We know that bridges play an important part in our daily lives. We know they are essential components of cities and the roadways between populations of people. Some bridges are simple and straightforward; others are amazingly complex. What are some bridges that you know that might be called simple bridges? (Possible answers: Log over a creek, bridges over streams.) What are some bridges you know that might be considered more complicated? (Possible answers: Golden Gate Bridge, other large bridges, bridges that carry both highway traffic and train traffic.) What makes some bridges simple and other complex? (Possible answers: Their size, multiple purposes, environmental conditions, environmental forces, material maintenance requirements, etc.)
It is not easy to create a bridge the size of the Sky Gate Bridge. Have you ever wondered how engineers actually go about designing an entire bridge? Bridges are often designed one piece at a time. Each pier (columns) and girder (beams) has to meet certain criteria for the success of the whole bridge. Structural engineers go through several steps before even coming up with ideas for their final designs.
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For designing safe bridge structures, the engineering design process includes the following steps: 1) developing a complete understanding of the problem, 2) determining potential bridge loads, 3) combining these loads to determine the highest potential load, and 4) computing mathematical relationships to determine the how much of a particular material is needed to resist the highest load.
One of the most important steps in the design process is to understand the problem. Otherwise, the hard work of the design might turn out to be a waste. In designing a bridge, for instance, if the engineering design team does not understand the purpose of the bridge, then their design could be completely irrelevant to solving the problem. If they are told to design a bridge to cross a river, without knowing more, they could design the bridge for a train. But, if the bridge was supposed to be for only pedestrians and bicyclists, it would likely be grossly over-designed and unnecessarily expensive (or vice versa). So, for a design to be suitable, efficient and economical, the design team must first fully understand the problem before taking any action.
Determining the potential loads or forces that are anticipated to act on a bridge is related to the bridge location and purpose. Engineers consider three main types of loads: dead loads, live loads and environmental loads:
Values for these loads are dependent on the use and location of the bridge. Examples: The columns and beams of a multi-level bridge designed for trains, vehicles and pedestrians should be able to withstand the combined load all three bridge uses at the same time. The snow load anticipated for a bridge in Colorado would be much higher than that one in Georgia. A bridge in South Carolina should be designed to withstand earthquake loads and hurricane wind loads, while the same bridge in Nebraska should be designed for tornado wind loads.
During bridge design, combining the loads for a particular bridge is an important step. Engineers use several methods to accomplish this task. The two most popular methods are the UBC and ASCE methods.
Every beam shape has its own cross sectional area calculations. Most beams actually have rectangular cross sections in reinforced concrete buildings, but the best cross-section design is an I-shaped beam for one direction of bending (up and down). For two directions of movement, a box, or hollow rectangular beam, works well (see Figure 3).
What are examples of load types? (Possible answers: Vehicles, people, snow, rain, wind, the weight of the bridge and its railings and signs, etc.) Why would the loads make a difference in how an engineer designed a bridge? (Answer: Engineers must figure out all of the loads that might affect bridges before they design them.) If you were an engineer, how would you go about designing a bridge to make sure it was safe? (Discussion points: First, fully understand the problem to be solved with the bridge, its requirements and purpose. Then figure out all the possible types of loads [forces] that the bridge might need to withstand. Then calculate the highest possible load the bridge might have to withstand at one time. Then figure out the amount of construction material required that can resist that projected load.)
Pairs Drawing: Divide the class into teams of three students each. Have each engineering team sketch a bridge to carry a train across a river that is 100-meters wide. Have them describe the type of bridge and where the compressive and tensile forces are acting on it.
Complete the Design/Presentation: Have student teams return to their bridge design from the pre-lesson assessment and think about the potential loads on their bridge, given the just-discussed engineering design process steps. Have them draw in the loads and the direction that they would act on the bridge. What do they think the highest load combination would be (how many of these loads could actually happen at the same time)? Then, ask for one or two engineering teams to volunteer to present the details of their bridge design to the class.
Human Bridge: Have students use themselves as the raw construction material to create a bridge that spans the classroom and is strong enough that a cat could walk across it. Encourage them to be creative and design it however they want, with the requirement that each person must be in direct contact with another class member. How many places can you identify tension and compression? How would you change the design if the human bridge had to be strong enough for a child to walk across it? What other loads might act upon your bridge?
Have the class participate in the yearly West Point Bridge Design Contest. Access excellent and free downloadable bridge design software and other educational resources at the US Military Academy at West Point website: bridgecontest.usma.edu/
Students take a hands-on look at the design of bridge piers (columns). They determine the maximum possible load for that scenario, and calculate the cross-sectional area of a column designed to support that load.
Students learn about the variety of materials used by engineers in the design and construction of modern bridges. They also find out about the material properties important to bridge construction and consider the advantages and disadvantages of steel and concrete as common bridge-building materials ...
A cable-stayed bridge is similar to a suspension bridge. However, the deck hangs directly from the piers on cables. The piers are in compression and the cables are in tension. The deck experiences both forces.
A truss bridge is a variation of a beam structure with enhanced reinforcements. The deck is in tension. The trusses handle both tension and comprehension, with the diagonal ones in tension and the vertical ones in compression.
While this seems simple enough, this bridge must deal with unique tension and compression issues. It leverages features of suspension and cable-stayed designs that are pushed (and stretched) to extreme limits when the bridge is in motion. This structure adds a new dimension to standard bridge engineering.
Bridge design is simple and complex at the same time. A bridge is constantly balancing compressive forces in certain locations with tensile ones in others so no overwhelming force, especially gravity, overcomes the structure at any time, leading to damage or collapse.
It would be easy to build bridges if the loads on them stayed static. The forces on them would never change. The reality is that the loads can vary dramatically and dynamically throughout the day and over time.
Traffic and construction accidents, boats hitting abutments, and explosions can lead to significant bridge stress and sometimes, failure. Builders can leverage strong, fire-retardant materials and isolating elements to limit the impact extreme events have on the balance of forces affecting a bridge.
What allows an arch bridge to span greater distances than a beam bridge, or a suspension bridge to stretch over a distance seven times that of an arch bridge? The answer lies in how each bridge type deals with the important forces of compression and tension.
Tension: What happens to a rope during a game of tug-of-war? It undergoes tension from the two sweaty opposing teams pulling on it. This force also acts on bridge structures, resulting in tensional stress.
Compression and tension are present in all bridges, and as illustrated, they are both capable of damaging part of the bridge as varying load weights and other forces act on the structure. It's the job of the bridge design to handle these forces without buckling or snapping.
The best way to deal with these powerful forces is to either dissipate them or transfer them. With dissipation, the design allows the force to be spread out evenly over a greater area, so that no one spot bears the concentrated brunt of it. It's the difference in, say, eating one chocolate cupcake every day for a week and eating seven cupcakes in a single afternoon.
In transferring force, a design moves stress from an area of weakness to an area of strength. As we'll dig into on the upcoming pages, different bridges prefer to handle these stressors in different ways.
When engineers build structures, they want to make sure that the structure can bear weight. In other words, they do not want the structure to fall down when a force is applied to it. For example, bridges must be able to hold up the materials that make the bridge, as well as all of the traffic travelling across it. One of the shapes that can bear weight very well is the triangle. But what makes triangles so good at this?
There are several different types of trusses used in bridge design. The type of truss depends on how the horizontal and diagonal beams are arranged. There are four main styles of trusses used to make bridges. 589ccfa754
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