Lesson Notes By Weeks and Term v5 - Grade 7
Structures: types of structures and simple frame structures – Week 1 focus
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Structures: types of structures and simple frame structures – Week 1 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: 1
Theme: General lesson support
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Structures are all around us! From the houses we live in to the bridges we cross, structures are essential for our daily lives. In South Africa, with its diverse landscapes and growing infrastructure needs, understanding structures is particularly important. We need strong, safe, and efficient structures to support our communities and economy. This week, we'll begin exploring the world of structures, focusing on different types and how they are built, particularly frame structures. Understanding the principles of structures will empower you to appreciate the engineering marvels around you and even design your own simple structures.
What is a Structure? A structure is anything that has a definite size and shape and can withstand forces acting on it. Think of it as something that can hold itself up and carry a load. This "load" can be anything from the weight of the structure itself (called self-weight or dead load) to people walking across a bridge, or furniture in a house (called live load). The primary purpose of a structure is to support a load or to enclose a space.
Types of Structures: We can categorize structures in different ways.
Here are a few common classifications: Natural Structures: These are structures found in nature, not made by humans. Examples include spider webs, termite mounds, rock formations like Table Mountain, and trees. Table Mountain is a mass or solid structure.
Man-Made Structures: These structures are built by humans. Examples include buildings, bridges, dams, vehicles, and furniture. Man-made structures can be further classified based on their form: Solid Structures (or Mass Structures): These are made from a solid mass of material. They rely on their own weight and strength to withstand forces. Examples include a brick wall, a dam, or a concrete pillar. These structures are typically very heavy and use a lot of material. They often resist forces through sheer bulk.
Frame Structures: These are made up of a framework of interconnected parts (usually beams and columns) that support a covering. The framework provides the strength and stability. Examples include a house frame, a bridge truss, or a steel tower. Frame structures are efficient because they use less material than solid structures. The strength relies on the arrangement of the frame elements, not just the individual parts.
Shell Structures: These are hollow structures with a curved shape that provides strength and rigidity. Examples include eggshells, domes, airplane fuselages, and car bodies. The curved shape allows shell structures to distribute forces evenly across the entire structure, making them very strong for their weight.
Focus on Frame Structures: This week, we are focusing on frame structures. A frame structure is built from individual pieces that are joined together to create a strong, stable framework.
The key elements of a frame structure are: Beams: Horizontal members that primarily resist bending forces. Imagine a beam supporting a roof.
Columns: Vertical members that primarily resist compression forces. Imagine a column supporting a beam.
Joints: Points where beams and columns are connected. The type of joint is crucial for the overall strength of the structure. Joints can be rigid (fixed) or pinned (allowing some rotation).
Braces: Diagonal members that add extra stability to the frame by preventing it from distorting or collapsing.
Importance of Shape and Materials: Shape: The shape of the frame significantly impacts its strength and stability. Triangles are a very strong shape because they are inherently rigid. This is why you often see triangles incorporated into frame structures, especially in trusses (a type of frame structure often used in bridges). Square or rectangular frames are more susceptible to distortion unless they are braced.
Materials: The material used to build the frame also affects its strength. Different materials have different properties, such as strength, stiffness, and weight. Common materials for frame structures include wood, steel, and concrete. The material must be suitable for the loads the structure will experience and the environment it is in. For example, steel is strong but can corrode in wet environments, so it needs to be protected. Wood is renewable but can be susceptible to rot and fire.
Example 1: Building a Simple Straw Frame Structure Imagine you're building a small shelter for toy animals using drinking straws and Prestik.
Planning: Decide on the basic shape (e.g., a rectangular prism).
Construction: Cut the straws to the desired lengths. Use Prestik to connect the straws at the corners to form a rectangular frame.
Testing: Gently push on the frame. Notice how easily it distorts.
Improvement: Add diagonal braces (straws connected diagonally across the rectangles) to make the frame more rigid. Test again. You'll notice a significant improvement in stability. This demonstrates the power of triangulation.
Example 2: Comparing Material Strength (Hypothetical) Let's say we want to build a small bookshelf that must support 10 kg of books.
We have two options for the shelves: Option A: Pine wood (known for being relatively soft).
Option B: Steel (known for being very strong). Although pine could be used if the shelf is short and wide, a thinner shelf made of steel would likely be a better choice because it can withstand the bending force caused by the weight of the books without sagging. This illustrates the importance of material strength.
Example 3: Bridge Design A bridge is a complex frame structure. Think about the N1 highway bridges you may have seen.