Solutions for all Technology

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2 Solutions for all Technology Grade 8 Learner s Book Debi Bosch Alphonso Hendricks Isabel Tarling

3 Solutions for all Technology Grade 8 Learner s Book D Bosch, A Hendricks, I Tarling, 2013 Illustrations and design Macmillan South Africa (Pty) Ltd, 2013 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, photocopying, recording, or otherwise, without the prior written permission of the copyright holder or in accordance with the provisions of the Copyright Act, 1978 (as amended). Any person who commits any unauthorised act in relation to this publication may be liable for criminal prosecution and civil claims for damages. First published Published by Macmillan South Africa (Pty) Ltd Private Bag X19 Northlands 2116 Gauteng South Africa Typeset by Tangerine Design Studio Cover design by Deevine Design Cover image from INPRA Illustrations by MPS, Greenhouse Cartoons Photos by AAI/Fotostock, Afripics, Alamy, Cape Argus, Gallo Images, Getty Images, Greatstock/Corbis, Greatstock/Masterfile, INPRA, Mayibuye Archives, Picturenet, The Bigger Picture, Science Photo Library, VMS Images The publishers have made every effort to trace the copyright holders. If they have inadvertently overlooked any, they will be pleased to make the necessary arrangements at the first opportunity. ISBN: WIP: 4571M000 e-isbn: It is illegal to photocopy any page of this book without written permission from the publishers. The publishers have made every effort to trace the copyright holders. If they have inadvertently overlooked any, they will be pleased to make the necessary arrangements at the first opportunity. The publishers would also like to thank those organisations and individuals for which we are anticipating permission.

4 Contents Introduction... v Term 1 Topic 1 Frame structures... 1 Unit 1 Definition of a frame structure... 3 Topic 2 Structural members Unit 1 Structures that span gaps Unit 2 Structural failure Topic 3 Purpose of graphics Unit 1 Drawing conventions Unit 2 Working drawing Unit 3 Artistic drawing Topic 4 Mechanical systems and control Unit 1 Mechanical systems revision Unit 2 Simple mechanisms Unit 3 Gears Unit 4 Mechanisms that change direction of motion Unit 5 Graphic skills Formal Assessment Task: Mini Practical Assessment Task Term Term 2 Topic 5 Impact of technology: plastics Unit 1 Investigating plastic Topic 6 Investigating cardboard packaging Unit 1 Investigating the impact of paper and cardboard packaging Unit 2 Cardboard packaging developments Formal Assessment Task: Mini Practical Assessment Task Term

5 Term 3 Topic 7 Mechanical systems and control: revision of levers and gears Unit 1 Revising single levers and levers linked in pairs Unit 2 Revising gear systems Topic 8 Mechanical advantage calculations Unit 1 Lever calculations Unit 2 Gear calculations Topic 9 Communication skills Unit 1 Representing gear systems graphically Unit 2 Design skills Topic 10 Investigation skills Unit 1 Analysing systems Topic 11 Impact of technology Unit 1 Investigating the negative impact of mining Formal Assessment Task: Mini Practical Assessment Task Term Term 4 Topic 12 Electrical systems and control Unit 1 Electric circuits Topic 13 Impact of and bias in technology Unit 1 Energy Topic 14 Electrical systems and control Unit 1 Electrochemical cells Unit 2 Series and parallel connections Unit 3 Photovoltaic cells Topic 15 Electrical systems and control Unit 1 Generating electricity for the nation Unit 2 The national power grid Formal Assessment Task: Mini Practical Assessment Task Term Glossary Isometric paper sample

6 Introduction Introduction Welcome to the Solutions for all Grade 8 Learner s Book. In Technology, you will be given a number of real-life scenarios. You will be taught to use your innovative skills and develop your creativity and critical thinking. You will also learn to manage time and resources effectively as you work through each task presented to you. This will provide opportunities for collaborative learning and working within teams. Learning Technology as a subject will give you the opportunity to: analyse problems and devise ways to work technologically when solving problems in context develop and apply specific design skills to solve technological problems understand the concepts and knowledge used in Technology education and use them responsibly and purposefully appreciate the interaction between people s values and attitudes, technology, society and the environment. How to use this Learner s Book The Solutions for all Technology series aims to give learners, no matter what their level of ability or background, the opportunity to be successful in developing the technological skills necessary to master this subject. In this Learner s Book, every activity is scaffolded in manageable steps. Within each topic you will find: What you will learn about in this topic CAPS extract Let s talk about about this topic a short overview of what will be covered in the topic What you already know prior knowledge of skills and knowledge already acquired Check yourself a check for you to use to ensure that you have acquired the relevant knowledge What you still need to know knowledge and skills you will learn, and what to expect in the topic Word bank difficult words and terms, which are explained Units and lessons each topic is broken down into manageable units and lessons Classroom, homework and extra practice activities and practical tasks these provide you with an opportunity to reinforce the learnt knowledge in a practical way Summary a review of what you have learnt in the topic. The publisher and authors wish you all the best in your study of Technology Grade 8! v

7 How to work through a Mini Practical Assessment Task This year you will complete four Mini Practical Assessment Tasks. In this section you will find out how to work through these tasks. It starts with a problem At the beginning of a Practical Technology Assessment Task, you will be given a problem to solve. Technology is about solving problems to meet people s daily needs and wants. The problem will be set in real life and you will design and make something to solve the problem. It is important, however, before you begin, to make sure you understand the problem properly. You need to understand the problem To understand the problem properly and work out exactly what it is that you need to design and make, it helps to write down a few important things. These are: a list of Specifications, a list of Constraints and a Design brief. Specifications, Constraints and Design briefs Whenever you read through a technology problem you will need to ask yourself questions such as: What do I have to make? What is it going to be used for? Who will be using it? How big does it have to be? Does it have to be made using anything specific? Is there anything specific that needs to be included in my design? Are there any limitations placed on me? Will it solve the problem? vi

