Lesson Notes By Weeks and Term v5 - Grade 7
Structures: types of structures and simple frame structures – Week 3 focus
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Structures: types of structures and simple frame structures – Week 3 focus
Download the Lessonotes Mobile South Africa app for faster lesson access on Android and iPhone.
Subject: Technology
Class: Grade 7
Term: 1st Term
Week: 3
Theme: General lesson support
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Structures are everywhere around us! From the classrooms we learn in, to the bridges we cross, to the shacks people build for shelter, structures play a crucial role in our daily lives. Understanding how structures work, what makes them strong, and the different types that exist is vital for solving real-world problems in South Africa. For example, knowing about different building techniques can help improve the quality and safety of homes, while understanding structural stability can help design safer roads and bridges. This week, we'll focus on the different types of structures and specifically delve into simple frame structures, learning how they work and how they can be strengthened.
What is a Structure? A structure is anything that has a definite size and shape and can support a load. The load is the weight or force that the structure has to carry. Structures can be natural, like a mountain or a tree, or human-made, like a building or a bridge.
Types of Structures: Solid Structures: These are structures made entirely of solid materials. They are strong and resist forces from all directions. Examples include a brick wall, a statue, or a concrete pillar. In South Africa, think of the traditional rondavel huts which often have thick mud walls.
Frame Structures: These structures consist of a framework of parts that are joined together to support a load. They are strong because the load is distributed through the frame. Examples include buildings, bridges, and cell phone towers. Think of the skeletal frame of a shack before the corrugated iron is added.
Shell Structures: These structures are hollow and curved, and they get their strength from their shape. Examples include eggshells, domes, and car bodies. Think of the Soccer City stadium in Johannesburg with its large bowl shape.
Mass Structures: These structures are held up by their own weight. They are strong because they are massive and stable. Examples include pyramids, dams, and earth retaining walls. Think of the massive concrete dams built to store water across South Africa.
Focus on Frame Structures: Frame structures are made up of interconnected members. These members can be made of different materials, such as wood, steel, or concrete. The key to a strong frame structure is how these members are joined together and how they distribute the load.
Advantages of Frame Structures: Strong for their weight: Frame structures can support a large load with relatively little material.
Versatile: They can be used to create structures of different shapes and sizes.
Easy to build: Compared to some other types of structures, frame structures can be relatively easy to construct.
Disadvantages of Frame Structures: Can be complex to design: Designing a strong and stable frame structure requires careful planning and engineering.
Susceptible to buckling: If not properly designed, the members of a frame structure can buckle under compression.
Joints can be weak points: The joints where the members are connected are often the weakest points in a frame structure.
Forces Acting on Structures: Tension: A pulling force that stretches or elongates a material.
Example: A rope being pulled.
Compression: A pushing force that squeezes or shortens a material.
Example: A pillar supporting a roof.
Shear: A force that causes one part of a material to slide past another part.
Example: Cutting paper with scissors.
Torsion: A twisting force.
Example: Turning a doorknob.
Bending: A combination of tension and compression that causes a material to curve.
Example: A beam supporting a load in the middle.
Material Strength and Stability: The choice of materials is crucial for the strength and stability of a frame structure.
Wood: A readily available and relatively inexpensive material, but it is susceptible to rot and insect damage.
Steel: A strong and durable material, but it is expensive and can corrode.
Concrete: A strong and versatile material, but it is heavy and can crack.
Example 1: Analyzing a Simple Frame Imagine a simple frame structure for a small shack in a rural South African community. The frame is made of wooden poles. The roof is made of corrugated iron.
When wind blows against the shack: Compression: The poles on the windward side (facing the wind) experience compression.
Tension: The poles on the leeward side (opposite the wind) experience tension.
Shear: The nails or screws connecting the poles may experience shear forces trying to cut them.
Bending: The roof (corrugated iron sheets) may bend under the force of the wind.
Example 2: Strengthening a Frame To strengthen the frame of the shack, you could: Use thicker poles: This increases the resistance to compression and tension.
Add diagonal bracing: This helps to distribute the load and prevent the frame from collapsing. Think of adding wooden planks at an angle to the corner poles.
Use stronger fasteners: This prevents the joints from failing. Use thicker nails or screws or even bolts.
Example 3: Analyzing a Bridge Frame Consider a small bridge made of steel beams.
When a car drives across the bridge: Compression: The beams supporting the car's weight experience compression.
Tension: The beams on the underside of the bridge experience tension.
Shear: The bolts holding the beams together experience shear forces.
Bending: The beams bend slightly under the weight of the car. Guided Practice (With Solutions)
Question 1: Identify the type of structure used to build a water tower. Explain your answer.
Solution: A water tower is primarily a frame structure. It relies on a framework of steel or concrete to support the large tank of water at the top.