Types of Bridges Source:

Arch Bridge Introduction: The essence of an arch is that ideally there should be no tendency for it to bend, except under live loads. It should be purely in compression, and for that reason it can be made of materials such as, masonry, cast iron and concrete, that perform poorly in tension.

Arch Bridge Questions Why must an arch be curved? In order to balance the downward forces, the forces from the side must not be exactly in line: the angle between them, repeated throughout the arch, is the reason for the curvature. Why is a true arch bridge not convenient? Must have a support system as it is being built until the keystone is placed.

Arch Bridge Pics

keystone abutment

Arch Bridge Advantages: The entire arch is in compression. The compression is transferred into the abutments, and ultimately resisted by tension in the ground under the arch. The absence of tension in the arch means that it can sustain much greater spans than beams can achieve, and it can use materials that are not strong in tension, such as masonry and cast iron. Disadvantages: An arch cannot stand until it is complete.

Beam Bridge Introduction: Like the arch, the beam is in principle very simple. As with the arch, we see the spreading of the force in the ground, but this time the ground has to support only the weight of the beam: there is no horizontal thrust. The nature of the ground is therefore not quite as critical as it is for an arch.

Beam Bridge Questions Why build an arch bridge when you can build a beam bridge? The fact is that forces within a beam can be far greater than they are in an arch of the same weight and span. The reason is that the beam is pulled down at all points by its local weight, which tends to make it bend.

Beam Bridge Pics

Beam Bridge Advantages: Because a beam rests simply on the supports, the effects of thermal expansion and movements of the ground are fairly easily sustained. In principle, a beam may be built away from the final position, and lifted swiftly into place with minimal disruption of traffic or navigation. Disadvantages: This means that a beam contains forces which are much larger than the load, and it needs to be relatively massive

Cantilever Bridge Introduction: The cantilever is rather like a bracket, projecting out into space. The two forces almost always act in opposite directions. A cantilever is really a large bracket, held rigidly at one end. The largest cantilever bridges are made of steel, though medium sized ones are sometimes in pre-stressed concrete. Ancient ones in Asia were made of wood.

Cantilever Bridge Questions How many cantilevers do most bridges have? Almost all cantilever bridges have only two.

Cantilever Bridge Pics

Cantilever Bridge Advantages: Building out from each end enables construction to be done with little disruption to navigation below. The span can be greater than that of a simple beam, because a beam can be added to the cantilever arms. Cantilever bridges are very common over roads. Because the beam is resting simply on the arms, thermal expansion and ground movement are fairly simple to sustain. Disadvantage: Like beams, they maintain their shape by the opposition of large tensile and compressive forces, as well as shear, and are therefore relatively massive.

Cable-Stayed Bridge Introduction: The cable-stayed bridge is related to the cantilever bridge. The cables are in tension, and the deck is in compression. The spans can be constructed as cantilevers until they are joined at the centre. The cable-stayed bridge is related to the cantilever bridge. The cables are in tension, and the deck is in compression. The spans can be constructed as cantilevers until they are joined at the centre.

Cable-Stayed Bridge Pics

Cable-Stayed Bridge Advantages: The two halves may be cantilevered out from each side. There is no need for anchorages to sustain strong horizontal forces, because the spans are self- anchoring. They can be cheaper than suspension bridges for a given span. Many asymmetrical designs are possible. Disadvantages: In the longer sizes, the cantilevered halves are very susceptible to wind induced oscillation during construction. The cables require careful treatment to protect them from corrosion.

Suspension Bridge Introduction: With the deck high above the floppy cables, this looks unstable, and it is. This construction can be used only for spans that are short enough for a stiff deck to transmit lateral forces to the anchorages. The cable of a suspension bridge is in tension, enabling it to be much narrower and cheaper than an arch of the same span.

Suspension Bridge Question What is the function of the following parts of a suspension bridge? Towers, Anchorage, Hangers, Deck

The functions of the various parts are easy to understand. The functions of the various parts are easy to understand. The towers hold the cable up. The anchorages pull the cable outwards and downwards. The anchorages pull the cable outwards and downwards. The hangers connect the deck to the main cable. The hangers connect the deck to the main cable. And the deck is there to carry the traffic.

Suspension Bridge Pics

Suspension Bridge Advantages: The main sustaining members, the cables or chains, are purely in tension, and are not required to be rigid, so they can be only as thick as needed to resist the tension. The towers are almost purely in compression, so their design is relatively simple. Disadvantages: They are only as rigid as the deck structure, which in older structures was usually of truss construction. This makes them generally unsuitable for railway traffic.