Physics of BridgesMr. HoaSTEM 2014

Beam Bridge

One of the simplest bridges

What are the forces acting on a beam bridge?

• There is the weight of the bridge

• The reaction from the supports

Forces on a beam bridge

Here the red represents the weight of the bridge and the blue represents the reaction of the supports

Assuming the weight is in the center, the supports will each have the same reaction

• With all bridges, there is only a certain weight or load that the bridge can support

• This is due to the materials and the way the forces are acted upon the bridge

Arch Bridge

The arch bridge is one of the most natural bridges

It is also the best example of dissipation

• In a arch bridge, everything is under compression

• It is the compression that actually holds the bridge up

• In the picture above, you can see how the compression is being dissipated all the way to the end of the bridge where eventually all the forces get transferred to the ground

Arches

Here is one more look at the compression lines of an arch

Truss Bridge

Another way to increase the strength of a bridge is to add trusses

What are trusses?

• A truss is a rigid framework designed to support a structure

• A truss adds stability to the beam; therefore, increasing its ability to dissipate the compression and tension forces

A truss is essentially a triangular structure

Trusses

Let’s take a look at a simple truss and how the forces are spread out

Assume that all the triangles are equilateral triangles

There is a mixture of both compression and tension forces and these forces are spread out across the truss

• The truss can easily hold up weights, but there is a limitation

• Truss bridges are very heavy due to the massive amount of material involved in its construction

• Eventually the bridge would be so heavy, that most of the truss work is used to hold the bridge up instead of the load

Suspension Bridge

Due to the limitations of the truss bridge type, another bridge type is needed for long spans

A suspension bridge can withstand long spans as well as a fairly decent load

• A suspension bridge uses the tension of cables to hold up a load. The cables are kept under tension with the use of anchors that are held firmly to the Earth

How suspension bridge works

The deck is suspended from the cables and the compression forces from the weight of the deck are transferred to the towers. Because the towers are firmly in the Earth, the forces gets dissipated into the ground

The support cables that are connected to the anchors experience tension. The cables stretch due to the weight of the bridge as well as the load it carries

Cable Stayed Bridge

A cable stayed bridge is a variation of the suspension bridge

Like the suspension bridge, the cable stayed bridge uses cables to hold the bridge and loads up

Forces in a cable stayed

A cable stayed bridge uses the cable to hold up the deck

The tension forces in the cable are transferred to the towers where the tension forces become compression forces

Limitations

With all cable type bridges, the cable must be kept from corrosion (rust)

If the bridge wants to be longer, the towers must also be higher. This can be dangerous in construction as well as during windy conditions

“The bridge is only as good as the cable”… If the cable snaps, the bridge fails