8 Introduction These questions can then be used to make a list of Specifications and a list of Constraints. Specifications are all those things that must be included in your product or system. Constraints are all the limits that have been placed on you as a designer. Then using your list of specifications and constraints, you will need to work out a short, brief statement of what it is you actually have to design and make to solve the problem. This is called a Design brief. A Design brief always starts with the following sentence: Design and make a The design brief, as well as your list of specifications and constraints will help you to understand the problem that you need to solve. You need to solve the problem Once you know what it is that you have to do to solve the problem, you are almost ready to begin, but you may need to gain some new technological skills. This will be taught through the different Enabling tasks. Enabling tasks An Enabling task will help you to learn all the skills that you might need when you set about solving a problem. These might be skills such as cutting, sawing, joining or hammering. You will complete an Enabling task with the help of your teacher. You may need to work through more than one Enabling task per problem. Problems are solved through the design process Once you understand the problem that needs to be solved and you have learnt all the skills needed to solve the problem, you can begin solving the problem. The way you do this is by working through the design process. You will learn more about this in Term 1, and how to use it. Remember that your teacher will use Mini Practical Assessment Tasks which will count towards your final mark. Always work well with your group and make sure you use all the knowledge, skills and values you have learnt to help you. Always do your best and have fun! vii

9 Term 1 Topic 1 Frame structures What you will learn about in this topic You will: learn the definition of frame structures learn about the purpose of structural members (components) in wood and steel roof trusses (king and queen post, strut, tie, rafter, tie beam) identify structural members and types of force (shear, torsion, tension, compression) acting on them in given frame structures learn about structural members under tension or compression. Let s talk about this topic What you will learn about in this topic We are surrounded by structures of different shapes and forms. Each structure is specifically designed to perform a particular function. Some structures, such as trees, the feathers of a bird and the human skeleton occur in nature. Some human-made structures were built by people who learnt from their forefathers. These structures have survived hundreds of years. Africa has many human-made structures. These show that people truly understood the technology needed to design and build these wonders. Examples include Great Zimbabwe in Zimbabwe (built in the early 1500s) and the Giza Pyramids in Egypt (built around 2000 BC). Let s talk about this topic The designs of some human-made structures have been influenced by nature. For example, the wing of an aeroplane is a human-made structure that has been inspired by the wing of a bird. An aeroplane wing has to be strong enough to support the engines and fuel tank of an aeroplane, but it should not be heavy and add to the weight of the aeroplane. 1

10 What you already know In previous grades you learnt what a structure is. You have also learnt about the differences between frame and shell structures; and the difference between a natural structure and a human-made structure. Remember that choosing the right material for a structure can add to its ability to hold a load. You learnt about different skeletons and how they provide a support system for animals. Skeletons help animals to move and stay upright. The skeleton structure is a collection of bones. Some of these bones are connected to muscles in the body. When these muscles are contracted or relaxed, they cause the bones to move. The skeleton structure also protects the soft internal organs. Check yourself 1. What is the difference between a frame structure and a shell structure? 2. Three basic shapes have been constructed by connecting strips of cardboard. square triangle pentagon Which shape can withstand pulling and pushing forces the best? 3. Name three natural structures and three human-made structures. Two of your answers must be examples from the area where you live. What you still need to know A structure is any collection of parts that are joined together to support a load. Structures have one or more of the following uses: they support a load (such as a water tower), they enclose or protect an object (such as a box or carton), or they span a gap (such as a bridge). 2

11 Topic 1 Unit 1 Definition of a frame structure Word bank A B C frame structure a frame that has supports on the inside of the structure structural members different parts of frame structures tie a structural member that holds two other members in place guy a rope or cable that is attached to columns strut solid members pushed up against columns to keep them in place roof truss a frame structure that gives the roof its shape trapezium a four-sided figure that has a set of parallel sides (one of these sides is shorter than the other) king post a single support column that carries the greatest load queen posts posts that are thinner than king posts, and which always work in pairs in a frame structure rafter a sloping beam that forms a roof truss tie beam a solid beam that acts like a tie and holds two structural members together tensile force a force that pulls on a structural member; the stretched member is therefore under tension compressive force a force that presses down on a structural member, putting that member under compression electric pylon a tall frame structure used for supporting heavy electricity-carrying cables regime a ruling system militant cross-bracing gusset brace column triangulation engaged in fighting used to reinforce building structures in which two diagonal supports cross each other; this forms four triangles a flat piece of rigid material such as wood or metal used to brace or hold frame members together an extra member in a structure (normally a strut) that joins the two opposite corners of a rectangle; the single brace divides the rectangle up into two triangles part of a frame structure that stands upright and offers support triangles that are formed in order to provide extra stiffness in the frame structure 3

12 A shell structure usually encloses something. One example of a shell structure is a milk carton. It provides a protective shell around the milk inside the container. A frame structure has supports on the inside of the structure. One example is the roof of a house. The inside of a roof has wooden struts, trusses and beams that give it form and make it sturdy. The outside covering could be materials such as metal sheets, polycarbonate plastic sheets or roof tiles. A shell structure A frame structure Lesson 1 The purpose of structural members Frame structures are made up of different parts. These different parts are called structural members or just members. They include columns, beams, ties, guys and struts. Columns and beams Some frame structures have uprights, which stand upright and are vertical. These structural members are called columns. Normally, cross members run from one column to the next. These members are called beams. They lie horizontally. Their ends are supported by the vertical columns. A long cross member also has more columns supporting it along its length. A beam spreads the load that columns are supporting across all the columns. A swing frame structure has a horizontal beam that spread the load of the swinging child across the supporting upright columns. 4

13 Topic 1 Ties, guys and struts A structure must keep its shape, even when a force is applied to it. Ties, guys and struts are structural members that help keep columns and beams in place. Ties are structural members that hold other members in place by pulling on them. A tie can be made using rope or other materials such as wood or steel. Guys are ropes or cables that are attached to columns. They are pulled tight and keep the columns upright. Struts are solid pieces of wood or steel that are pushed against columns in order to keep them in place. The pole (upright column) is held in position by a guy (cable) that is anchored firmly to the ground. Study the three frame structures in the following illustrations. They look different from one another but each one has structural members in common. Can you identify all the structural members? Three different frame structures Purpose of structural member in roof trusses When the foundation and walls of a structure have been completed, it is time to construct the roof. Before the roof tiles or metal sheets are placed on a roof, you can usually see several large triangles with lots of little triangles inside them. We call these frame structures roof trusses. Hoisting the roof trusses into place before the roof can be completed 5

14 What is a roof truss? A roof truss is a rigid frame structure made up of members. These members are held together by metal connector plates. The frame gives the roof its shape. It also supports the roofing material that will be placed on top of it. The more triangular shapes within such a structure, the stronger it is. Classroom activity 1 Identifying triangles Can you identify all 18 triangles in the drawing below? Triangulation makes the roof truss a very stable frame structure. The purpose of a roof truss is to ensure the load placed on it spreads down to the end supports. These end supports are placed on top of the loadbearing walls. The walls of the structure carry most of the load of the roof. The roof frame structure gives the roof its shape. Many bridges that have to support the weight of heavily laden trucks are also constructed using the same design principles. For example, the Bloukrans Bridge near Nature s Valley in the Western Cape is constructed using trapezium and semicircular geometrical shapes. The Bloukrans Bridge is more than 200 m high and almost 300 m across 6

