DESIGN & TECHNOLOGY UPPER SECONDARY
STRUCTURES
Understand what a structure is. Understand the need for manufactured structures. classify natural and man-made structures as in plants, trees, honeycombs, webs, animal skeletons vs. that of bridges, cranes, pylons, roofs, domestic furniture
7051 syllabus-intro
Structures are in all shapes and sizes; from a molecule to the largest volcano. But they have 3 things in common :
Molecules
Volcano
(1) They must resist forces (2) They must not collapse during use (3) They must hold parts in the right place
STRUCTURES
ď‚— Structures
are things that are made or exist naturally to resist forces or loads.
Definition of Structures
STRUCTURES EVERYWHERE Structures can be big and small Show can samples to students
STRUCTURES EVERYWHERE Can you name these famous structures?
ANCIENT STRUCTURES How were they build? Which country can you find them?
MODERN STRUCTURES
FUTURE STRUCTURES
QUIZ NO:1 The pictures earlier show mammoth future structures in mega projects. With the advance of technologies, the structures can be constructed. But do you think that structures in future will also become smaller and smaller? Discuss about a few structures that have shrunk over the years. (eg. Diskette to memory card)
STRUCTURES
NATURAL & MAN-MADE STRUCTURES
NATURAL STRUCTURES Spider Web
Ant Hill
What about our skeleton ?
Beaver’s Nest
Leaf Show leaf samples to students
There are also many structures found in nature. These natural structures have evolved over years due to the changes in the climate and the environment.
MAN-MADE STRUCTURES Thumb drive
Human Transporter
Bullet Train
Swimming pool
A structure that satisfy one or more of the functions of a structure and does not exist in nature will be classified as a man-made structure.
QUIZ NO:2 Where, in nature, do you think the designers of this structure get their ideas from?
QUIZ NO:3 Why does the swimming pool classified as a structure? The walls of the swimming pool satisfy the conditions of a structure: 1.Able to resist loads and forces of the water acting on its walls. 2. Does not collapse when the pool is filled with water. 3. The walls stand erect.
STRUCTURES ACTIVITY 1 Construction of a tall structure Challenge : You are to construct a standing structure using the materials provided on the table. The structure is to be as tall as possible and able to stand firmly. Discussion : How does each part of the structure support one another? Do the joints play an important role? Duration : 10 mins
STRUCTURES
FRAME & SHELL STRUCTURES
FRAME & SHELL STRUCTURES Man-made Structures
Frame Structures
Members joined to form frames
Shell Structures
Outer shell provides strength
NATURAL FRAME STRUCTURES Frame Structures
Spider Web
Skeleton
Bird’s Nest
The frame provide support and strength. It provides protection to maintain its form.
MAN-MADE FRAME STRUCTURES Frame Structures
A frame structure is constructed by joining separate pieces of materials, called members to support and protect various parts. The frame structure provides support and strength for the house.
Structural frame building
Scaffolds
Truss roof
MAN-MADE FRAME STRUCTURES Frame structures often consist of multiple pieces of members joining together to hold up the structures.
Before
member
4 members are joined to form a simple frame structure.
After
members
Add a diagonal member to Strengthen the frame structure.
NATURAL SHELL STRUCTURES Shell Structures
Terrapin shell
Snail shell
Egg shell
Coconut shell
The hard shell provides support, strength and protection.
SHELL STRUCTURES Shell Structures
Safety helmet
Litter bins Vacuum cleaner
Drink can
The outer shell provides support and strength. They do not have a frame.
QUIZ NO: 4 Does the computer external cover qualify as a structure? Yes. -The external cover is the outer shell that provides support and strength. -It also protects the internal parts of the computer.
QUIZ NO:5 Designers look to nature for inspiration when creating today’s man-made structures. Where in nature, do you think the designers of this structure get their ideas from?
Honeycomb.
