Physics of Bridges Mr. Hoa STEM 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 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