15 Topic 1 Classroom activity 2 Identifying geometric shapes A semicircle is one half of a circle. A trapezium looks almost like a rectangle except that one of its short sides is slanted. 1 How many semicircles can you identify in the bridge structure on page 6? 2 How many trapezium shapes can you find in the bridge structure? 3 If you could make only two uprights stronger than the rest, which two uprights would you choose and why? In the practical task that follows, you will determine the purpose of each structural member. Practical task 1 Investigating the purpose of roof truss structural members Imagine that a fire broke out in an empty shed and spread to the roof. As parts of the roof truss frame structure burned, they would not be able to support the roofing material. The roof would start to cave in. When the members that bear the most weight are destroyed, the roof structure begins to collapse. Materials: A4 size rolled-up cylindrical paper beams masking tape (or brass paper fasteners) to connect beams together at least five heavy books to place on top of your structure, acting as roofing material a pair of scissors. WARNING: Be aware of the danger of the sharp ends of the pair of scissors at all times. Handle with care. 7

16 Step 1: Roll a sheet of paper tightly around a wooden dowel. Spread some paper glue on the end of the paper. Step 2: Once the paper is rolled up, use a spoon to firmly press down along the glued section. Once dried, carefully slip the rolled-up paper off the wooden dowel. Flatten the ends and use a punch to make a hole at each end. 10 cm 7,5 cm 15 cm 20 cm Step 3: Make several tubes (follow Steps 1 and 2). Check the roof frame specifications as shown in the diagram. Connect the flattened ends of the tubes using paper fasteners. Making a structure Method: Use the paper beams to construct the flat roof truss structure. It should be 20 cm long, 15 cm wide and 10 cm high as shown above. Load the structure by stacking the heavy books, one by one, on top of it. This weight represents the roofing material (in the real world this may be metal, asbestos sheets or clay tiles). Stop loading immediately if you find that the structure has started to buckle under the weight. Test the structure by removing one upright structural member. You can cut it in half using a pair of scissors. Continue weakening the roof truss structure by removing more structural members in turn. Note at what point the roof tumbles in completely. 8

17 Topic 1 1. What can you conclude from this practical task? Write down your findings. 2. If you want your roof structure to tumble quickly, which beams should you remove first? 3. If you want your roof to last as long as possible, which beams should you remove first? 4. Imagine that the roof structure started to fail during a fire. What type of roofing material (roof tiles, roof metal sheets or roof asbestos sheets) would cause the least injuries to people trapped inside the burning house? Something interesting Fire is a hazard that we must always be aware of, regardless of the kind of building. It is dangerous, whether the building is old or new. Take immediate action if you discover any fire. People live in shacks for many reasons. In South Africa, for the last five years, there have been ten shack fires every day. Someone dies in a shack fire every second day. Components of a roof truss The components of a roof truss frame structure include the king post, queen post, strut, tie, rafter and tie beam. King post A king post stands upright on a crossbeam and connects with the apex of a triangular truss. The king post is the most important post and is designed to carry a huge load. It must be able to withstand compressive forces. It can be identified because it is thicker and stronger than any other supporting post. King post 9

18 Queen post Instead of using a king post, two queen posts can be used. These are positioned along the base of the truss and stand upright. However, they do not connect with the apex of roof frame structures. Instead, they provide support for the sloping sides of the truss. For every one king post, there are always at least two queen posts in a frame structure. The two queen posts will share the compressive load of a single king post. That is why queen posts are not as thick and strong as king posts. Queen posts A queen post will allow space for people to walk through the truss. This makes it possible to have an additional room inside the roof. Strut A strut is a rod, bar or beam that forms part of a roof framework. A strut always forms a triangle when connected into a framework structure. It supports beams and sometimes upright columns. A strut is designed to withstand compression. That means it can take a heavy load pushing down on it without breaking. Tie A tie is a structural member that holds other members in place by pulling on them. A tie can be made of rope, but it can also be made of other materials such as wood or steel. Ties are always being pulled on and have to be able to withstand tension forces. load strut beam A strut holds a beam in place by pushing on it. column tie column load A tie holds a beam in place by pulling on it. 10

19 Rafter Topic 1 Any one of a set of sloping beams that form the framework of a roof is called a rafter. Usually rafters extend all the way from the outside supporting wall of the house to the main horizontal beam (ridge) at the top of the roof. Rafters must help support the weight of the roof and are always experiencing compressive forces. hip rafters common rafters Rafters Tie beam A tie beam is a horizontal beam. It connects two rafters in a roof truss and prevents them from separating. A tie beam is always experiencing a tensile force. Tie beams Tie beams connect rafters in a roof truss. 11

20 Lesson 2 Identifying structural members and the forces acting on them All structures have forces acting on them. Tensile and compressive forces Tensile (or pulling forces) and compressive (or pushing forces) act to pull apart and push down on structural members. Remember that the tie always experiences a tensile force, while the strut always experiences a compressive force. a) A tensile force pulls on a member and lengthens it. b) A compressive force pushes down on a member and makes it shorter. Shear forces and torsion forces Shear forces act in a way that pulls parts of a single beam in two opposite directions at the same time. Sooner or later the beam will snap. Think of driving a broomstick firmly into the ground and then pushing on the top part of the stick. While you are pushing the top part of the stick one way, the ground pushes the bottom part in the opposite direction. With time and enough shear force, the broomstick will snap. A structural member experiences a shear force when two forces act on it in opposite directions. These two forces do not act along the same straight line and force the member out of shape. A torsion force twists a beam, either in a clockwise or anticlockwise direction. If this force is big enough, the rigid beam will snap. Tension and compression forces on structural members Applying a force on an object can make that object A torsion force twists a member out of shape. either move or change its shape. Sometimes a force can act for a long time before it has any visible effect. For example walls of a new house hold up the roof with no visible sign of stress. 12