EGG TRAY
NATURAL VS MAN MADE STRUCTURES
Natural Structures • Materials - sticks, grass, string, paper, and pine needles. • Bonding agent - Materials are bonded with mud, sticky saliva, tent caterpillar silk and spider webs.
Man-Made Structures • Materials - bricks • Bonding agent - Materials are bonded with mixture of sand and cement
FUNCTIONS OF STRUCTURES
FUNCTION OF STRUCTURES 1. SPANNING - BRIDGES Beam Bridge
Suspension Bridge
Truss Bridge
Arch Bridge
Cable-stayed Bridge
FUNCTION OF STRUCTURES 2. SUPPORTING
FUNCTION OF STRUCTURES 3. PROTECTING
FUNCTION OF STRUCTURES 4. CONTAINING
STRUCTURES ACTIVITY 2 Construct and test a Bridge using computer modeling; West Point Bridge Designer. Procedure : Download the West Point Bridge Designer programme from http://bridgecontest.usma.edu/download.htm. Construct a truss bridge and test the bridge using the computer model. Discussion : Identify which cross-section of the truss members will make the strongest bridge. How do you compromise between cost and strength? Duration : 15 mins
Explain the terms ◦ Loads ◦ Forces (tension, compression, bending, shear and torsion) ◦ Struts ◦ Ties ◦ Beams & cantilever beam ◦ Describe their relation to structures
7051 syllabus-Forces
LOADS AND FORCES
FORCES 2 types of forces acting on a structure
External forces External forces on the structure.
Internal forces Internal forces act within the structure.
FORCES
Tower crane-strong enough to support weight and the containers
Rare Earth Magnets
High pressure steam in power station provide force to drive the turbines.
Tops of skyscrapers can sway due to strong winds blowing against them
TYPE OF LOADS
Loads Static Loads Loads that are stationary & has constant size, position and direction on or within a structure.
Dynamic Loads Loads that are in motion & changes in size, position or direction.
QUIZ NO:6 The term forces also known as loads and vice versa. But what’s the difference between forces and loads? Forces are external forces applied to a structure. Loads are forces due to gravitational pull or weight applied on a structure vertically through the centre of gravity of the structure.
QUIZ NO:7 Bridges are subjected to dynamic loads most of the times as vehicles continuously cross over it. How do engineers prevent the bridge from collapsing due to the dynamic loads?
Both ends of the bridge or one end will be supported on rollers. The rollers will be able slide and will not cause a structural damage At the ends.
FORCES & LOADS SI UNITS OF FORCE
NEWTONS (N)
Force = Mass (m) x Gravitational pull (g) Where
Mass (m) in kg Gravitational pull in 9.81 m/s2 (or 10 m/s2)
Example: Amount of force produced by 1 kg of weight on a table = 1kg (mass) x 10 (g) = 10N 1,000 N = 1kN (kilonewton) 1,000,000 N = 1MN (meganewton) 1,000,000,000N = 1 GN (giganewton)
TYPE OF FORCES 1. Compression
Compression – part of the structure is squashed. member
In compression, the member of a structure (eg: leg of a table or truss in a bridge) will experience forces pushing towards its centre from two or more directions. Forces that cause compression are called compressive forces. A member experiencing compression is known as a strut.
TYPE OF FORCES 2. Tension member
In tension, the member will experience forces pulling away from its centre from two or more directions. Another word for tension is stretched. Forces that cause tension are called tensile forces. A member experiencing tension is known as a tie.
TYPE OF FORCES 3. Torsion
Torsion- part of the structure ‘twists
member
An member in torsion will experience a ‘twisting’ effect from the torsional forces. A force that causes torsion in a structure is called torque.
TYPES OF FORCES 4. Shear
member
Forces that causes a member of a structure to try to slide past another part are called shear forces.
TYPE OF FORCES 5. Bending
member
A member in bending often experience compressive forces on one surface and tensile forces on the opposite surface. In the case above, the member would experience compression on the top surface and tension at the bottom surface.
