Structures. Key Terms. Objectives. This sample chapter is for review purposes only. Copyright The Goodheart-Willcox Co., Inc. All rights reserved.

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1 6 Structures Key Terms abutment arch bridge cantilever bridge compression dynamic pier reinforced concrete shear static stay structure strut suspension bridge tension tie truss Objectives Structures come in many different shapes and sizes, but they all have one common purpose: to support a given. Skyscrapers support many floors of offices, restaurants, and businesses. fter reading this chapter you will be able to: Recognize many different types of structures, both natural ones and those made by humans. Recall that structures made by humans include bridges, buildings, dams, harbors, roads, towers, and tunnels. Identify the s acting on structures. nalyze the forces acting on a structure. Demonstrate how structures can be designed to withstand s. Design and make a product that incorporates structural principles.

2 148 Technology: Shaping Our World S tructures are all around us. We build them to live in or to cross a river. We build them to carry wires, to receive radio waves, and to transport people. Houses, bridges, and towers are not the only structures; airplanes, boats, and cars are structures, too. The main purpose of a structure is to enclose and define a space. t times, however, a structure is built to connect two points. This is the case with bridges and elevators. Other structures are meant to hold back natural forces, as in the case of dams and retaining walls. Everyone has built some kind of structure. Have you ever used a cardboard box to build a playhouse large enough to crawl inside? Have you ever constructed a ramp for a skateboard? Perhaps you built a treehouse from a variety of scrap materials. Maybe you have made a model crane, a dollhouse, a tunnel for a model railroad, or a sand castle on the beach. Figure 6-1 shows two structures. What is the purpose of each? Not all structures are made by humans. Living organisms, such as trees and our bodies, are natural structures. giant redwood tree must be rigid enough to carry its own weight.yet it is able to sway in high winds. Grass is flexible, because it springs back after it is stepped on. The bones of a skeleton have movable joints. They permit activities such as running and lifting. Figure 6-2 shows both natural and human-made structures. Figure 6-1 scaffold supports workers while they build structures. Scaffolds are structures, too! They have connected parts and carry workers without collapsing. (hristopharo) What Structures Have in ommon What do all structures have in common? They all have a number of parts, which are connected. The parts provide support so the structures can serve their purpose. One important job of all structures is to support a. is the weight or force placed on a structure, Figure 6-3. For example, a on a bridge would be a heavy vehicle crossing it, Figure 6-4. These vehicles must also carry s, such as the weight from their own frame and the passengers they carry, Figure 6-5. hapter 6 Structures 149 Wasp s nest Shells Magnified bone Lighthouse Snow Den Stonehenge Figure 6-2 Structures are found all around us. Top Some are found in nature. (Ecritek, TE) ottom Others are planned and built by humans. (Ecritek) Tower Platform Figure 6-3 Towers and platforms are important structures used every day.what other similar structures have you seen? What do they lift or support? (TE, Seeds)

3 150 Technology: Shaping Our World hapter 6 Structures 151 The for a dam is the force of the water behind it. oth must also support the materials from which they are built. This is part of the. Types of Structures Structures vary greatly in size and type. Look at the photographs in Figure 6-6. s you look, think about the s that each of the structures must withstand. Think of the materials used in their construction. Think how the parts are connected together. ll structures must be able to support a without collapsing. roof must not only support its own mass but also a heavy blanket of snow. dining chair must carry the of a person sitting or fidgeting, Figure 6-7. There are two types of s: static and dynamic. Geodesic dome Figure 6-4 Roads, sidewalks, and bridges are important structures.they help us travel from place to place. Pipes that supply water, electricity, and fuel are often built under sidewalks or roads. (Ecritek) Tunnels may provide a walkway under obstructions. (Ecritek) ridges span other obstacles. D Highways must be kept in good repair. (Jack Klasey) D Modern tent frame Figure 6-5 These are a few structures for transportation vehicles. oth structures must carry people and support other parts of the vehicle. (Ford, Ecritek) High-rise building House frame Figure 6-6 How is the framework of each building like a skeleton? (Ecritek,TE, Ecritek, Ecritek)