21 Topic 1 As the house becomes older, the more the walls start buckling under the weight of the roof. The roof is quite heavy. Remember that the part of the structure that has a tensile force acting on it is called a tie. The tie holds two parts together and is always being pulled on by these two parts. It is as if these two parts want to stretch the tie. Because the tie does not stretch, the two parts being held together do not move. The part of the structure that has a compressive force acting on it is called a strut. column beam column tie load strut beam load Tension forces act on ties Compression forces acting on a column A long horizontal beam that only has two columns supporting it at its ends could be bent if a weight that is too heavy is placed on it. The bending beam experiences both a tensile and compressive force. Compression forces act on struts A beam resting on a column exerts a downward force on the column. The column is supporting a load and this load squashes the column to make the long column shorter. This downward force on the column is known as a compression force. In the picture alongside, the beam bench is supported by two columns. If a weight is placed on top of the beam, the two columns will experience a downward compression force. heavy load A bending beam experiences both a tensile and compressive force. 13

22 Shear and torsion forces on structural members Two other types of forces that act on structural members are shear forces and torsion forces. A structural member experiences a shear force when a part of it is pulled in one direction while another part of it is pulled in the opposite direction. Look at the picture below. It shows two metal plates that have been riveted together, being pulled in opposite directions. The rivet that holds the plates together will experience a shear force as the plates are being pulled in opposite directions. Shear force in action A solid member that experiences a twisting force is experiencing a torsion force. Think about how you wring out your wash cloth to dry, after taking a bath. As you twist the cloth, it experiences a torsion force. Torsion force in action 14

23 Topic 1 Classroom activity 3 Structural members and the forces acting on them Study the diagrams below. 1 Identify all the structural members (labelled i xii). 2 State whether they are experiencing a tension, compression, torsion or shear force. a) (ii) b) c) (iv) (iv) (i) (iii) (vi) (vii) (v) d) e) (x) f) (xii) (viii) WALL (ix) (xi) Case study Electric pylons Electric power stations are often situated far from the factories and houses that need the electricity. This electricity has to be transported in copper cables for many kilometres. These cables can be buried underground, but if a mountain is in the path of the copper cables, it will create an obstacle. Tall electric pylons are erected above the ground. A pylon is a humanmade structure that is very tall. Pylon structures keep these high-voltage cables high off the ground and are an effective solution for this problem. All pylons have the following in common: they are very tall, sturdy and strong enough to carry heavy copper cables. These pylons support the electrical cables and transport the electricity in this way. The cables are then easier to reach if there is a problem than those cables that are buried underground. 15

24 The Nelson Mandela (Rivonia) Trial In 1963, Nelson Mandela and ten other leading opponents of South Africa s apartheid regime faced trial for their lives. This trial is said to be the trial that changed South Africa. Mandela led a militant wing of the African National Congress called Umkhonto we Sizwe. They knew that electricity was essential to keep a country running. Any interruption in the supply of electricity could cause people to become very upset. These upset people could in turn start an uprising against the government. Their strategy was to blow up electric pylons to bring about a change of government in South Africa. They were careful not to harm anyone in the blast. (Source: adapted from Douglas O. Linder, The Nelson Mandela (Rivonia) Trial: An Account, 2012.) Toppled electric pylon bombed by Umkhonti we Siswe 1. Why do you think pylons must be such tall structures? 2. A structure that stands upright is more likely to topple over, the taller it is. Two ways of preventing a tall structure from toppling is to: (i) make sure the structure is heavier at the base than at the top; and (ii) make the structure wider at the base than at the top. a) Study the structure of a typical electric pylon and see if you can identify which method ((i) or (ii)) above has been used to make the pylon more stable. b) Mention one advantage and one disadvantage of using methods ((i) and (ii)) to secure the pylon. 3. Pylons, unlike the trunk of a tree, are not solid structures. Instead, they are frame structures made up of a combination of many structural members such as posts, struts, rafters and ties. Study one of the pylon structures in the photo above and identify as many different structural members as you can. 16

25 Topic 1 Different designs to solve the same problem effectively High voltage cables that bring electricity from the power stations to our houses are extremely dangerous. These electricity-carrying copper cables should never be touched by people or animals because they will shock and kill them. These cables must be kept out the reach of people, animals and any tall vehicles passing under them. Classroom activity 4 Redesign the pylon that has been around since 1922 Pylons can be seen in different areas in our country. These structures can spoil the views of our beautiful landscapes. Recently, Britain held a competition where designers had to come up with new pylon designs. Here are a few examples of different pylon designs: Various pylon designs Study the pylon designs and then: 1 Place them in order of best design to worst design (according to your taste). Say why you like the best design and why you dislike the worst design. 17

26 2 Arrange the designs in order of strongest pylon to weakest pylon. a) What makes the strongest one so strong? b) Why do you think the weakest one will not support a heavy load? 3 What type of material would you use to make your pylon? Explain. List any ideas about how to redesign a pylon so that its shape looks better but it will still be strong enough to carry heavy copper cables. 4 Sketch your design and point out any internal cross-bracing and triangulation you included to increase the pylon s stiffness. 5 List three structural members in your design that will experience tensile forces and three that will experience compressive forces. Remember: The members that experience the most force must be stronger than the rest. Use of internal cross-bracing and triangulation to provide stiffness A structure must hold its shape even when a big load is placed on it. One way to do this is to use cross-bracing in the structure. Two diagonal support beams inserted into a frame structure cross each other giving the structure extra stiffness. This cross-bracing forms triangles. It is this triangular shape that provides stiffness to the entire structure. You should have seen a number of triangular shapes in almost all the pylon and roof truss structures that we have discussed so far. Look back at some of the diagrams to indentify these. In the next practical task, you will investigate how cross-bracing that forms triangles works to stiffen the shape of a structure. Practical task 2 Making structures more rigid You will need: rolled-up A4 sheets of paper forming cylindrical beams drawing pins to join various beams together triangular pieces (gussets) paper glue. WARNING: Drawing pins have very sharp points. Handle with care. 18

27 Topic 1 1. Use various lengths of the cylindrical beams to construct the shapes shown alongside. 2. Flatten the ends of the beams and use drawing pins to join two beams together. 3. When each shape has been completed, exert the following forces on the shape and records what happens: a) pushing force (directed sideways) b) pulling force (directed sideways) c) pushing force (directed downwards). 4. Suggest improvements to each of the structures. Sketch your suggestions. 5. Now use extra beams to bring these design changes and test the structures by applying the forces again. 6. Record what happened. Did the structure keep its shape better than the last time (or not)? 7. Attach gussets to each corner of your structures. Record whether this design change had any positive effect on the way your structures handled the applied forces. Square shape before being subjected to pushing and pulling forces House shape before being subjected to pushing and pulling forces An alternative to triangulation is to use a gusset plate. A gusset is a flat piece of material used to brace and join the members in a structure. Gussets can come in a variety of shapes. The pictures below show the difference between bracing, cross-bracing and using triangular gusset plates. Square structure being pushed to the right Brace Cross-brace Gusset Using bracing, cross-bracing and gussets to strengthen a structure Which method do you think makes the square-shaped structured most stiff? 19