STRUCTURES ACTIVITY 3 Video presentation on forces. Procedure : Make a 2 mins video presentation explaining the FIVE forces acting on a structure. Use your mobile phone camera to record your presentation. Use your creativity and innovation to present your findings. Discussion : Do you think that a structure will experience only one type of force or a combination of other forces? Provide examples. Duration : 15 mins
QUIZ NO:9 A concrete beam is strong in compression but weak in tension. When a concrete beam is subjected to bending forces, the lower part in tension will crack. How do engineer’s strengthen it?
Engineer’s add steel bars inside the beam at the tension area that is at the bottom layer.
CANTILEVER BEAM Cantilever
Warehouse Storage Shelves A cantilever is a beam that is supported only on one end. It isn't the strongest structure but is used as a structural element when the designer does not want to see a support under an object.
QUIZ NO:10 Study the diagram of a computer desk. Each member of the structure is under some type of force. PART A: Is in tension because the weight of the computer is stretching it.
PART B: Is under compression because the weight of the computer unit and the members above that make up the desk, are pushing downwards and compressing it. PART C and D: This is the same member but on the inside compression is taking place and on the outside it is being stretched (under tension).
ď‚—
apply the concept of equilibrium as a result of applied load and reaction
7051 syllabus-equilibrium
STRUCTURES IN EQUILIBRIUM
EQUILIBRIUM
There are forces balancing the structures
EQUILIBRIUM Why the objects do not fall?
The forces acting downwards is/are equal to forces acting upwards.
EQUILIBRIUM
200 N
200 N
5N 10 N
5N 10 N
400 N
If several forces are applied to an object and the object remains stationary, or if the object continues to move with uniform velocity, the forces are said to be in equilibrium. Forces in equilibrium add to produce a resultant of zero. Equilibrium Means "Zero Acceleration"
VECTORS A force can be represented in a line diagram called a VECTOR. Vector diagram  Each length of the line, drawn to scale, represents the size of the force  The arrow head and angle of each line represents the direction of the force F1 = 20 N
1 cm F2 = 40 N 2 cm F1 + F2 = 60 N 3 cm
SCALE : 20 N to 1 cm
VECTORS Parallelogram of Forces Force A = 60 N Direction of force is 45 0 to force B Force B = 40 N Direction of force is horizontal
40 N (2 cm)
Scale: 200 N to 1 cm Find resultant force.
45 0 60 N (3 cm)
Resultant force
= 4.7 x 20 = 94 N
4.7
cm
.
VECTORS Scale: 200 N to 1 cm
Triangle of Forces The triangle of vectors addition method is : 1. Draw 40 N to scale and in the direction of its action.
40 N (2 cm) 45 0 60 N (3 cm)
From the nose of the 400 N force vector draw 600 N force vector using the same scale and in the direction of its action. 3.
The resulting vector is represented in both magnitude and direction by the vector drawn from the tail of 40 N vector force to the nose of 60 N vector force.
4.7
cm
Resultant force
= 4.7 x 20 = 94 N
MOMENTS
State of Equilibrium Anti-Clockwise moments = Clockwise moments (Rotational Balance)
MOMENTS
Moment = force x perpendicular distance
A 1000N load, at a distance of 15m from the fulcrum causes a moment of 15000 Nm. The further the distance between the load and the fulcrum, the greater is the turning effect.
To stop the crane from toppling over, a counterbalance is used on the counterjib on the other side of the fulcrum.
QUIZ NO:10 A 2m long shelf weighing 20N supports a speaker weighing 40N placed 0.5m from the left hand edge. Find the reactions at the supports.