4 152 Technology: Shaping Our World hapter 6 Structures 153 Static Dynamic Dynamic Figure 6-11 Foam rubber with parallel lines drawn on it will show what happens when a is placed on a beam. Load ompression Static Figure 6-9 Moving objects create dynamic s. Figure 6-7 The structure of a chair must be such that it can carry the of a person sitting on it. (hristopharo) Static Loads Static s are s that are unchanging or change slowly. They may be caused by the weight of the structure itself. olumns, beams, floors, and roofs are part of this. They are also caused by objects placed in or on the structure. Figure 6-8 is an example of such a. Dynamic Loads dynamic is a that is always moving and changing on a Figure 6-8 Objects at rest create static s. given structure. For example, the mass of a person walking across the floor creates a dynamic. Other dynamic s include the force of a gust of wind pushing against a tall building and a truck crossing a bridge, Figure 6-9. Forces cting on Structures oth static and dynamic s create forces, which act on structures. To understand these forces and what they do, imagine a plank placed across a stream, Figure When you (the ) walk across the plank (the structure), what would you expect to happen? The plank bends Figure 6-10 person standing on a plank is a static. ending will cause compression on its top surface and tension on its bottom surface. in the middle. The forces acting on the bridge may be shown by the foam rubber in Figure Notice that parallel lines have been marked on it. Support the foam at each end. vertical applied to the center of the foam bends it, Figure Tension Figure 6-12 ending causes compression and tension stress. Notice what has happened to the parallel lines. t the top edge, the lines have moved closer together. The lines at the bottom edge have moved farther apart. The top edge of the plank is in compression (being squeezed) and the bottom edge is in tension (being stretched). long the center is a line that is neither in compression nor in tension. It has no force acting along it. This line is called the neutral axis. The design and construction of structures must minimize the effects of bending. Parts must be shaped so the forces of tension and compression are balanced. These energies are then said to be in a state of equilibrium, and there is little chance to bend.

5 154 Technology: Shaping Our World hapter 6 Structures 155 Designing Structures to Withstand Loads s was shown by the foam rubber in Figure 6-12, the top and bottom surfaces of a beam are subject to the greatest compression and tension. These surfaces are where the greatest strength is needed. The shapes shown in Figure 6-13 strengthen a beam along these surfaces. fter members have been shaped to resist compression and tension, they must be connected in a way that minimizes bending. Look at the structures in Figure What shape appears Figure 6-13 The shapes shown here will support heavy s. Figure 6-15 Why will frames and collapse when the shown is applied? Figure 6-16 The frames retain their shape from s at,, and. Pylon Geodesic dome Figure 6-14 Some shapes can support heavier s better than other shapes can.what supporting shape appears most often in these two pictures? (TE) most often? You can see that the triangle appears most often. To understand why the triangle is important in structures, look at Figure The frame is made of four connected members. If a is applied at, the frame retains it shape. However, if a is applied at a corner ( or ), the frame will collapse. Now compare this frame to the one in Figure rigid diagonal member (running from corner to corner) has been added, Figure Once again, when a is applied at, the frame retains its shape. This time, however, it also retains its shape when a is applied at corners or. t corner, the causes the diagonal to be in tension. rigid member in tension is called a tie. When the is Figure 6-17 Why will a rope or chain work as a tie but not as a strut? applied at corner, the diagonal is in compression. rigid member in compression is called a strut.

