out on a country walk. Traditionally, the deck area comprises wood planks or stone slabs (often referred to as a clapper bridge). Two beams run between abutments/piers on either side to support them. Other beams are frequently found in between the main beams, providing additional support and stability. A simple decking will be positioned vertically across the underlying beams and over which people or vehicles will ride. A “simply supported” structure is a term used to describe this type of building. There is no stress transmission, as seen in arch constructions and other bridge designs.
Truss Bridges – It’s a popular bridge design that distributes pressures throughout practically the whole bridge structure using a diagonal mesh of most often triangle-shaped posts above the bridge. Individual elements of this construction (typically straight beams) can withstand dynamic tension and compression stresses, but by dispersing those loads across the entire structure, the complete bridge can withstand significantly stronger forces and higher loads than other bridge designs. The king posts (two diagonal posts supported by a single vertical post in the centre) and queen posts are the two most popular truss types (two diagonal posts, two upright posts and a horizontal post that connect two vertical posts at the top). Allan, Bailey, Baltimore, Bollman, Bowstring, Brown, Howe, Lattice, Lenticular, Pennsylvania, Pratt, and other truss types are in use.
Cantilever Bridge – The first cantilever bridge was considered a tremendous engineering achievement when it was built. Cantilevers, which might be simple beams or trusses, are used to support the bridge. When utilised to accommodate traffic, they are composed of prestressed concrete or structural steel. Considering that the horizontal beams that make up the cantilever arm are only supported on one side sounds a little risky. However, the two cantilever arms are joined by a suspended span, which is essentially a centre with no direct support underneath. Rather than traditional vertical bracing, the bridge weight is sustained by diagonal bracing using horizontal beams.
Suspension Bridge – Because of the magnificent Golden Gate Bridge in San Francisco, a suspension bridge is possibly the most known bridge type in the United States. The twin 746-foot orange towers and sweeping Art Deco impact, built in 1937, attract more than 10 million people each year. Suspension bridges are exactly what they sound like: they’re supported by vertical pillars or pylons linked by suspension cables. More diminutive, vertical suspenders are attached to these main cables and use tension to hold the bridge deck up. Tension is the main force that sustains suspension bridges. Even though the original suspension bridges were composed of simple ropes supporting wooden planks, the suspension technique now allows vast spans across wide canals. However, these bridges are only attached to the ground in a few locations (the towers or pillars). They might sway in the wind or shake when heavy traffic passes beneath them. Wind or movement across a bridge can produce vibrations. When these reach a specific frequency, resonance occurs, the exact mechanism that causes the glass to shatter when a trained vocalist hits a high enough note. Bridge crossings can be disrupted and collapsed if vibrations are strong enough.
Torsion, a twisting force commonly generated by external variables such as wind, can also impact suspension bridges, causing unsafe movement. Travellers can be thrown off a bridge if the surface twists significantly while they are on it. While torsion causes tension in a vertical plane, shear causes stress in a horizontal plane. It occurs when negative environmental pressures act on a single, fixed bridge component, causing it to break like a stick between two hands.
Cable-Stayed Bridge – A suspension bridge with a cable-stayed bridge connects the crossbeam or bridge deck directly to pillars or towers. There is no main cable, only a slew of vertical suspenders attached to the tower’s summit. These suspenders use tension to keep the bridge deck solid and in place. Sweden’s Strömsund Bridge is regarded as the world’s first modern cable-stayed bridge. In 1956, the three-span construction was completed. The steel and concrete deck is suspended from two pylons by diagonal cables. The glass-decked Sundial Bridge, built in 2004 across the Sacramento River in Redding, California, uses the cable-stay technique in conjunction with elements of cantilever and suspension. The famed Brooklyn Bridge, which opened in 1883, is a hybrid cable-stayed and suspension bridge.
Tied Arch Bridge – The tied-arch bridge is intriguing, with a metal arch structure supported by vertical ties between the arch and the deck. A bottom chord connects the tips of the arch construction. This works in the same way that a bowstring does. The vertical ties convert the downward pressure from the arch structure to the bridge deck into tension. Many people believe that the abutments are responsible for keeping the linked arch bridge and arch construction. The decking/strengthened chord, on the other hand, is what binds the tips of each end of the arch together.
When you consider the various bridges and how they function, you’ll discover a whole new world of design engineering. Many of us think an aesthetic aspect of a modern bridge is frequently an essential design component. These characteristics can aid in the management of tension and stress in a variety of ways. It’s also fascinating to watch how different bridge designs are appropriate for various terrains. For generations, these basic designs speak something about their viability, longevity, and safety.
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