Structures

Structures come in many differentshapes and sizes, but they all haveone common purpose: to support agiven load. Skyscrapers supportmany floors of offices, restaurants,and businesses.

6Structures

Key Termsabutmentarch bridgecantilever bridgecompressiondynamic loadloadpierreinforced concreteshear

static loadstaystructurestrutsuspension bridgetensiontietruss

ObjectivesAfter reading this chapter you will be able to:

� Recognize many different types of structures, both natural onesand 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.

� 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.

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

148 Technology: Shaping Our World

S tructures are all around us. Webuild them to live in or to cross ariver. We build them to carry

wires, to receive radio waves, and totransport people. Houses, bridges,and towers are not the only struc-tures; airplanes, boats, and cars arestructures, too.

The main purpose of a structure isto enclose and define a space. Attimes, however, a structure is built toconnect two points. This is the casewith bridges and elevators. Otherstructures are meant to hold back nat-ural forces, as in the case of damsand retaining walls.

Everyone has built some kind ofstructure. Have you ever used a card-board box to build a playhouse largeenough to crawl inside? Have you everconstructed a ramp for a skateboard?Perhaps you built a treehouse from avariety of scrap materials. Maybe youhave made a model crane, a doll-house, a tunnel for a model railroad, ora sand castle on the beach. Figure 6-1shows two structures. What is the pur-pose of each?

Not all structures are made byhumans. Living organisms, such astrees and our bodies, are natural struc-tures. A giant redwood tree must berigid enough to carry its own weight.Yetit is able to sway in high winds. Grass isflexible, because it springs back after itis stepped on. The bones of a skeletonhave movable joints. They permit activ-ities such as running and lifting.Figure 6-2 shows both natural andhuman-made structures.

What Structures Havein Common

What do all structures have in com-mon? They all have a number of parts,which are connected. The parts pro-vide support so the structures canserve their purpose. One important jobof all structures is to support a load. Aload is the weight or force placed on astructure, Figure 6-3. For example, aload on a bridge would be a heavyvehicle crossing it, Figure 6-4. Thesevehicles must also carry loads, suchas the weight from their own frame andthe passengers they carry, Figure 6-5.

Figure 6-1 A scaffold supports workerswhile they build structures. Scaffolds arestructures, too! They have connected partsand carry workers without collapsing.(Christopharo)

Chapter 6 Structures 149

Lighthouse Snow Den Stonehenge

Wasp’s nest Shells Magnified bone

Figure 6-3 Towers and platforms are important structures used every day.What othersimilar structures have you seen? What do they lift or support? (TEC, Seeds)

Tower Platform

Figure 6-2 Structures are found all around us. Top—Some are found in nature. (Ecritek,TEC) Bottom—Others are planned and built by humans. (Ecritek)


Figure 6-4 Roads, sidewalks, and bridges are important structures.They help us travel fromplace to place. A—Pipes that supply water, electricity, and fuel are often built under sidewalks orroads. (Ecritek) B—Tunnels may provide a walkway under obstructions. (Ecritek) C—Bridgesspan other obstacles. D—Highways must be kept in good repair. (Jack Klasey)

A

C D

B


Figure 6-5 These are a few structures for transportation vehicles. Both structures mustcarry people and support other parts of the vehicle. (Ford, Ecritek)


The load for a dam is the force of thewater behind it. Both must also supportthe materials from which they are built.This is part of the load.

Types of StructuresStructures vary greatly in size and

type. Look at the photographs inFigure 6-6. As you look, think aboutthe loads that each of the structures

must withstand. Think of the materialsused in their construction. Think howthe parts are connected together.

All structures must be able to sup-port a load without collapsing. A roofmust not only support its own massbut also a heavy blanket of snow. Adining chair must carry the load of aperson sitting or fidgeting, Figure 6-7.There are two types of loads: staticand dynamic.

