1. Physics of Bridges
Mr. Hoa
STEM 2014

2. 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

3. 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

4. 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

5. Arch Bridge The arch bridge is one of the most natural bridges
It is also the best example of dissipation

6. 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

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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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