1. Bridges A bridge has a deck, and supports
Supports are what holds the bridge up
Forces exerted on a support are called reactions
Loads are the forces acting on the bridge

2. Bridges
A bridge is held up by the reactions exerted by its supports and the loads are the forces exerted by the weight of the object plus the bridge itself.

3. Beam Bridge Consider the following bridge The beam bridge
One of the simplest bridges

4. What are the forces acting on a beam bridge?
So what are the forces?
There is the weight of the bridge
The reaction from the supports

5. 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, then the supports will each have the same reaction

6. Forces on a beam bridge Lets try to add the forces
Horizontal forces (x-direction): there are none
Vertical forces (y-direction): the force from the supports and the weight of the bridge

7. Limitations
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

8. What is happening?
There are 2 more other forces to consider in a bridge.
Compression forces and Tension forces.
Compression is a force that acts to compress or shorten the thing it is acting on
Tension is a force that acts to expand or lengthen the thing it is acting on

9. There is compression at the top of the bridge and there is tension at the bottom of the bridge
The top portion ends up being shorter and the lower portion longer
A stiffer material will resist these forces and thus can support larger loads

10. Bridge Jargon
Buckling is what happens to a bridge when the compression forces overcome the bridge’s ability to handle compression. (crushing of a pop can)
Snapping is what happens to a bridge when the tension forces overcome the bridge’s ability to handle tension. (breaking of a rubber band)
Span is the length of the bridge

11. How can deal with these new forces?
If we were to dissipate the forces out, no one spot has to bear the brunt of the concentrated force.
In addition we can transfer the force from an area of weakness to an area of strength, or an area that is capable of handling the force

12. A natural form of dissipation
The arch bridge is one of the most natural bridges.
It is also the best example of dissipation

13. In a arch bridge, everything is under compression
It is the compression that actually holds the bridge up
In the picture below you can see how the compression is being dissipated all the way to the end of the bridge where eventually all the force gets transferred to the ground

14. Compression in a Arch Here is another look at the compression
The blue arrow here represents the weight of the section of the arch, as well as the weight above
The red arrows represent the compression

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

16. A Stronger 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
How does a truss help the bridge?
A truss adds rigidity to the beam, therefore, increasing it’s ability to dissipate the compression and tension forces

17. So what does a truss look like?
A truss is essentially a triangular structure.
Consider the following bridge (Silver Bridge, South Alouette River, Pitt Meadows BC )

18. Trusses
We can clearly see the triangular structure built on top of a basic beam bridge.
But how does the truss increase the ability to handle forces?
Remember a truss adds rigidity to the beam, therefore, increasing it’s ability to dissipate the compression and tension forces

19. Trusses
Lets take a look at a simple truss and how the forces are spread out