Suspension bridges "bridge the unbridgeable". In a suspension bridge, cables which are suspended via towers which support them hold up the road deck. The weight is transferred by the cables to the towers, which then in turn transfer the weight to the ground.
Cable-stayed bridges are similar to suspension bridges because cables support the roadway. Two or more towers have cables hanging from them. These cables support the roadway by transferring the weight to the towers. In turn, the towers transfer the weight to the ground. Construction of a cable-stayed bridge is similar to the construction of a regular suspension bridge, except on how the cables are positioned.
Materials and Construction
The amount of towers on suspension bridges can vary, but a suspension bridge MUST have at
least two towers. When possible, these towers are built on ground. However, there are
methods for making "floating towers" that are secure. Towers are usually built with hollow steel boxes, but some are built with concrete.
How do they make "floating towers"?
Floating towers are made by caissons. Caissons are hollow structures that are filled with concrete until they hit the ground. Gravity does the work on keeping the bridge up.
Here is one way that towers are built:
After the towers are completed, the cables must be put in place.
- The first portion of the tower is built. Hoists and temporary supports are built also, so a crane (called a jib crane, which lies on the beam that will be at the top of the bridge) can be lifted up and be used to complete the tower.
- The crane is lifted up to the highest level on top of the upper cross-beam.
- The crane puts the lower cross-beam in place on its way up.
- The crane builds the tower up to its final height.
- The upper cross-beam is finally laid in place.
How do they get the cables in place?
The air-spinning method
A method called air-spinning is sometimes used. Here are the steps in the air-spinning method:
- A continuous loop of rope is hung across the wheel, and two spinning wheels are
attached to that rope at both ends.
- Wire is looped on to the spinning wheel from its reel, and the other end is anchored.
- When a reel of cable is exhausted, the end of the first wire is joined to a new
reel of cabling.
- This process is repeated until the desired amount of strands has been spun. Then,
the strands are packed together, covered with wrapping wire, and painted. Clamps are
connected at certain intervals to carry the ropes that will suspend the deck.
The pre-assembled method
The pre-assembled method is simpler and easier to apply than the air-spun method. It
consists of pre-assembling the cables and then placing them on the towers. This is usually only
used for small bridges, but may totally replace the air-spinning method someday. All traffic
in the area must be kept clear of the cables to avoid accidents.
The road deck
A while ago, the common design for the road deck was a truss; an arrangement of horizontal
and vertical girders (steel bars, see photo), stiffened (or strengthened) by diagonal
bars. The truss design allows wind to pass through it. A truss design still takes lots
This design was replaced by the aerodynamic box-girder. The aerodynamic box-girder design
allows wind to pass around it. They use less steel, reducing weight and cost. Construction
and painting is therefore easier. In fact, much of the construction can be done at a
factory in large sections, making things a whole lot easier for everyone. (You can see an aerodynamic box-girder to the left.)
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