Solution to Quiz no:10 Weight of the shelf is acting at its centre. Let reaction force at right support be Rr Assuming moments at left support= 0 Rr x 2 = 40 x 0.5 + 20 x 1.0 Rr = 20N ď‚—
Therefore the Reaction force on the right support is 20N. Since total downward force is 60N, the Reaction force on the left support is 60N-20N = 40N
use different methods of reinforcing such as ◦ ◦ ◦ ◦ ◦ ◦
Gussets Ribs Braces Laminating Honeycomb triangulation
7051 syllabus- reinforcing
REINFORCING STRUCTURES Braces RIGIDITY
Stiffened fold
Lamination
Ribs MATERIALS
Gusset plates
Honeycomb
REINFORCING STRUCTURES 1. Rigidity • triangle is one of the strongest shapes • provide strength & rigidity •Unlike a circle or rectangle, it will not distort easily
TRIANGLES
REINFORCING STRUCTURES 1. Rigidity-Frameworks • TRUSS – triangular parts to provide strength and rigidity. Examples: Supporting roofs & Bridges
Frameworks made of triangular parts are called triangulated frameworks. Example: Geodesic dome
ACTIVITY NO: 4 Activity Build 4 types of polygons as shown and test the rigidity of each shape. While all of the other polygons can be bent into many different forms that are NOT regular polygons (with many different angles in each polygon), the triangle always keeps the same shape. It is the strongest polygon. Why is that?
ACTIVITY NO:4 SOLUTION The reason is because in all of the other polygons, all of the angles can change. There is nothing to stop them. However in the triangle, the angles can not change once the triangle is built. The angles are fixed. This is because a triangle has three sides and three angles, and each angle is fixed by the side opposite to it. If you look at the following picture, you can see that there is only one angle where the two free sides can connect to the third side, and that once connected, all of the angles are fixed
REINFORCING STRUCTURES 2. Stiffened Folds A material can be stiffened by folding.
Because of the fold, the can is able to withstand 70 kg of mass.
REINFORCING STRUCTURES 3. Lamination Thin sheet materials (are generally weak) can be strengthened by laminating several sheets together.
Layers of laminate Laminated sheets of wood were used to stiffen the back rest of a seat and a table top.
REINFORCING STRUCTURES 4. Honeycomb • Light but strong when under compression • Used in cavities of doors- sides of door apart and keeps the weight of the door to a minimum. Top left corner of a honeycomb door
REINFORCING STRUCTURES 5. Gusset Plates Used to add strength to a joint
Gusset Plates
They help to distribute the stress of the joint more evenly by increasing the area of the joint.
REINFORCING STRUCTURES 6. Ribs Used to add strength to a wall of a structure
Ribs They help to stop the walls from caving in. They allows objects to be light but strong.
REINFORCING STRUCTURES 7. Braces Add strength to a joint especially in brackets and cantilevers
Braces
STRUCTURAL FAILURE **HIGHER THINKING ORDER
Many factors contributed to a structural failure: •Poor selection/wrong use of material. •Lack of understanding of the forces involved in design. •Failure of a joint. •Fatigue resulting from changes in the properties of a material over time. •Excessive loads, more than the design load. •Incomplete knowledge of the conditions a structure is likely to face.
MODELLING MATERIALS 1. BALSA Very light hardwood that is easy to cut and shape.
2. PLASTICS SECTIONS & TUBINGS Ideal for modelling hitech furniture. Available in range of colours and sizes.
MODELLING MATERIALS 3. ARTSTRAWS Light and can be painted
4. Corrugated plastic sheets (Corriflute) Good for modelling shell structures
MODELLING MATERIALS 5. FOAMBOARD Light, rigid material made from a sheet of foam sandwiched between two sheets of card.
6. RIPSTOP NYLON Flexible, tear resistant plastic sheet
JOINING OF MATERIALS 1. SCREWS AND NUTS 2. PLASTICS RIVETS 3. SPLIT PINS
4. HOT GLUE (semipermanent joint)
JOINING OF MATERIALS 5. DOUBLE SIDED TAPES 6. EYELETS / POP RIVETS 7. SPOT WELDING
End of structures module