Lesson Notes By Weeks and Term v5 - Grade 9
Structures: advanced structural systems and forces – Week 4 focus
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Structures: advanced structural systems and forces – Week 4 focus
Download the Lessonotes Mobile South Africa app for faster lesson access on Android and iPhone.
Subject: Technology
Class: Grade 9
Term: 1st Term
Week: 4
Theme: General lesson support
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This week, we delve deeper into the world of structures, moving beyond basic shapes and support systems to explore more advanced structural systems and the forces that act upon them. Understanding these concepts is crucial because structures are all around us - from the houses we live in and the schools we attend, to bridges, dams, and even the cell phone towers that connect us. These structures need to be strong and stable to withstand various forces, ensuring the safety and well-being of our communities.
2.1 Advanced Structural Systems Trusses: A truss is a structure composed of members connected at joints to form a rigid framework. These members are typically arranged in triangular units because triangles are inherently stable. Trusses are incredibly efficient at distributing loads, making them ideal for bridges, roofs, and towers.
Advantages: High strength-to-weight ratio, efficient use of materials, capable of spanning long distances.
Disadvantages: Can be complex to design and fabricate, susceptible to buckling if members are not adequately supported.
Example: Imagine the roof structure of a large warehouse or shopping centre in South Africa. These often use trusses to support the roof over a wide, open space without needing many supporting columns.
Arches: An arch is a curved structure that spans a space and supports a load primarily through compression. The curved shape allows the arch to transfer the load downwards and outwards to the supports (abutments).
Advantages: Can span large distances with minimal material, aesthetically pleasing.
Disadvantages: Requires strong abutments to resist the outward thrust, can be challenging to construct.
Example: Think about the old stone bridges you might see in some parts of South Africa. These bridges rely on the arch's ability to handle compressive forces.
Suspension Bridges: Suspension bridges use cables suspended between towers to support the deck. The cables are anchored at each end, and the deck is hung from the cables using suspender cables.
Advantages: Can span incredibly long distances, aesthetically impressive.
Disadvantages: Expensive to construct, susceptible to wind forces, requires strong anchorages.
Example: While South Africa does not have suspension bridges on the scale of the Golden Gate Bridge in San Francisco, smaller versions might be used in pedestrian walkways in areas with difficult terrain. 2.2 Types of Forces Tension: A force that pulls or stretches a material. Think of pulling on a rope – the rope is under tension.
Real-life example: The cables in a suspension bridge are under tension. The guy wires supporting a cell phone tower are also under tension.
South African context: The cables used in elevators in tall buildings in Johannesburg experience tension.
Compression: A force that squeezes or compresses a material. Think of pushing down on a column – the column is under compression.
Real-life example: The columns in a building are under compression. The legs of a table are also under compression.
South African context: The concrete pillars supporting bridges and buildings throughout the country are constantly under compressive forces due to the weight they bear.
Shear: A force that causes one part of a material to slide past another part. Think of cutting paper with scissors – the paper is experiencing shear.
Real-life example: Bolts holding two plates together experience shear when a force is applied parallel to the joint.
South African context: Shear forces are present in the joints of structures built to withstand seismic activity, which is a consideration in some parts of the country.
Torsion: A force that twists a material. Think of twisting a screwdriver – the screwdriver is experiencing torsion.
Real-life example: A driveshaft in a car is under torsion when it transmits power from the engine to the wheels.
South African context: The axles of mining equipment in South Africa experience torsion due to the rotational forces involved in mining operations. 2.3 Force Distribution in a Truss Understanding how forces are distributed in a truss is critical to ensuring its stability. Each member of the truss experiences either tension or compression. By analysing the forces at each joint, we can determine whether a member is under tension (being pulled) or compression (being pushed).
Method of Joints: This is a common method used to analyze truss structures. It involves isolating each joint and applying the equations of equilibrium (sum of forces in the x and y directions equals zero). By solving these equations, we can determine the forces in each member connected to the joint.
Example: Consider a simple A-frame truss supporting a weight of 1000N at the apex (top point). Let's assume the two members of the "A" are of equal length and are supported at the base.
Symmetry: Due to symmetry, each support at the base will carry 500N (1000N / 2).
Joint Analysis (Apex): At the apex, the 1000N load is acting downwards. The two members of the "A" must provide an upward force to balance this load. Since they are angled, we need to consider the vertical component of the force in each member.
Assume Angle: Let's assume the angle each member makes with the horizontal is 45 degrees. The vertical component of the force in each member is F * sin(45°), where F is the force in the member.
Equilibrium: 2 F sin(45°) = 1000N => F = 1000N / (2 * sin(45°)) => F ≈ 707.1N Conclusion: Each member of the "A" experiences a compressive force of approximately 707.1N.