“Why do bridges collapse?”, is a common question asked by most. As humans, we are more prone to think about what could go wrong rather than why something does not go wrong. Yet, at the same time, neither of these questions may even cross our mind. We walk and drive across bridges almost every day without so much of a thought that we are even on a bridge. Why should we trust them and what do we need to take into account when building a stable bridge? The answer is actually quite simple: F1 + F2 = 0 and M1 + M2 = 0. However, we must also bear in mind how cost, materials, and design affect these equations.
Two simple equations can sum up why a bridge does not crumble as hundred of cars drive across it: F1 + F2 = 0 and M1 + M2 = 0, or, the sum of the forces and the sum of the moments (what keeps a bridge from toppling over) must add up to zero.(Evertson). Newton’s first law states: “An object in motion will stay in motion.” So, if something is not moving, the sum of the forces and moments are equal to zero (Evertson). The goal for a bridge is exactly this, for it to be stable and strong. In order for this to happen, all forces involved must not outweigh each other. If this happens, the bridge will not be able to hold the loads that travel across it, along with tension and compression, and will then fall or sway. However, it is not just these equations to focus on. The cost and materials used must be taken into account.
Building a bridge obviously costs money. The materials used will determine whether it will be a high or low price. For example, provided by Terron Evertson, a simple span bridge made of concrete will result in a high cost because the bridge must be really thick in order for it not to crack. Concrete takes a lot of energy and water to make which hurts the environment. The heaviness of the concrete will also negatively affect the environment and cause pollution (Mladjov). Because of this, many bridges are made out of steel. Steel is a cheaper and lighter metal than concrete. Steel is made more naturally or from recycled metal and its light weight is less harming to the surface of the earth (Mladjov). How the steel is designed to make a bridge can also determine the stableness of a bridge, not just the material itself. One of the strongest designs for a bridge is the truss bridge.
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The triangle is a very strong shape and is commonly used in architecture because of its strength. The truss bridge relies on triangles as it is an essential part to the design. In fact, the difference between all truss bridges is simply the type and placement of the triangles. The Pratt truss is made with only congruent triangles while the Paddleford truss includes many different types and sizes. Even though they have different triangles, they are both still very strong designs. With all bridges come the forces tension and compression. Using the example from earlier, a simple span bridge must handle both on the only the top and bottom surfaces, unable to handle any forces between these surfaces (Lamb and Morrissey). The adjacent, steel triangles in a truss almost evenly distribute these forces between them. That is why the truss is so strong. The forces are not fought in one or two places but all through the bridge, making the effort needed significantly less. This results in a strong, stable bridge.
We trust bridges without even thinking. There is a lot that goes into building the steady bridges we travel across frequently. Bridge architects and engineers have a lot of pressure in their field. They are in charge of building a structure that will carry thousands of people across it but they are able to do it with these equations: F1 + F2 = 0 and M1 + M2 = 0. Even though cost, materials, design, and forces are involved with these equations, a trustworthy bridge has been proven possible many times.
