My Bridge

My Bridge

In order to construct a bridge that will be able to support a mass well over fifty times its own, a balsa wood bridge must be carefully crafted with supports in order to dissipate force throughout the bridge, not just in the middle sector where the two-by-four will rest. In order to do that, the bridge that I am building will have trusses, or framework that supports a bridge or other structure, that will equalize the tension created by the weights. The trusses dissipate the force so that the bridge shares the force equally, so it does not bend and break such as under this beam bridge.

The weight of the elephant causes tension on the bottom of the bridge, with the bridge trying to support the weight. But without trusses, the part of the bridge where the elephant is would not be able to support that weight and it would eventually break. 

This is a picture of the way forces dissipate in a truss bridge, such as the one I am constructing. The forces now can be shared throughout the entire bridge.

This is a basic design of the bridge that I am constructing. The tresses and abutments on the ends help equalize the forces so that the bridge does not bend and break as easily under high amounts of pressure.

Static Equilibrium

Static Equilibrium

When all forces on an object are balanced, then an object is said to be in a state of equilibrium. Objects that are in a state of equilibrium must have an acceleration of 0 M/S/S. This does not necessarily mean that the object is at rest, though. An object could be moving at a constant 75 MPH but still be in a state of equilibrium as it is not accelerating. This derives from Newton's first law, or: 

•  An object is in rest and stays at rest, or
• An object is in motion and stays in motion with constant speed and direction

If an object is at rest and in a state of equilibrium, then the object would be in a state of "static equilibrium" with static meaning stationary or at rest. Bridges use static equilibrium to balance their forces. Structures on the ends of bridges, or abutments, help equalize the forces on the bridge so that the bridge does not collapse. In an arched bridge, the abutments at the end of the arch take the brute of the force and help dissipate it throughout the bridge. Some bridges such as suspension bridges try to dissipate it through the ground with the help of anchorages, towers, and cables.  

Forces at work in an arched bridge.

Another example of static equilibrium are balancing toy birds. The birds have weights in their wings and beaks that help the forces spread out and balance. This helps them reach static equilibrium and helps them balance on almost all surfaces.

 

Bridges

Bridges

Bridges are structures that span over some sort of obstacle such as a body of water, a trench, or a valley in order to provide transportation over said obstacle. Certain bridges must be able to withstand certain conditions such as the terrain or weather in which the bridge is anchored. For instance, in San Francisco, bridges must be able to withstand the frequent earthquakes that occur as the city is placed on a major fault line.

The Golden Gate Bridge in San Francisco

There are many different types of bridges that have spanned throughout history such as the beam, truss, arch, suspension, and many others.

Beam Bridge

The beam bridge is constructed of horizontal beams which are supported by structures at each end. These components support the downward weight of the bridge and any traffic traveling upon it. In supporting weight though, the bridge endures compressional and tensional stress. To try and spread out the forces, sometimes a longer beam is used. Sometimes the pressure is too much though and a truss may need to be used.

These are both examples of beam bridges.

Truss Bridges

A truss bridge is a bridge whos lattice-like superstructure that bears the load of the force. The support truss adds rigidity to the structure, greatly increasing its ability to dissipate the compression and tension caused by weight on the bridge. Once the beam begins to compress, the force is spread through the truss. 

Arch Bridge

The arch bridge has been used for over 2,000 years. Its elegant structure is composed mainly of two abutments, or structures at the end of the bridge where the superstructure mainly rests, at the end of each side of the arch that take the main amount of force. The curve of the arch and abutments greatly dissipates the amount of tension on the underside of the bridge. 

The second picture shows how the tension dissipates throughout the bridge.

Suspension Bridge

Suspension bridges suspend the roadway in between two towers held by cables. The towers support much of the weight as weight pushes down on the roadway, is transferred to the cables and then to the towers. The towers then dissipate the force directly into the earth. The cables however, receive most of the bridges' tensional force. The cables are attatched to anchorages, or large slabs of rock or concrete in which the bridge is anchored to, and the tensional force travels through the cables, to the anchorages, and then is dissipated into the ground.

Bridges come in all shapes and sizes, but they all basically do the same job: support weight over a certain obstacle. In order to do that job, they must find a way to dissipate the forces so that they do not buckle and break.