6 156 Technology: Shaping Our World hapter 6 Structures 157 Figure 6-18 Shear is a force that causes one part to slide over an adjacent part. Shear firm foundation. Other ways have to be found to strengthen the beam bridge. One solution is to make the beam much thicker. This, however, would make the beam very heavy. Its own mass would make it sag in the middle. The beam could also be strengthened at the center where it is most likely to bend or break, Figure Once again, notice that the strongest shape is the triangle. s we saw in Figure 6-16, a triangle does not have to be solid; it can be a frame and still be very rigid. ompression Load ompression Figure 6-22 This diagram shows how a simple truss bridge works. Is the center (vertical) beam under tension or compression? What would be the effect of replacing the rigid diagonal member with a nonrigid member such as a rope, chain, or cable? Would the frame retain its shape when ed in each of the three positions? When is the rope in compression, and when is it in tension? In addition to compression and tension, there is a third force acting on structures. This force is called shear. Shear is a multidirectional force that includes parallel and opposite sliding motions. To understand how shear takes place, imagine you are pulling the wagon in Figure Suddenly, the wheels hit a rock. The effect is a sharp jolt on the pin. This force causes the material to shear. Let us see how bending and the forces of compression, tension, and shear are resisted in the design of structures. Then we will see why bridges are built the way they are. major problem with bridges is that they bend, Figure One common way to prevent a beam bridge from bending is to support the center with a pier as in Figure However, it is not always possible to build piers under a bridge. Piers may not allow the passage of ships. Sometimes the river is too deep, runs too swiftly, or has a soft bed with no Figure 6-19 simple beam bridge bends easily. ompression Load Pier Figure 6-20 The pier of a beam bridge is compressed by the on it. Point most likely to break Figure 6-21 One way to strengthen a beam bridge is to make it thicker in the middle. Truss bridges make use of the triangle in their design, Figure s the truck crosses the bridge, its mass causes the bridge roadway to bend. Member moves down. This pulls down on members and, pulling them towards the end of the bridge and carrying the forces out to the bridge supports. Most truss bridges are more complex than the simple truss. Many triangular frames are used to construct them, Figure bridge deck can also be supported from above. ables, called stays, provide the support, Figure Notice that the pylons are in compression and the stays are in tension. The same principle is used for suspension bridges. Suspension bridges are the longest bridges, Figure The bridge deck is suspended from hangers attached to a continuous cable. The cable is securely anchored into the ground at both ends. The cables transfer the

7 158 Technology: Shaping Our World hapter 6 Structures 159 mass of the deck to the top of the towers. From there, compression transfers the mass to the ground. There are many other types of bridges. Their design follows the same Load Warren Girder Figure 6-27 This shows the principle of the cantilever. Load at is transferred to. Lattice Girder Figure 6-23 truss is a long beam made up of shorter beams or girders that give strength to one another. ables Figure 6-25 These examples of suspension bridges show how huge they can be. Top Note the steel cables supported by the tower. ottom The Humber ridge in England, the longest single-span suspension bridge in the world, stretches across the Humber Estuary. Stay Pylon Figure 6-24 To where is the mass (weight) of the truck transferred, when the truck travels over the bridge? Figure 6-26 Top Here is a simple arch bridge. rch transfers back to its ground supports. ottom World s longest arch bridge spans the New River Gorge in West Virginia. It is 3030 (923.5 m)long. (Ecritek) general principle: try to reduce bending. Two of the most common types are arch bridges and cantilever bridges. In an arch bridge, the compressive stress created by the is spread over the arch as a whole. The mass is transferred outward along two curving paths. The supports where the arch meets the ground are called abutments. They resist the outward thrust and keep the bridge up, Figure beam can support a at one end provided that the opposite end is anchored or fixed. This is known as a cantilever beam. The principle of a cantilever is seen in Figure cantilever bridge has two cantilevers with a short beam to complete the span, Figure ridges are made from many materials. The most common are steel and concrete. Steel is fairly inexpensive, strong under compression and tension, but needs maintenance to prevent corrosion. oncrete is economical and resists fire and corrosion. It is strong under compression but weak under tension. However, it can be strengthened with steel rods. Reinforced oncrete Most modern bridges use steel and concrete. Steel cables made of wire rope are used to support the mass of the roadway and the traffic on it. The towers of many bridges are made of steel. Steel trusses give rigidity to the bridge deck. They also resist bending. Many bridges use concrete even though it is weak in tension. To overcome this weakness, the concrete is