Figure 6-6 How is the framework of each building like a skeleton? (Ecritek,TEC,Ecritek, Ecritek)

High-rise building

Geodesic dome

Modern tent frame

House frame



Static LoadsStatic loads are loads that are

unchanging or change slowly. Theymay be caused by the weight of the structure itself. Columns, beams,floors, and roofs are part of thisload. They are also caused byobjects placed in or on the structure.Figure 6-8 is an example of such a load.

Dynamic LoadsA dynamic load is a load that is

always moving and changing on a

given structure. For example, themass of a person walking across the floor creates a dynamic load.Other dynamic loads include theforce of a gust of wind pushingagainst a tall building and a truckcrossing a bridge, Figure 6-9.

Forces Acting onStructures

Both static and dynamic loads cre-ate forces, which act on structures. Tounderstand these forces and whatthey do, imagine a plank placedacross a stream, Figure 6-10. Whenyou (the load) walk across the plank(the structure), what would youexpect to happen? The plank bends

Figure 6-7 The structure of a chair mustbe such that it can carry the load of a per-son sitting on it. (Christopharo)

Staticload

Staticload

Figure 6-8 Objects at rest create static loads.


in the middle.The forces acting on thebridge may be shown by the foamrubber in Figure 6-11. Notice thatparallel lines have been marked on it.Support the foam at each end. A ver-tical load applied to the center of thefoam bends it, Figure 6-12.

Notice what has happened to theparallel lines. At the top edge, thelines have moved closer together.The lines at the bottom edge havemoved farther apart. The top edge ofthe plank is in compression (beingsqueezed) and the bottom edge is intension (being stretched). Along thecenter is a line that is neither in com-pression nor in tension. It has no forceacting along it. This line is called theneutral axis.

The design and construction ofstructures must minimize the effects ofbending. Parts must be shaped so theforces of tension and compression arebalanced. These energies are thensaid to be in a state of equilibrium, andthere is little chance to bend.

Figure 6-10 A person standing on a plankis a static load. Bending will cause compres-sion on its top surface and tension on itsbottom surface.

Figure 6-11 Foam rubber with parallellines drawn on it will show what happenswhen a load is placed on a beam.

Load

Compression

Tension

Figure 6-12 Bending causes compressionand tension stress.

Dynamicload

Dynamicload

Figure 6-9 Moving objects createdynamic loads.


Designing Structuresto Withstand Loads

As was shown by the foam rubberin Figure 6-12, the top and bottomsurfaces of a beam are subject to thegreatest compression and tension.These surfaces are where the great-est strength is needed. The shapesshown in Figure 6-13 strengthen abeam along these surfaces. Aftermembers have been shaped to resistcompression and tension, they mustbe connected in a way that mini-mizes bending.

Look at the structures inFigure 6-14. What shape appears

Figure 6-13 The shapes shown here willsupport heavy loads.

Figure 6-14 Some shapes can support heavier loads better than other shapes can.Whatsupporting shape appears most often in these two pictures? (TEC)

Pylon Geodesic dome


A

B

C

Figure 6-15 Why will frames B and Ccollapse when the load shown is applied?

A

B

C

Figure 6-16 The frames retain theirshape from loads at A, B, and C.

A B

C

Figure 6-17 Why will a rope or chainwork as a tie but not as a strut?


most often? You can see that the triangle appears most often. Tounderstand why the triangle is impor-tant in structures, look atFigure 6-15. The frame is made offour connected members. If a load isapplied at A, the frame retains itshape. However, if a load is appliedat a corner (B or C), the frame willcollapse. Now compare this frame tothe one in Figure 6-16.

A rigid diagonal member (runningfrom corner to corner) has beenadded, Figure 6-17. Once again,when a load is applied at A, the frameretains its shape. This time, however, italso retains its shape when a load isapplied at corners B or C. 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 incompression. A rigid member in com-pression is called a strut.


What would be the effect of replac-ing the rigid diagonal member with anonrigid member such as a rope,chain, or cable? Would the frame retainits shape when loaded in each of thethree positions? When is the rope incompression, and when is it in tension?

