2. 6
Structures
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3. Objectives
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 loads acting on structures.
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4. Objectives Analyze the forces acting on a structure.
Demonstrate how structures can be designed to withstand loads.
Design and make a product that incorporates structural principles.
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5. Introduction Structures 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.
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6. Introduction
The main purpose of a structure is to enclose and define a space.
At 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.
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7. Introduction
Living organisms, such as trees and our bodies, are natural structures.
A 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.
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8. Discussion
What structures have you built? What was their intended purpose? What materials and tools did you use?
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9. Commonalities 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 load.
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10. What is the purpose of each structure?
Activity
Make a list of three animals.
Make an illustrated poster showing the structures they create.
Include the materials used to build each.
What is the purpose of each structure?
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11. Loads Bridges support vehicles. Vehicles support passengers.
Both also support the materials from which they are built.
(Ford, Ecritek)
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12. Types of Structures Structures vary greatly in size and type.
All structures must be able to support a load without collapsing.
(Ecritek, TEC)
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13. Static Loads
Static loads may be caused by the weight of the structure itself.
They are also caused by objects placed in or on the structure.
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14. Dynamic Loads
The mass of a person walking across the floor creates a dynamic load.
Other dynamic loads include the force of a gust of wind pushing against a tall building and a truck crossing a bridge.
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15. Compression and Tension
Bending causes compression and tension stress.
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16. Minimize Bending
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.
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17. Discussion
What geometric shapes can you identify in structures? Which geometric shape appears more often than others?
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18. What shape appears most often?
Withstanding Loads
What shape appears most often?
(TEC)
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19. Withstanding Loads
The top and bottom surfaces of a beam are subject to the greatest compression and tension.
After members have been shaped to resist compression and tension, they must be connected in a way that minimizes bending.
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20. The Importance of Triangles
The frame in the figure is made of four connected members. If a load is applied at A, the frame retains shape.
However, if a load is applied at a corner, the frame will collapse.
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21. The Importance of Triangles
A rigid diagonal member (from corner to corner) has been added.
The frame now retains its shape when a load is applied at the corners.
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22. Tie and Strut
At corner B, the load causes the diagonal to be in tension. A rigid member in tension is called a tie.
When the load is applied at corner C, the diagonal is in compression. It is a strut.
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23. Shear A third force acting on structures is called shear.
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24. Bending Bridges A major problem with bridges is that they bend.
One common way to prevent a beam bridge from bending is to support the center with a pier.
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25. Piers Piers may not allow the passage of ships.
Sometimes the river is too deep, runs too swiftly, or has a soft bed with no firm foundation.
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26. Other Solutions
Making the beam thicker would cause it to sag from its own weight.
The beam could be strengthened in the middle.
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27. Simple Trusses Truss bridges make use of the triangle in their design.
The mass of a truck crossing causes the bridge roadway to bend. Member “A” moves down. This pulls down on “B” and “C,” pulling them toward the end of the bridge.
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28. Trusses Most truss bridges are more complex than the simple truss.
Many triangular frames are used to construct them.
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29. Stays Stays are cables that provide support from above.
Notice that the pylons are in compression and the stays are in tension.
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30. Suspension Bridges
The bridge deck is suspended from hangers attached to a continuous cable.
Cables secured to the ground transfer mass from the deck to the top of the towers.
Compression transfers the mass to the ground.
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31. Other Bridges There are many other types of bridges.
Their design follows the same general principle: try to reduce bending.
Two of the most common types are arch bridges and cantilever bridges.
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32. Arch Bridges Mass is transferred outward along two curving paths.
The supports where the arch meets the ground are called abutments.
They resist outward thrust and keep the bridge up.
(Ecritek)
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33. Cantilever Bridges
A beam can support a load at one end provided that the opposite end is anchored or fixed.
A cantilever bridge has two cantilevers with a short beam to complete the span.
(Ecritek)
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34. Steel and Concrete
The most common bridge materials are steel and concrete.
Steel is fairly inexpensive, strong under compression and tension, but needs maintenance to prevent corrosion.
Concrete is economical and resists fire and corrosion but is weak under tension.
It can be strengthened with steel rods.
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35. Steel
Steel cables made of wire rope are used to support the mass of the roadway and the traffic load on it.
The towers of many bridges are made of steel.
Steel trusses give rigidity to the bridge deck.
They also resist bending.
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36. Reinforced Concrete
To overcome the weakness of concrete, it is reinforced with steel rods wherever it is in tension.
Reinforced concrete is used in many bridges.
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37. Activity Go to http://www.pbs.org/ Search for “Building Big.”
Click on “Bridges.”
Click on “The Bridge Challenge.”
Do one of the bridge challenges and write a page on a real-life bridge that would work like the one in the activity.
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38. Review Both skeletal systems and scaffolds are ________. structures
During design: can we hide “structures” and then animate so it is on the blank.
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39. Review Name some man-made structures.
Bridges, buildings, dams, harbors, roads, towers, tunnels, airplanes, boats, and cars.
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40. Review List and explain the two types of loads.
Static loads are unchanging or slow changing.
Dynamic loads are always moving and changing.
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41. Review Explain the three forces that act on structures.
Tension stretches out a structure.
Compression squeezes the structure.
Shear includes parallel and opposite sliding motions.
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42. Review Name one way to prevent a bridge from bending.
Support the center with a pier.
Strengthen the beam at the middle.
Use trusses.
Use cables to distribute the load.
Use arches and abutments.
Apply the principle of a cantilever.
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43. Review Why are steel rods used to reinforce concrete?
Steel rods can resist tension, unlike concrete.
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44. End of Chapter 6
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45. Glossary Abutments Arch bridge Cantilever bridge
The supports where a bridge arch meets the ground; they resist the outward thrust (push) and keep the bridge up.
Arch bridge
A type of bridge in which the compressive stress created by the load is spread over the arch as a whole.
Cantilever bridge
A type of bridge in which a beam is capable of supporting a load at one end when the opposite end is anchored or fixed.
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46. Glossary Compression Dynamic load Load Pier A squeezing force.
A load on a structure that is always changing.
Load
The weight, mass, or force placed on a structure.
Pier
A structure used to support the center of a bridge.
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47. Glossary Reinforced concrete Shear Static load Stays
Concrete in which steel rods have been embedded to increase the concrete’s resistance to tension.
Shear
A multidirectional sliding and separating force.
Static load
A load that is unchanging or changes slowly.
Stays
Cables that support a bridge deck from above.
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48. Glossary Structure Strut Suspension bridge
Something that encloses and defines a space; also, an assembly of separate parts that is capable of supporting a load.
Strut
A rigid structural member that is in compression.
Suspension bridge
A bridge in which the deck is suspended (hung) from hangers attached to a continuous cable, which passes over towers and is anchored to the ground at each end.
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49. Glossary Tension Tie Truss A pulling force.
A rigid structural member that is in tension.
Truss
A structural element made up of a series of triangular frames.
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