The end is in sight, and things are starting to come together now; the pressure is indefinitely experienced by all TALONS learners. In this meeting with my mentor, he and I worked on was that I could show and deliver my learnings. After some brainstorming, we both decided that I should present the things I learned about types of bridges and the way they work. This meeting was more of about research and teachings rather than the previous meeting which was more about the model bridge and how to build it. So, let’s go through what I learned this weekend!
Before we start, we must identify what are the names of the bridges we see around us today. Well, there are six main types of bridges; Beam, Arch, Truss, Cantilever, Suspension, and Cable Stayed. In order to work, these bridges must be able to endure multiple forces such as gravity, the load, and weather but how do they withstand all this? They do it by carefully balancing two main kinds of forces called compression (a pushing or squeezing force, acting inward) and tension (a pulling or stretching force, acting outward), channeling the load onto the beams, joints, and towers.
A beam is the simplest (and often cheapest) kind of bridge: a deck, spanning a relatively short distance, held up by a pair of abutments (the vertical supports at either end). Putting pressure on the deck makes it flex downward in the middle, so it’s slightly longer underneath and slightly shorter on top. That tells us that the bottom of a beam is in tension (pulled longer than it would ordinarily be), while the top is in compression (squashed shorter). The load on a bridge like this is transmitted through the beam to the abutments at either end, which is also compressed (squashed downward). The longer the beam, the more likely it is to sag in the middle, which is why basic beam bridges are usually quite short
Arche are the only kinds or bridges supported entirely by forces of compression. A bridge deck resting on an arch, pushes down on the curve of stones (or metal components) underneath it, squashing them tightly together and effectively making them stronger. The load on a stone arch bridge is transmitted through the central stone (called the Keystone), around the curve of other stones, and into the abutments, where the solid ground on either side pushes back upward and inward. Like beam bridges, arches are relatively simple and cheap to construct and don’t need to block a road or river with central piers. They can easily exceed the span of a basic beam, though their big drawback is that they need large abutments, so they’re not always an efficient way of bridging something like a highway if a lot of clearance is needed underneath.
One way to extend the reach of a basic beam bridge is to reinforce it—and engineers have found the best way to do that is with a system of diagonal, triangular bars on the sides, which are called trusses. There are many ways of arranging trusses to support a bridge, giving a variety of intricate and often attractive lattice patterns. A typical truss bridge looks like a hollow box with open or closed vertical sides and roof, the sides reinforced with diagonal trusses, and the base resting on girders.
Two back-to-back beams extending outward from a pier can balance one another—just as a tightrope walker can balance by holding both arms straight out from her body. That’s the basic idea behind the cantilever bridge. Normally, when we talk about a cantilever, we mean a beam supported at only one end, like a diving board or see-saw only much more rigid. In a cantilever bridge, there’s usually a pair of cantilevers extending from each pier, with a short beam bridge in between, linking them together; alternatively, some have a cantilever extending out from each abutment toward the middle, with a beam bridging them. Cantilever bridges are sometimes hard to recognize because they’re typically reinforced with girders and trusses, but easier to spot if you remember that they have multiple sections and often have at least one pier in the middle.
If you need a bridge that spans even further, a suspension bridge of some kind is really your only option. The genius of a suspension bridge lies in using very tall piers with huge, curving main cables strung between them. Dozens of thinner vertical suspension cables of varying length hang down from the main cables and support the immense weight of the deck and the loads it carries. (And although people always notice the cables in a suspension bridge, they often fail to spot the girders and trusses reinforcing the deck underneath. This is a subtle and quite important point: most bridges are actually composites of two or more of the basic bridge types.)
A big drawback of suspension bridges is that they need to be anchored to the ground on either side. That’s not always possible if there isn’t room for the cables or appropriate bedrock to anchor them into. A different kind of suspension bridge, known as a cable-stayed bridge, does away with this by balancing two sets of suspension cables either side of each pier, which supports the load. In a “normal” suspension bridge, the deck hangs from cables of varying length that are themselves supported by the immensely strong main suspension cables. In a cable-stayed bridge, there’s only one set of cables that fan out, diagonally, from each pier to the bridge deck, which tends to be stronger and bulkier than in a suspension bridge.
This is just the tip of the iceberg, a little sneak-peek when it comes to the what I’ll be displaying on In-depth night. Let’s talk more about the meeting regarding Beautiful Mind questions.
One big idea that my mentor and I talked about a lot and was brought up this meeting was about the primary purpose of bridges in their certain locations. To a structural engineer, it’s like asking the for the meaning of life. It takes a lot of brainstorming and contemplation for a head engineer to decide which bridge should be used for the crossing. However, just as it takes a village to raise a child, it takes a great team of engineers and architects to develop a bridge. You have to look at the distance, the cost of materials at the time, the land, the things the bridge will cross over, and its general purpose. It’s all about location, location, and location.
Another concept to look at in structural engineering is accuracy. In a field like this, ballpark numbers get you nowhere; it’s all about getting the exact length, weight, cost, etc. This is why math plays the biggest role in bridge building.
Throughout this project, my mentor and I have just been problem-solving and finding alternatives to speed bumps that we face in our project. There was a time when I had to get stuff done, but my mentor was unable to meet that week, so I found an alternative and made a bridge on a software to add onto my project. With the cable on my suspension bridge, I used plastic tubing to solve the problem of extra movement on the bridge itself. The scaling of the bridge was also a pain because with trying not to exceed a certain length caused our bridge to have a small deck which means we couldn’t include all details we hoped to. Even though this project came with many problems, I was able to find a solution every time, and this is why alternatives have been working for me.
As for that, it’s time to finish things up.