28 Lesson 3 Structural members under tension or compression Complete the following activity in your workbook. Classroom activity 5 Identifying structural members Study the three frame structures below and then answer the questions that follow. Frame structure 1 A beam supporting a hanging signpost The horizontal beam is held in position by a steel rod. A signpost is attached to the beam by means of two small chains. Wall Under construction A hanging sign post Frame structure 2 The roof of a house The roof beams are under pressure from the weight of the roof tiles on the roof. The floor beam is mounted on top of two walls. Floor beam Roof of a house Frame structure 3 Flagpole The wires on either side of the vertical flagpole are tied down to the ground. wire wire Flagpole 20

29 Topic 1 1 Identify as many beams, struts and ties as possible in all three frame structures on page Indicate what type of force each of the following will experience: a) wall b) signpost beam c) chains supporting signpost d) floor beam e) roof strut f) flagpole g) ties supporting flagpole. Summary In this topic, you learnt about: the definition of frame structures the purpose of structural members (components) in wood and steel roof trusses identifying structural members shear, torsion, tension and compression, which are forces that act on different structural members of frame structures recognising structural members under tension or compression. Extra practice activites You are called in by a ship builder who shows the basic ship design shown alongside to you. 1 How can you stiffen the structural design of this shape? 2 What design changes will you suggest in order to stiffen the structural design of the ship? Explain. 21

30 Topic 2 Structural members What you will learn about in this topic You will: learn the definition of frame structures learn about: steel I-beams (girders), concrete lintels, beam and column bridges alternate bridge supports: suspension bridges, cable-stayed bridges arches in buildings, bridges and dam walls simple and cable-stayed cantilevers learn about the three most likely ways in which structures fail: fracture of a member bending toppling over. Let s talk about this topic We are surrounded by structures. These structures take on many shapes and forms. Each structure is specifically designed to perform a particular function. 22

31 Topic 2 What you already know Members are the parts of a structure that make up the structure. In Topic 1, you learnt that members of a structure have forces acting on them. Therefore, it is very important to choose the best member to do a job in a structure. In Grade 4, you learnt about different structures, including natural, humanmade, shell and frame structures. In Grade 5, you learnt about choosing the correct material for a structure. Structures must be strong, rigid and stable. They must support a load. Examples of structures that support loads are electricity pylons and frames built to hold water tanks. All structures must be able to support the loads for which they are designed. Structures must also be able to withstand forces such as compression. Check yourself 1. Copy and complete the following sentences: a) A structure is something that has been put together in a particular way in order to. b) It can hold, enclose, protect or. 2. Give two examples of structures that are found in nature. 3. Copy the following sentences and supply the missing words: a) In nature, a bird will build a nest to hold its eggs, but supermarkets use to hold eggs on the shelf. b) A spider will spin a web to span a gap, while humans make use of to span a gap across a river. c) A snail has a shell to enclose and protect itself. Humans do not have a shell to protect and cover themselves, so they build instead. What you still need to know Roof trusses and bridges perform completely different functions, even though they have certain structural members in common. In this topic, you will learn to identify the different structural members in a frame structure. 23

32 Unit 1 Structures that span gaps Word bank A B C beam a solid structural member, much like the log of a tree I-beam (girder) a solid beam that has the shape of the capital letter I lintel a solid beam that spans a gap, such as the beam on top of a door or window aqueduct an ancient Roman water canal structure that was used to transport water over long distances cantilever a solid beam that is supported only on one side anchorage anything to which something is fastened Before people used the ruler and other measuring tools, they used their body parts to measure distances. For example, stretching out the fingers on one hand as far as possible and then taking the distance between the tip of the thumb and the tip of the little finger was known as a hand span. One hand span is the distance between the tip of the thumb and the tip of the little finger of an outstretched hand. The Nelson Mandela cable-stayed bridge spans 176 m. As time passed, people starting using the term span to mean the distance between two points. For example, we say a bridge spans 50 metres. This means that the distance between the two main supports of the bridge measures 50 metres. 24

33 Topic 2 The wingspan of a bird is a good indication of whether the bird can glide easily for long distances or fly for short periods of time. Seagulls can glide longer than pigeons because their wingspan is so much wider. Something interesting Your attention span is the time interval from when you begin to concentrate to when you start losing concentration. When your teacher talks and you listen, you are busy concentrating. Your teacher continues talking but at some point you start thinking about something completely different, now you are losing your concentration and are no longer focused. Your concentration has ended. The listening time is a measure of your attention span. Normally, learners who get good grades are those with a long attention span. Learners with a long attention span normally do better in exams. 25

34 Lesson 1 Beams Any member of a structure that must resist the force of bending is called a beam. Beams offer support to a load and they do so while spanning a gap. A log bridge across a river is a good example of a beam. The picture on the right shows a beam over which a hiker is crossing a river. Clearly the beam must be strong enough to support the weight of the hiker. If it is not strong enough, it will snap and the hiker will fall into the water below. A hiker crossing a gap by walking across a fallen tree that lies across the gap; the tree can be considered to be a beam The upper half of the beam feels a weight acting down upon it. It is under a compressive force. But what do you think is happening to the lower half of the beam while the hiker is crossing? Look at the illustration alongside. It shows how the lower half of the beam is being stretched as the beam sags under the weight of the hiker. upper section of the beam experiences compression support load lower section of the beam experiences tension span support The upper section of the beam experiences compression while the lower section experiences tension. Similarly, any member in a frame structure that is being stretched experiences a tensile force. When building a long bridge it is important to construct the beam in such a way that parts of the beam can withstand compression, while other parts of the same beam can withstand tension. This is not easy to achieve, and bridge-building engineers have to be well informed on the subject. 26

