Snapping is what happens when tension surpasses an object's ability to handle the lengthening force. 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. Sign up for our Newsletter!
Mobile Newsletter banner close. Mobile Newsletter chat close. They are one of the oldest types of bridges and have extraordinary natural strength. Instead of pushing straight down as beam bridges do, the weight of the arch bridge and any additional load on the bridge is carried outward along the curve of the arch to the supports at each end. These supports are called abutments. Abutments distribute the load from the bridge and keep the ends of the bridge from spreading out.
The Romans were masters of the arch bridge. Many of their arch bridges used little or no mortar, or "glue," to hold the stones together. The goal of an arch bridge is to carry all loads in compression, without any tensile loads present. The stones in the structures stay together by the sheer force of their own weight and the compression transferred between them. The size of the arch, or the amount of curvature, has a major effect on the effectiveness of this type of bridge.
Sometimes, in very large arch bridges, the arch is often reduced in size or flattened down, which results in significant tensile forces that must be factored into the design. Most modern arch bridges span between , feet m. Two categories of suspension bridges are: modern suspension bridges and cable-stayed bridges.
Modern suspension bridges are characterized by an M-shaped cable pattern. Cables are strung over two towers and then anchored on both ends. The roadway is suspended from the cables by thinner cables or rods. The roadway's weight and any additional load are transferred to the cables, creating a tension force in the cables. The cables then transfer their force to the towers and anchors. Typical modern suspension bridges span distances from 2, to 7, feet ,m.
Cable-stayed bridges are characterized by an A-shaped cable pattern. Cables are anchored directly to the towers and eliminate the need for an anchorage system. The same tensile and compressive forces are seen in a cable-stayed bridge as they are in a modern suspension bridge. Typical cable-stayed bridges span distances from to 3, feet m , fast becoming the bridge of choice for medium length spans.
Cable-stayed bridges also look cool! Today, we are going to create simple models of each type of bridge that we just discussed to help us learn more about how the forces of tension and compression act on each one. We are also going to think about the situations when an engineer might decide to use each type of bridge when designing roadways. Figure 1. The beam bridge model.
Figure 2. Arch bridge model. Figure 3. Suspension bridge model. Pressing down on the string between two connected books shows how unstable they are; they easily fall inwards. Figure 4. With the addition of anchorages to each side, pressing down on the string between the two connected books shows that they are more stable.
Figure 5. Cable-stay bridge model. Characterized by A-shaped cable patterns. Review their answers to gauge their mastery of the concepts. Design Your Own : After the activity, describe for students how when a community needs a new bridge, teams of engineers decide on the bridge type, design and materials to best distribute the load across an obstacle, and then draw detailed design plans that are used for construction of the bridge.
Assign student teams to each choose a bridge type and draw their own detailed design of the bridge, specifying materials, measurements, shapes and angles, just like engineers. Engineering Scenarios : Engineers use their knowledge of bridge types to select the most appropriate design for a new area. Have students discuss in pairs and share with the class which bridge types they would choose for transportation through the following scenarios:.
Have students cut out pictures from magazines of the three different bridge types and draw arrows showing where the tensile and compressive forces are acting.
Accessed March 21, Super Bridge: Build a Bridge. The third factor refers to environmental factors that go beyond normal weather conditions, factors such as sudden gusts of wind and earthquakes. All three factors must be taken into consideration in the design of a bridge.
For example, suppose that it is necessary to build a bridge across a span that is ft m wide.
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