My Bridge

The class was given 5 pieces of information:
1) The bridge must weigh less than 100 grams
2) The bridge must span a space of 16 inches
3) The bridge must be at least 3.5 inches wide
4) If the bridge is able to hold 120lbs, you will receive a 90%
5) The bridge can be made only out of balsa wood and adhesive

Based upon these criteria, I have chosen to build a Warren Truss Bridge. If, after I build this bridge, I have enough supplies to build a second or third bridge, I will. My bridge will be approximately 18 inches longs, 4 inches high, and 3.5 inches wide.

Here is an analysis of how the Warren truss distributes forces:

Each number represents a percent of the load applied to the bridge. My bridge will utilize equilateral triangles to spread the force out evenly across the bridge.

Bridges

We all depend on bridges. They are an easy way to get from point A to point B when there really was no path to cross at all before the bridge. Though there are many different kinds of bridges, including truss bridges, suspension bridges, and arch bridges, they all were made to accomplish the same goal - to spread out the forces (weight) acting upon it in such a manner that the bridge does not collapse or crumble. 

History of Bridges 

      The first bridges were made in Mesopotamia, the birthplace of modern society. These bridges were able to cover only short distances and became weaker with age due to environmental stress. Breakthroughs in bridge design first occurred in Ancient Rome. Roman engineers discovered that volcano rocks, if ground down, would become mortar. Mortar was much stronger than any other bridge 'glue' available at the time. Now bridges could become longer, carry more weight, and were becoming more efficient. The Romans also developed the arch bridge, a bridge that was able to hold a load of of its own weight. During the time of the Roman Empire, over 900  bridges were built. During the Middle Ages another critical development in bridges came about. They began to build bridges with living quarters on the bridge itself. Finally, in 1779 the first iron bridge was built. It was built by Abraham Darby and had a single span over 100 feet. As technology advanced, bridges became more and more efficient. 

Roman Arch Bridge 

Iron Bridge Built in 1779

Beam Bridges 

      A beam bridge, also known as a girder bridge, is one of the more simpler kinds of bridges. It is a flat, horizontal expanse supported by columns on either end, or throughout its length. These columns do all of the work in supporting the downward force applied by the weight of the bridge and the bridge's load. The columns push into the Earth and the Earth pushes back with an equal amount of force in order to keep the bridge from collapsing. Beam bridges utilize concrete or steel to build the vertical columns. The size and height of the column are directly related to the distance the bridge can span. When the height and size of the beam are increased, there is more space for the tension to spread out, allowing for a higher amount of tension before the beam snaps. 

Beam Bridge

Truss Bridges 

      Truss bridges are essentially beam bridges, but with added structural support. The trusses are built to create a bigger mass in which the tension can be spread throughout. There are many different kinds of trusses, including the Warren truss, the Pratt truss, and the Howe truss. A truss is essentially a latice that is used to disperse force. A famous example of a truss bridge is the Bollman Truss Bridge in Maryland. 

Bollman Truss Bridge,  Maryland

Arch Bridges 

      Arch bridges follow the same principle as a beam bridge. The arch directs the force into the Earth and the Earth pushes back, to keep the bridge from falling. As according to Newton's third law, every action has an equal and opposite reaction. 

Arch Bridge in the Japanese Tea Garden at Golden Gate Park, San Francisco 

Suspension Bridges  

      Suspension Bridges are the most recent development in bridge design. Suspension bridges spread the force out through hanging cables which in turn move the force into towers. These towers dissipate the force into the Earth. Suspension bridges may also have a truss support system in addition to the suspended cables; this is called the deck truss. It helps to prevent the roadway from swaying. A famous example of a suspension bridge is the Golden Gate Bridge in California.

Golden Gate Bridge, San Francisco

Static Equilibrium

Equilibrium is the point reached when all forces acting upon an object equal zero in all directions.
For instance, if there is 20 newtons acting upon a box from the right and 20 newtons acting upon a box from the left, that box is said to be in static equilibrium. Another example of static equilibrium would be a free body diagram.

These objects are in equilibrium because the net force in each direction is zero. 

In other words, equilibrium is the point at which an object is acting with an acceleration of zero. 

Static equilibrium has an added criteria. The object or system must be at rest; the object must be static. There are many objects in a state of static equilibrium. For example, a stationary table. Because this table has a constant 0 velocity (and therefore a 0 acceleration), it is not crumpling in on itself due to gravity, nor is it being pushed on the left or right, it is in a state of static equilibrium. The force of gravity is acting down upon the table, but as Newton's law states, every action has an opposite and equal reaction, meaning that the earth is pushing back up, mitigating the force applied downwards by gravity. Bridges are another example of static equilibrium. The bridge itself is unmoving, and if the bridge does not fail, it is in a state of static equilibrium. When a car drives along a bridge, a new force has been introduced to the bridge. The bridge dissipates this new force, so that it is constantly in a state of static equilibrium. Suspension bridges, arch bridges, and truss bridges all redistribute forces differently, but they all still remain in a state of static equilibrium.  

Suspension Bridge 

Above is an example of how a suspension bridge distributes force so that it maintains static equilibrium. Force is distributed into the columns and outwards into the far suspension cables. The Earth pushes up against the force downwards in each column, resulting in a net force of 0. The suspension cables, to the far left and right, push the force into the short columns where the Earth again pushes up, mitigating the force downwards. Because this bridge has a net force of 0, and it is stationary, or static, it is in a state of static equilibrium.