35 Topic 2 Classroom activity 1 Strengthening a beam structure Task Make a beam using only cardboard. This beam must be able to support as many matchboxes filled with dry sand as possible, without completely sagging under the weight. You may use only three A4 sized pieces of thin cardboard (manila board), glue and sticky tape to make the beam structure. Your beam must span a gap of at least 200 mm while supporting the matchboxes. All matchboxes must be placed in the middle of the supported beam. circular beam semi-circular beam corrugated beam Challenge Triangular beam Square beam I-beam Possible beam designs 1 Sketch the design of your choice. It may be one shown in the picture above, or a combination of two or more. Your choice of beam design may be nothing like those shown above. It might be a totally original design. Include all sketches in your project report. A simple rough sketch will do. 2 The strengths and weaknesses of your design must be pointed out before a good decision can be made. List these strengths and weaknesses in your project report. 3 When you have made a decision about your design, collect your three A4 sized cardboard pieces from your teacher and start to constructing your beam. 27

36 4 When your beam has been completed, set up the two supports. Two bricks, spaced 200 mm apart, should work well as supports for your beam. Lay your beam down across the two supports. Place matchboxes filled with sand one by one on top of the beam. Did your beam handle the extra weight or was it in danger of buckling and collapsing? 5 Based on your observations in 4 above, suggest improvements to your beam design. What could you have done differently to make it stronger? 6 The beam that sags the least while carrying the most weight (number of filled matchboxes), wins. The final two points in your report should note which design performed best (provide sketches) and how this winning design was different from your design. The winners will say why they think their design won and why the others lost focus only on the design when writing your report. Girders (steel I-beams) The I-beam is so called because when you look at the beam end-on, it appears to be a capital letter I. It is also sometimes called an H-beam. This shape is ideal for carrying bending and shearing loads. Many railway stations (or old warehouses), use I-beams. The steel railway track is actually a very long I-beam. Look at the photo alongside. Take a look around you when you are visiting different places and see whether you are able to recognise the I-beams used in different constructions. I-beam web flange Railway track The steel I-beam structure is used in many of the tall skyscraper buildings in our big cities. They provide the structural support and stability that is needed for a tall structure. The photograph, by an unknown photographer, depicts eleven men eating lunch, seated on a girder with their feet dangling hundreds of metres above the New York City streets. The photo was taken on 20 September 1932 on the 69th floor of the RCA Building during the last months of construction. 28

37 Concrete lintels Topic 2 Doors and windows are important parts in the construction of a house. They allow people and sunlight to enter the home. However, there is a section above the door and window that has to be supported as the building of the house continues upward. One way of solving this problem is to include a lintel or an arch (which we will deal with later in the topic). Lintel The lintel is simply a solid horizontal beam that spans the gap above a door or a window. The two ends of the lintel transfer the weight to the walls supporting it on either side. Since the lintel must be able to support part of the weight of the wall above the gap, it must be stiff and strong. It must not buckle under the weight placed on top of it. Lintel above a door The lintel, like any beam that spans a gap and has to support a weight, will experience two types of forces at the same time. It will experience compressive forces on its upper section and tensile forces on its lower section. In years gone by, people used natural materials such as hard dense tree trunks and quarried stone to make lintels. Hardwood is a good material but it can rot and burn, leading to the collapse of parts of the building structure. Stone is longer lasting and is able to handle forces that weigh down on it. However, it does not cope well with forces that want to stretch it. Modern-day buildings use concrete lintels. They are strong and can be reinforced with steel rods placed inside their bottom sections. In this way, the bottom section is more able to deal with tensile forces that the lintel has to bear. Using concrete lintels also means that fewer hardwood trees have to be chopped down. It is much easier to cast a concrete lintel into a decorative shape than for stonemasons to carve a horizontal beam out of quarry rock. 29

38 Beam and column bridge The beam bridge was probably the earliest way of spanning a gap across a river. A fallen tree probably formed the first natural beam bridge. A beam bridge is the simplest design for crossing a distance. It consists of a horizontal beam supported at each end by columns (piers). The weight of the beam pushes straight down on the piers. The farther apart its piers, the weaker the beam becomes. This is why beam bridges rarely span more than 80 metres. When a weight is placed on a beam, it exerts a load. However, the weight is The Kaaimans River Bridge, near George in the Western not directly in contact with the ground. Cape is a fine example of a beam bridge. The load reaches the ground by passing along the beam and down the supporting columns, as shown in the picture alongside. Advantages of a beam bridge quick and easy to construct easier to fix when repair work is required relatively cheap to construct and maintain. Disadvantage of a beam bridge cannot span great distances. load Loading a beam bridge load 30

39 Lesson 2 Alternative bridge supports Topic 2 The simple beam bridge has undergone quite a few changes. The biggest and most striking change is the use of an arch structure to add support to the bridge. Arch support in bridge construction Suspension bridges A suspension bridge has at least two queen posts. Two very strong cables are spun between these two towers, one on either side. The bridge deck is then suspended from these strong cables. The deck hangs from the cables. Cars travel on the bridge deck as they cross the bridge. Cable-stayed bridges A cable-stayed bridge looks much like a suspension bridge. It also has one or more columns (usually called towers or pylons) with metal cables supporting the bridge deck. These support cables are all tied directly to the tops of the towers and the bridge deck. There is no single cable that spans between the two towers as in a suspension bridge. Suspension bridge These bridges are much stiffer than suspension bridges. The bridge is less likely to sway from side to side in a heavy wind. Compared to other bridge types, the cable-stayed bridge is best suited for spans longer than cantilever bridges, and shorter than suspension bridges. Cable-stayed bridge in Amman, Jordan 31

40 Lesson 3 Arches (in buildings, bridges and dam walls) One of the earliest known construction methods used to span a gap is the arch. Before the invention of the arch, the only way to span a gap was with a wooden or stone beam between two supports. However, with this method there is a limit to what the length of the span could be, because a flat beam bends in the middle (and breaks if too much weight is placed on it). Arches were built because they formed a strong structure to frame the top of openings in a wall, such as windows and doors. The ancient Romans also used arches to support roadways (bridges) and in many of their structures that were designed to carry water over vast distances. These water canal structures are known as aqueducts. Something interesting Aqueducts operate on a very simple principle. The Romans just built an extremely long canal that was higher at one end and gradually became lower until it reached the city that needed water. They didn t bother with complicated pressure systems or pumps they relied on gravity. The water sources for these aqueducts were natural bodies of water, such as lakes and springs. The water was then poured into cisterns (large structures for storing water). With a constant supply of fresh water, the populations of the cities were healthy and could thrive. Arched doorway lintels Arched bridge supports Arched aqueduct frame structure Roman architectural arches One advantage of an arched structure is that it experiences little or no tensile force. This is useful since many building materials such as stone, cast iron and concrete can handle large compression loads but not tensile forced. The Roman arch (which has a hemispherical shape) can support a huge load. 32

