Large constructions: Bridges

This town lies on two sides of a river. The only way to cross the river right now is by boat. As the city grows, there are more and more people who live on one side of the river, but who work, shop or study on the other side. So the city authorities decide it’s time to build a bridge, to make travelling across the river easier for the citizens. They hire a team of engineers for the job.

Building a bridge usually takes a lot of time, money and resources, so there are many things the engineers need to consider before they can start building. They need to think of how long the bridge needs to be to join the two river banks; how many people are going to travel back and forth; and what modes of transport people will use to cross the bridge: will they travel by car, by bus, by train, or should the bridge accommodate all of these options? Getting answers to these questions helps the engineers better understand the loads the bridge will need to support and withstand. The engineers also need to consider the geographic conditions in the area. Are earthquakes or hurricanes common here?

Does the water level change during certain seasons? What type of land and soil is there? They also need to know how much the city wants to spend, so they can plan their budget. All these different factors affect the design of the bridge. Once they know the answers to these questions, the engineers can start thinking about possible solutions.

In the next step, the engineers take the data they’ve collected and analyse it in terms of forces. There are two types of forces that are always present inside a bridge. One is a “squeezing” force that pushes parts of the bridge together. This is compression. The other one is the “stretching” force that pulls the parts outwards.

This is tension. Different structures and materials have different properties when it comes to tension and compression. Too much tension or compression can cause parts of the bridge to bend or crack. The bridge might collapse as a result! So the engineers need to calculate these forces carefully and plan what type of bridge, and which materials, would be best.

They also need to determine what supports to use and where to place them, so that forces are best distributed and balanced, making the bridge sturdy and strong. The bigger and sturdier the bridge, the more expensive it is to build. So, to save money, resources, and effort, bridges are designed to be only as strong as necessary. In this town, the engineers have come up with several possible solutions. They have involved architects to make the bridge look good, tested the different solutions using virtual and physical models, and estimated the costs.

Now, they consider the pros and cons of each solution, and finally choose the best project to present to the city authorities. They have decided to build a suspension bridge with long vertical cables that support the whole structure. This bridge is relatively easy to build and maintain, and strong enough to withstand the predicted load. Once the city approves the design and the budget, construction can begin. Depending on the size and complexity, constructing a bridge can take from a couple of weeks for simple over-road bridges for pedestrians, to years, for building a large suspension bridge over water.

This particular bridge should be finished before next summer. The locals in town are already looking forward to it!