15. Structures: Compression and Tension in Different Types of Bridges

Arch Bridge

- great natural strength because instead of pushing straight down the load of an arch bridge is carried outward along the curve of the arch to the supports at each end - supports carry the load and keep the ends of the bridge from spreading out - the load at the top of the key stone (the stone in the middle) makes each stone on the arch press on the one next to it until the end supports - the ground around the supports is squeezed and pushes back on the supports so resistance is created that is passed until it pushes on the key stone which is supporting the load

Truss Bridge (Girder Bridge)

A beam bridge with extra support for the main beam as the trusses increase the structural strength of the bridge so it can span longer distances. Downward force is not pushing in the middle of the beam but the force is distributed across the two different sticks in the truss.

Suspension Bridge

A bridge where cables (ropes or chains) are strung across the river (or another obstacle) and the deck is suspended from these cables - suspension bridges can span distances longer than any other kind of bridges (ideal for busy waterways) - the most expensive of bridges

Structure

A framework of connected parts that is built or constructed.

Tension

A pulling force that makes things longer by stretching the material .

Compression

A pushing (compressing) force that makes things shorter by squeezing of the material.

What is a truss?

A triangle-shaped frame that helps spread the compression and tension forces around.

Bridge

An elevated structure designed to support the movement of objects over a span (distance the bridge covers).

Why do bridges fail?

Because they have faulty parts, are used in ways that exceed what was intended by the design or poor design. But the most common reason is because of poor maintenance.

What are some examples of natural structures?

Biological organisms, minerals, chemicals, bird's nest...

What are some examples of man-made objects?

Buildings, bridges, machines...

How are bridges kept stable?

By balancing the load forces with the supporting forces of the structure

Tension and Compression in a Truss Bridge

Compression pushes along the trusses, while tension pulls it apart to the piers at the sides.

Compression and Tension in a Suspension Bridge

Compression: The force of compression pushes down on the suspension bridge's deck, but because it is a suspended roadway the cables transfer the compression to the towers which dissipate (loss of energy) the compression directly into the earth where they are firmly entrenched. Tension: The supporting cables, running between the two anchorages (supports on the sides), are the recipients of the tension forces. The cables are literally stretched from the weight of the bridge and its traffic as they run from anchorage to anchorage. The anchorages are also under tension, but since they, like the towers, are held firmly to the ground the tension they experience is dissipated.

How do you ensure that a bridge is strong?

Make sure that not all weight is in one place.

What are the two different types of material structures?

Man-made objects and natural structures.

How do tension and compression work?

Tension and compression are internal forces inside a material where the molecules are trying to get back to their original kind of equilibrium spacing: squashed parts push away from one another to get back to their spacing, and stretched out parts pull on each other to their original spacing.

Beam Bridge

The simplest type of bridge. It is made from a rigid, straight structure resting on supports at either end. - the further apart its supports the weaker a beam bridge gets - not used for great distances - balances the load by concentrating it entirely onto the two piers that support the bridge (or piers distributed throughout the span) - force pushes -> beam bends, the amount of bend depends on the length of the beam - cheap and simple

What weight do bridges have to support?

Their own weight (dead load) and the weight of those objects that will cross over them or other forces that will act on them from time to time, such as wind, snow or ice (live load).

Compression and Tension in a Beam Bridge

Under load the beam's top surface is pushed down or compressed while the bottom edge is stretched or placed under tension.

Compression and Tension in an Arch Bridge

compression: - always under compression as the force of compression is pushed outward along the curve of the arch towards the end supports tension: - negligible as the natural curve of the arch makes it possible to dissipate the force outward which greatly reduces the effects of tension - the greater the semicircle -> the greater the effect of tension on it

What are some materials that are commonly used in bridges?

wood, rope, aluminium, concrete, steel