8 160 Technology: Shaping Our World Figure 6-28 The theory used in the design of a cantilever bridge is shown on the left. n example of a cantilever bridge is seen in the photo on the right. (Ecritek) reinforced with steel rods wherever it is in tension. The embedding of steel rods to increase the resistance to tension is the basic principle of reinforced concrete, Figure oncrete is weak in tension and cracks will occur at an unsupported center. Reinforced concrete uses steel rods to resist tension. If these rods are stretched while the concrete is hardening, prestressed concrete is produced. Reinforcing bar Figure 6-29 oncrete is made stronger with steel-reinforcing rods. Summary hapter 6 Review Structures ll structures comprise a number of connected parts. These parts provide support and withstand a without collapsing. There are two types of s: static and dynamic. These s create the forces of compression, tension, and shear. Individual members of a structure must be designed to minimize the effects of these forces. The members are then connected together in such a way as to minimize bending. ridges provide an example of how structures are designed to resist forces. truss bridge uses the rigidity of the triangle to resist the forces of compression and tension. ables and pylons in a suspension bridge resist these same forces. There are many other types of bridges. s with all structures, they are designed to withstand s and minimize bending. Modular onnections The information in this chapter provides the required foundation for the following types of modular activities: Structural Engineering pplied Physics ridge Design Tower Design Truss Design 161

9 162 Technology: Shaping Our World hapter 6 Structures 163 Test Your Knowledge Write your answers to these review questions on a separate sheet of paper. 1. Name three natural structures and three structures made by humans. 2. Which of the following is NOT a natural structure?. Spider s web.. ridge.. Tree. D. eaver s dam. 3. ll structures are.. built to withstand heat. made in factories. built to withstand a D. designed to house people 4. Name the two types of s acting on structures. Give one example of each. 5. What forces are acting on the top and bottom surfaces of a beam ed from above? 6. To strengthen a beam ed on the top surface, it must be reinforced at the.. top surface only. bottom surface only. center D. top and bottom surfaces 7. Which geometric shape gives the greatest rigidity to a structure?. Square.. ircle.. Rectangle. D. Triangle. 8. beam in compression is called a.. strut. tie. post D. stay 9. beam in tension is called a.. strut.. tie.. post.. stay. 10. bridge that uses a series of triangular frames is called a(n) bridge. 11. The world s longest bridges are bridges. 12. Using notes and diagrams, explain how an arch bridge resists s. 13. Using notes and diagrams, explain the principle of a cantilever bridge. 14. What are the most common materials from which bridges are built? 15. oncrete is weak in tension. How is this problem overcome? pply Your Knowledge 1. Look at the natural structures in the illustrations. Next look at the structures made by humans. For each of the structures made by humans, name the natural structure it most closely resembles. 2. Look at the structures in Figures 6-4 and 6-5. Write the location or address of a structure in your town that most closely resembles each one. 3. Name five different structures. For each structure, list the s to which it is subjected. State whether each is static or dynamic. 4. Draw a diagram of a plank bridge with a on it. Label your diagram to show the forces of tension and compression. 5. Using only one sheet of newspaper and 4 (10 cm) of clear tape, construct the tallest freestanding tower possible. 6. Using drinking straws and pins, construct a bridge to span a gap of 20 (508 mm) and support the largest mass possible at midpoint. 7. Research one career related to the information you have studied in this chapter and state the following:. The occupation you selected.. The education requirements to enter this occupation.. The possibilities of promotion to a higher level at a later date. D. What someone with this career does on a day-to-day basis. You might find this information on the Internet or in your library. If possible, interview a person who already works in this field to answer the four points. Finally, state why you might or might not be interested in pursuing this occupation when you finish school.