41 Topic 2 Under compression it wants to fall inwards from each side. This in turn squeezes the top of the arch tight, holding it up. It is as if the more weight loaded on top of the arch, the stronger the arch becomes. Both an arch structure and a beam structure spans a gap. The beam structure requires materials that can handle both compressive and tensile forces, whereas an arch structure requires materials that need to withstand only compressive forces. Let s see why that is so. Forces that beams experience when loaded Imagine the long beam as one that consists of smaller chunks all connected to one another. Long rectangular beam If a small load is placed on the beam, the beam can easily transfer the load to the two supporting columns on either side of it. It does so and still keeps its shape. When you apply a large load at the centre of the beam, it causes the beam to buckle. Load Buckled beam Each chunk at the upper section of the beam is squashed narrower as the beam buckles. This means that this section is under compression. Each section at the lower section of the buckled beam is stretched wider. This means that this section is under tension. 33

42 Forces that arches experience when loaded An arch consists of smaller wedge-shaped chunks all connected to one another. Both small and large loads are easily transferred to the two supporting columns on either side of the arch. Only compressive forces are experienced by the arch and not tensile forces. This is why arches can be constructed with a wide variety of materials that are not suited for beam construction. Lesson 4 Cantilevers A cantilever is a beam that is anchored at only one end. This is unlike the simple beam bridge where the beam is supported at both ends. The beam of the cantilever bridge carries the load to the single support. A balcony is a typical example of a cantilever structure. When you step on a balcony, your weight is transferred by the balcony to the wall supporting it. load supporting walls wall An arch transfers any load on top of it to the two supporting walls on which it rests. Each wall has to bear only half the load. Simple cantilever bridge A simple cantilever bridge has two cantilevers extending over a gap from either side. The gap in the centre is simply a beam that is suspended from the ends of these cantilevers. Balcony cantilever Simple cantilever bridge with middle section suspended between two cantilevers 34

43 Cable-stayed cantilever bridge Topic 2 The cable-stayed cantilever bridge is very similar to the simple cantilever bridge. The big difference is that the centre piece has a single king post onto which cables are attached. These cables act as the main support for the centre beam. cable stays Cable-stayed cantilever bridge Classroom activity 2 Case study In order to explain the principle involved in his proposed cantilever bridge design, Benjamin Baker set up a human cantilever as an example. His idea is shown in the photograph taken at the lecture that he gave. The background shows a diagram of the Forth Bridge drawn to the same scale as the human cantilever. Two men sat with their chairs positioned in front of the main columns (piers) they represented. On either side of them were a pile of bricks A human cantilever; the Forth Bridge, a cantilever bridge, is in the background that simulated the anchorage support provided to the columns. The outstretched arms of the seated men were supported by wooden struts that were butted against their seats. The tops of the two outer wooden struts were steadied by guy ropes down to the anchorages. A central span was suspended between the top ends of the sticks held by the two men s inner hands. 35

44 This arrangement easily supported the weight of a third person, seated on the central span. The arms and ropes carried tensile forces, whereas the body of each man and the wooden struts carried compression forces. 1 Do you think it would be possible to set up a similar human cantilever demonstration in the class? 2 a) In this human cantilever, must the two seated people always be big and strong while the person being supported always be small and light? b) Can it be the other way around? c) Will the human cantilever work if the seated people are small and weak while the supported person is big and heavy? d) What do you think? Challenge The third man chosen to sit on the central span of the human cantilever was a young engineering student, Kaichi Watanabe. Find out more about this man and why he became famous. A good place to start your research is: news/scotland/glasgow_and_west/ stm 36

45 Topic 2 Unit 2 Structural failure Word bank topple fatigue elastic A B C to lean over and then fall weakened material as a result of stress capable of returning to its original shape after being stretched, deformed, compressed or expanded When a structure is placed under severe stress, it can fail and collapse. The three most common reasons why a structure fails are: the material fractures (tears apart) the structure starts to bend a bit and eventually buckles the structure simply topples over. Engineers need to ensure they know what forces their structure is meant to cope with. They also need to consider the forces of nature which can be very big and are also often unpredictable. If they do not consider all forces, people inside the structures could get hurt or die. Toppled flats in Manenberg, Cape Town, after a tornado In the following lessons, you will learn more about the three most common reasons why structures fail. Lesson 1 Fracture Often, when we hear the word fracture, we think of a bone fracture. This is when a bone in the body breaks and the two parts of the bone have shifted apart. This can happen when the bone is not strong enough to withstand some huge force, such as an accident or a fall. A tear in the bone is an indication that it will completely fracture if more force is applied. A small tear in bone can later lead to complete fracture. 37

46 In the same way, we say that a structure has fractured when parts of the structure have completely broken apart. It is as if parts of the structure became tired (fatigued) after carrying a heavy load for too long and tore apart. Usually, some tearing occurs before the structural member completely fractures. These small tears in the structure are early warning signs that the structure will soon completely fracture. Bending is another good indicator that a structure will fracture if you continue applying force. The wing structure of an aeroplane is regularly checked for any tears. These early warning signs show that the wing is starting to lose its strength. If any tears are found, the aeroplane will be grounded until the problem is fixed. Lesson 2 Bending A horizontal beam bridge will probably bend when too heavy a load is placed on it. The beam loses its stiffness and buckles under a large load. This is true of all beams and supports in a structure. An elastic-type material will return to its original shape when the force is removed. A non-elastic type material will be permanently deformed by the bending even after the force causing it to bend has been removed. However, if a force is too large, then even an elastic-type material will reach its breaking point. Thereafter it just starts stretching and becoming thinner and weaker. It will eventually break under the huge force. Something interesting Plastic, when stretched too far, will become thinner and eventually break. Scientists have produced a plastic that turns red when it is stretched too far. They want to coat plane wings and bridges with a layer of this plastic. It is colourless when not stretched. When the wing or bridge stretches with age, the plastic layer will turn red and this will be an early warning to engineers that the structure is getting weak. Fantastic Plastic changes colour when it is close to its breaking-point. Can you see where the stretched plastic begins to develop a small tear? That tear eventually leads to a complete fracture as the plastic is stretched too far. 38