In addition to compression andtension, there is a third force actingon structures. This force is calledshear. Shear is a multidirectionalforce that includes parallel and oppo-site sliding motions. To understandhow shear takes place, imagine youare pulling the wagon in Figure 6-18.Suddenly, the wheels hit a rock. Theeffect is a sharp jolt on the pin. Thisforce causes the material to shear.

Let us see how bending and theforces of compression, tension, andshear are resisted in the design ofstructures. Then we will see whybridges are built the way they are.

Shear

Figure 6-18 Shear is a force that causes one part to slide over an adjacent part.

Figure 6-19 A simple beam bridgebends easily.


A major problem with bridges isthat they bend, Figure 6-19. Onecommon way to prevent a beambridge from bending is to support thecenter with a pier as in Figure 6-20.However, it is not always possible tobuild piers under a bridge. Piers maynot allow the passage of ships.Sometimes the river is too deep, runstoo swiftly, or has a soft bed with no

firm foundation. Other ways have to befound to strengthen the beam bridge.

One solution is to make the beammuch thicker. This, however, wouldmake the beam very heavy. Its ownmass would make it sag in the middle.

The beam could also be strength-ened at the center where it is mostlikely to bend or break, Figure 6-21.Once again, notice that the strongestshape is the triangle. As we saw inFigure 6-16, a triangle does not haveto be solid; it can be a frame and stillbe very rigid.

Truss bridges make use of the tri-angle in their design, Figure 6-22. Asthe truck crosses the bridge, its masscauses the bridge roadway to bend.Member “A” moves down. This pullsdown on members “B” and “C,” pullingthem towards the end of the bridgeand carrying the forces out to thebridge supports.

Most truss bridges are more com-plex than the simple truss. Many tri-angular frames are used to constructthem, Figure 6-23. A bridge deck canalso be supported from above.Cables, called stays, provide the sup-port, Figure 6-24. Notice that thepylons are in compression and thestays are in tension.

The same principle is used for suspension bridges. Suspensionbridges are the longest bridges,Figure 6-25. The bridge deck issuspended from hangers attached toa continuous cable. The cable issecurely anchored into the ground atboth ends. The cables transfer the

Load

Point most likely to break

Figure 6-21 One way to strengthen abeam bridge is to make it thicker in the middle.

Compr

essio

n

Compression

Load

B

A

C

Figure 6-22 This diagram shows how asimple truss bridge works. Is the center (ver-tical) beam under tension or compression?

CompressionPier

Figure 6-20 The pier of a beam bridge iscompressed by the load on it.



mass of the deck to the top of the towers. From there, compressiontransfers the mass to the ground.

There are many other types ofbridges. Their design follows the same

Cables Stay

Pylon

Figure 6-24 To where is the mass (weight) of the truck transferred, when the truck travelsover the bridge?

Figure 6-25 These examples of suspen-sion bridges show how huge they can be.Top—Note the steel cables supported bythe tower. Bottom—The Humber Bridge inEngland, the longest single-span suspensionbridge in the world, stretches across theHumber Estuary.


Warren Girder

Lattice Girder

Figure 6-23 A truss is a long beam madeup of shorter beams or girders that givestrength to one another.

general principle: try to reduce bending.Two of the most common types arearch bridges and cantilever bridges.

In an arch bridge, the compressivestress created by the load is spreadover the arch as a whole. The mass istransferred outward along two curvingpaths. The supports where the archmeets the ground are called abut-ments. They resist the outward thrustand keep the bridge up, Figure 6-26.

A beam can support a load at oneend provided that the opposite end isanchored or fixed. This is known as

a cantilever beam. The principle of acantilever is seen in Figure 6-27.A cantilever bridge has two cantileverswith a short beam to complete thespan, Figure 6-28.

Bridges are made from manymaterials. The most common aresteel and concrete. Steel is fairly inex-pensive, strong under compressionand tension, but needs maintenanceto prevent corrosion. Concrete is economical and resists fire and corro-sion. It is strong under compressionbut weak under tension. However, itcan be strengthened with steel rods.

