There are many different types of bridges although typically their structures can be traced back to one of the seven main types. It is the way in which the vertical/horizontal stresses are managed which dictates the structure of different bridges. In some cases the deck area will be the load-bearing element while in others it will be the towers. There are also designs that transmit tension through bridge cables which allow a degree of flexibility for different terrains.
One interesting factor when looking at different bridge designs is their longevity and the fact they have been around for centuries. Many of the world’s greatest engineers have failed to add any significant improvements to the basic load-bearing designs of years gone by. We will now take a look at the different types of bridges and how they work.
A beam bridge is one of the simplest types of bridge. A perfect example being a basic log bridge – something you may see while out on a country walk.
The deck area traditionally consists of wood plank or stone slabs (often referred to as a clapper bridge). These are supported either side by two beams running between abutments/piers.
Very often you will find other beams, positioned in between the main beams, offering additional support and stability.
The area over which people or vehicles travel will be a simple decking positioned vertically across the underlying beams. This is often referred to as a “simply supported” structure. There is no transfer of stress which you see in arch structures and other types of bridges.
The truss bridge has been around for literally centuries and is a load-bearing structure which incorporates a truss in a highly efficient yet very simple design. You will notice an array of different variations of the simple truss bridge but they all incorporate triangular sections. The role of these triangular elements is important because they effectively absorb tension and compression to create a stressed structure able to accommodate dynamic loads. This mixture of tension and compression ensures the structure of the bridge is maintained and the decking area remains uncompromised even in relatively strong winds.
When the first cantilever bridge was designed it was seen as a major engineering breakthrough. The bridge works by using cantilevers which may be simple beams or trusses. They are made from pre-stressed concrete or structural steel when used to accommodate traffic. When you consider that the horizontal beams making up the cantilever arm are only supported from one side it does begin to sound a little dangerous. However, the two cantilever arms are connected by what is known as the “suspended span” which is effectively a centrepiece which has no direct support underneath. The bridge load is supported through diagonal bracing with horizontal beams as opposed to typical vertical bracing. Extremely safe and very secure, the design of cantilever bridges is one which still lives on today.
There are many different types of arch bridge but they all have central elements in common. Each bridge has abutments, which are used to support the curved arch structure under the bridge. The most common type of arch bridge is a viaduct, a long bridge made up of many arches. The lateral pressure created by the arch span is transferred into the supporting abutments. It is therefore essential that these parts of the bridge remain solid, intact and well founded. You will see many arch bridges with decorative brickwork which is an integral part of the design. Simple yet so very effective an arch bridge can carry everything from pedestrians to heavy rail.
Tied Arch Bridges
The tied arch bridge is a fascinating design which incorporates an arch structure (usually metal) supported by vertical ties between the arch and the deck. The tips of the arch structure are connected by a bottom chord. This acts in a similar fashion to the string of a bow. The downward pressure from the arch structure to the deck of the bridge is translated into tension by the vertical ties. Many people assume that the abutments ensure that the tied arch bridge and arch structure stay in place. However it is the decking/strengthened chord which connects the tips of each end of the arch together. The best example of this is a bowstring which absorbs pressure, keeping both sides of the bow in contact, until it eventually flattens out.
The structure of a stereotypical suspension bridge looks very simple but the design is extremely effective. The deck of the suspension bridge is the load-bearing element of the structure. This is held in place by vertical suspenders which support the cables. The suspension cables extend out beyond each side of the bridge and are anchored firmly into the ground. It will depend upon the size of the bridge but a number of towers will be installed to hold up the suspension cables. Any load applied to the bridge is transformed into tension across the suspension cables which are the integral part of the structure. As there is some “give” in the suspension cables this can translate into slight, but measured, bridge movement in difficult weather conditions.
A cable stayed bridge is dependent upon towers/pylons which are the load-bearing element of the structure. Cables are connected from the pylons to the deck below. Either directly from the top of the tower or at different points of the column. When connected at different points of the column this creates a fan like pattern. This is the feature many people associate with cable stayed bridges. This type of structure tends to be used for distances greater than those achieved with a cantilever bridge design but less than a suspension bridge. One of the main issues with this type of bridge is that the central connection of the cables can place horizontal pressure on the deck. Therefore, the deck structure needs to be reinforced to withstand these ongoing pressures.
What is the strongest type of bridge and why?
Even though the truss bridge design has been around for literally centuries it is widely regarded as the strongest type of bridge. The design itself looks extremely simple, so what makes it the strongest type of bridge and why?
This is a load-bearing bridge which consists of an array of triangular structures. Interestingly, the triangular beam structures are pinned in place rather than rigidly connected which is important when spreading the load. The vibrations caused by traffic moving over the bridge or even weather conditions are not isolated; instead they are spread right across the bridge structure, moving between triangular sections. As the load is spread right across the bridge this also increases overall stability and reduces flexing.
What is the most expensive type of bridge to build?
If you look at the vast majority of expensive bridges you will see a pattern, they tend to be suspension bridges. So, the answer to the question, what is the most expensive type of bridge to build is simple, a suspension bridge!
There are a number of reasons why they tend to be so expensive. Firstly they offer the ability to span huge distances (up to 7000 feet) – a span which is out of the reach of other bridge designs. The size of the towers, materials used and the installation of what is known as a deck truss beneath the bridge deck all add to the significant costs. We have come a long way from the first suspension bridges which were apparently made of twisted grass. The cost of today’s larger suspension bridges will regularly exceed $1 billion!
When you look at the different types of bridges and how they work, it opens up a whole new area of design engineering. What many of us assume to be an aesthetic feature of a modern day bridge is often an integral part of the design. These features often help control tension and stress in a variety of ways. It is also interesting to see that different bridge designs are suitable for different terrains. The fact that many of these basic designs go back centuries says everything about their viability, durability and safety.
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