47 Topic 2 Classroom activity 3 Case study Skyscrapers that sway The tops of skyscraper buildings sway from side to side. They move up to 30 cm in either direction in high winds. This side-to-side motion actually protects the skyscraper from damage during highwind gusts and earthquakes. If they were not capable of bending slightly and swaying, they would simply topple over completely. Toppling over is the last thing that we want to happen to a skyscraper full of people. Despite the fact that the sway is sometimes a few metres in either direction, you may barely notice the smooth, almost rhythmic movements. This is because engineers go to great lengths to design skyscrapers in such a way that the people inside feel no disruption. Generally, a tall structure needs a wide base in order to be stable. Skyscrapers are very tall buildings that do not have a particularly wide base. How do these skyscrapers manage to be so stable if they do not have this wide base for support? Lesson 3 The narrow base bus will topple over because its centre of gravity has shifted beyond the wheel that is balancing the bus. Toppling over The wide base bus will not topple over because its centre of gravity has not shifted beyond the wheel that is balancing the bus. A tall, narrow structure has the tendency to topple over when pushed. It topples over even more quickly when the top section of the tall structure is heavier than its bottom section. We say the structure is top-heavy. 39

48 The Eiffel Tower and the Great Pyramid of Giza, as shown below, are both tall structures with a very wide base. The wide base ensures that these structures are bottom heavy and stable. If any of these structures were to fail, it would not be because of toppling over. Eiffel Tower Pyramid of Giza Summary In this topic, you learnt about: structural members that span over space, which include: steel I-beams (girders), concrete lintels, beam and column bridges alternate bridge support: suspension bridges, cable-stayed bridges arches in buildings, bridges and dam walls simple and cable-stayed cantilevers the three most likely ways in which structures fail: fracture of a member due to a lack of strength bending due to a lack of stiffness or rigidity toppling over due to a lack of stability because of the top being too heavy or the base being too narrow. Extra practice activities Your group will be given two complete decks of playing cards (104 cards in total). Your task is to build as tall a structure as possible using the playing cards. Use structural design principles of wide bases, avoiding a top-heavy structure and aim for structure stability. 40

49 Topic 3 Purpose of graphics What you will learn about in this topic You will: learn about the purposes of graphics learn about different drawing conventions use working drawing techniques for planning: single view flat 2D drawing with dimensions, line types and scale isometric using underlying isometric grid and simple instruments learn to use double vanishing point perspective with colour, texture and shading in: sketching using pencil, ruler and blank paper enhancing drawing to promote realism using colour, texture, shading and shadows. Let s talk about this topic You can define Technology as the use of knowledge, skills, values and resources to meet people s needs and wants. You do this by developing practical solutions to problems. You must take social and environmental factors into account throughout this process. When you develop a practical solution, many different ideas have to be generated and the most suitable one needs to be communicated to the people who are experiencing the problem. 41

50 What you already know You can use words to explain what the possible solution will look like. However, it is much better to have a drawing of what it will look like. It communicates your ideas more clearly than just using words. The technological process has communicate as one of its major elements. Investigate Evaluate Design Problem The design process skills Communicate Make The purpose of graphics in technology is for the development and communication of ideas. We give structure to thoughts and ideas through the use of graphics. Check yourself 1. When do you use thick, dark lines when drawing? 2. When do you use thin, feint lines? 3. When do you use dashed lines? 4. Explain what is meant by single vanishing point perspective. 5. How does a 3D drawing differ from a 2D drawing? What you still need to know Henry Ruiters makes a living as a designer. He designs many different kinds of products that are used by the South African Navy. These include radio antennae, underwater vehicles and torpedo launches. Henry Ruiters does technical sketches at IMT 42

51 Topic 3 Henry listens carefully to the client, who is an officer from the navy. Henry then puts a few possible designs down on paper. These are normally quick, rough drawings so that he can make sure that he understands what the client wants. He does not want to make a mistake and design something that the client does not want. When the client is happy with one of the possible designs, Henry takes time to draw up a set of detailed plans. The workshop technicians will follow Henry s plans to the last millimetre. There is no margin for error in the navy. The safety of the nation depends upon it, explains Henry. To make sure that his designs are not misinterpreted by the technicians, Henry will include drawings from all different angles. These are the front view, side view and 3D perspective view. He even includes oblique and isometric drawings of the final product. Sometimes designs can be misinterpreted. Look at this illustration: Marketing s concept How Operations installed it What Sales wanted The designer s proposal What the customer really required Plans and photos of a High Frequency Radio Antenna Henry also includes an artistic drawing so that his client can see what the product will look like when it is completed. This drawing is almost like a photograph. Because the product has not yet been built, the drawing gives the client a good idea what he or she will be paying for. 0URPOSE OF GRAPHICS s 43

52 Unit 1 Drawing conventions Word bank A B C technical drawing a drawing that is different from an artistic drawing, it shows how something is designed construction lines lines that show how a product is constructed scale a ratio to show the size of something, compared with something else Every line on a technical drawing (working drawing) has meaning. We use only thick, dark lines to outline the product. Thin, feint lines are used for construction lines. If parts of the object are hidden, then we use dashed lines to show that hidden detail. Whenever a piece of material has to be folded, then the line around which the folding must take place is represented by chain dash-dot lines. The centre of a circle is indicated by the intersection of two short dashed lines. Type of line Purpose of line Example Broken lines Show hidden detail Centre lines Show centre of a circle Construction line Object line Feint lines used to aid construction process Shows outline of an object Table with a summary of conventions 44 Scale 1:20 An image created using technical drawing conventions

53 Topic 3 The drawing should display a scale that will allow everyone to get an idea of how big the final product will be. It is impossible to draw a 20 m long side of a ship on an A4 page without using an appropriate millimetre scale. The dimensions of every drawn line must be indicated so that the technicians can scale up correctly when building the real object. Classroom activity 1 Incorrect drawing conventions A number of drawing conventions are incorrect in this drawing. Can you identify them? An image in which technical drawing conventions have been used incorrectly 0URPOSE OF GRAPHICS s 45

54 Unit 2 Working drawing Word bank A B C 2D a two-dimensional drawing is a drawing on a flat surface isometric drawing 3D drawing that show three sides of an object, all in proportion to one another double vanishing point (VP) perspective a realistic way of drawing objects in 3D two point perspective uses two vanishing points on the horizon To practise these conventions, you will use the simple computer stiffy box as your object. Study the following pictures of the stiffy box. Measurements: Closed: 115 mm long, 50 mm wide, 100 mm high Open: 115 mm long, 50 mm wide, 65 mm high (front), 100 mm high (back) Single view flat 2D drawing Two-dimentional (2D) drawings show two sides of an object. Front view outline and construction lines Single view flat 2D drawing 46