Reinforced ConcreteMost modern bridges use steel

and concrete. Steel cables made ofwire rope are used to support themass of the roadway and the trafficload on it. The towers of many bridgesare made of steel. Steel trusses giverigidity to the bridge deck. They alsoresist bending.

Many bridges use concrete eventhough it is weak in tension. To over-come this weakness, the concrete is

Load

Figure 6-26 Top—Here is a simple arch bridge. Arch transfers load back toits ground supports. Bottom—World’slongest arch bridge spans the New RiverGorge in West Virginia. It is 3030′(923.5 m)long. (Ecritek)

AB

Figure 6-27 This shows the principle ofthe cantilever. Load at A is transferred to B.



reinforced with steel rods wherever it is in tension. The embedding of steelrods to increase the resistance to ten-sion is the basic principle of reinforced concrete, Figure 6-29.


Concrete is weak in tension and cracks willoccur at an unsupported center.

Reinforcingbar

Reinforced concrete uses steel rods to resisttension. If these rods are stretched whilethe concrete is hardening, prestressed

concrete is produced.

Figure 6-29 Concrete is made strongerwith steel-reinforcing rods.

Figure 6-28 The theory used in thedesign of a cantilever bridge is shown onthe left. An example of a cantilever bridgeis seen in the photo on the right. (Ecritek)

Chapter 6 Review

161

SummaryAll structures comprise a number of connected parts. These

parts provide support and withstand a load without collapsing.There are two types of loads: static and dynamic. These loadscreate the forces of compression, tension, and shear. Individualmembers of a structure must be designed to minimize the effectsof these forces. The members are then connected together insuch a way as to minimize bending.

Bridges provide an example of how structures are designed toresist forces. A truss bridge uses the rigidity of the triangle toresist the forces of compression and tension. Cables and pylonsin a suspension bridge resist these same forces. There are manyother types of bridges. As with all structures, they are designed towithstand loads and minimize bending.

The information in this chapter provides the required founda-tion for the following types of modular activities:

� Structural Engineering

� Applied Physics

� Bridge Design

� Tower Design

� Truss Design

Modular Connections

Structures



Write your answers to these review questions on a separatesheet of paper.

1. Name three natural structures and three structures made by humans.

2. Which of the following is NOT a natural structure?A. Spider’s web.B. Bridge.C. Tree.D. Beaver’s dam.

3. All structures are _____.A. built to withstand heatB. made in factoriesC. built to withstand a loadD. designed to house people

4. Name the two types of loads acting on structures. Give oneexample of each.

5. What forces are acting on the top and bottom surfaces of abeam loaded from above?

6. To strengthen a beam loaded on the top surface, it must bereinforced at the _____.A. top surface onlyB. bottom surface onlyC. centerD. top and bottom surfaces

7. Which geometric shape gives the greatest rigidity to a structure?A. Square.B. Circle.C. Rectangle.D. Triangle.

8. A beam in compression is called a _____.A. strutB. tieC. postD. stay

9. A beam in tension is called a _____.A. strut.B. tie.C. post.C. stay.

10. A bridge that uses a series of triangular frames is called a(n) _____ bridge.

Test Your Knowledge


11. The world’s longest bridges are _____ bridges.12. Using notes and diagrams, explain how an arch bridge

resists loads.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. Concrete is weak in tension. How is this problem overcome?

Apply Your Knowledge1. Look at the natural structures in the illustrations. Next

look at the structures made by humans. For each of thestructures 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 mostclosely resembles each one.

3. Name five different structures. For each structure, list the loads to which it is subjected. State whether each load isstatic or dynamic.

4. Draw a diagram of a plank bridge with a load on it. Label yourdiagram 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 havestudied in this chapter and state the following:A. The occupation you selected.B. The education requirements to enter this occupation.C. 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. Ifpossible, interview a person who already works in this field toanswer the four points. Finally, state why you might or might notbe interested in pursuing this occupation when you finish school.


Structures

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