Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Identify and classify structures (frame, shell, solid).
• Define a frame structure and its pros and cons.
• Identify components of a frame structure (struts, ties, joints).
• Explain how forces are distributed in a frame.
• Design and sketch a simple frame structure.

Lesson summary

Structures are all around us. From the houses we live in to the bridges we cross, and even the cellphone towers that allow us to communicate, structures provide support, stability, and protection. Understanding how structures work is essential because it allows us to design and build safe and functional objects. In a South African context, knowledge of structures is vital for addressing issues like housing shortages, infrastructure development, and building sustainable solutions for our communities. This week, we will focus on identifying different types of structures and understanding simple frame structures, which are common in many buildings and other constructions.

Lesson notes

What is a Structure? A structure is something that has a definite size and shape, which serves a purpose, and can withstand forces (loads) acting on it. Structures can be natural (like mountains, trees, or termite mounds) or human-made (like buildings, bridges, or fences).

Types of Structures: There are three main types of structures: Solid Structures: These structures are made entirely of solid material. They are strong and can withstand a lot of force, but they can also be heavy and use a lot of materials.

Examples: A brick wall: Common in many South African homes.

A concrete dam: Like the Gariep Dam, providing water and electricity. A sculpture made of stone.

Frame Structures: These structures are made up of separate parts that are joined together to create a strong framework. Frame structures are lighter than solid structures but can still be very strong. They are often used when a structure needs to span a large distance.

Examples: A bicycle frame. A steel bridge. A wooden roof truss in a house. Cell phone towers – ubiquitous landscape features.

Shell Structures: These structures are hollow and have a curved shape. This shape makes them very strong for their weight.

Examples: An eggshell. The roof of a stadium like Soccer City. A plastic bottle. Traditional mud huts with curved roofs are good examples of shell structures adapting to local resources.

Focus on Frame Structures: Frame structures are particularly important in construction and engineering. They are efficient in terms of material usage and can be adapted to a wide variety of designs.

Key Components of a Frame Structure: Struts: These are structural members that are designed to resist compression (being pushed together). Struts are typically vertical or angled.

Ties: These are structural members that are designed to resist tension (being pulled apart). Ties are often horizontal or angled.

Joints: These are the points where the struts and ties are connected. Joints must be strong enough to hold the structure together under load. The type of joint can affect the stability of the whole structure.

Forces in Frame Structures: When a load is applied to a frame structure, the force is distributed throughout the structure. The struts resist compression, and the ties resist tension. The joints must be strong enough to transfer the forces between the members. Imagine a simple triangular frame structure supporting a weight at the top vertex. The two inclined members (sides of the triangle) act as struts, resisting compression. The base of the triangle acts as a tie, resisting tension. The joints at the vertices of the triangle must be strong enough to prevent the frame from collapsing.

Advantages of Frame Structures: Lightweight: Use less material than solid structures for the same strength.

Strong: Can withstand significant loads.

Versatile: Can be used for a variety of applications.

Relatively easy to construct: Components can be prefabricated.

Disadvantages of Frame Structures: Complexity: Can be more complex to design and build than solid structures.

Joints: Joints are critical and must be carefully designed.

Susceptible to Buckling: Struts, under compression, can buckle (bend).

Example: Designing a Simple Wooden Frame for a Small Chicken Coop Imagine you need to build a small chicken coop for your backyard in a rural area of KwaZulu-Natal. You decide to use a simple frame structure made of wooden planks.

Frame: You would likely use a rectangular frame as the base, with vertical supports (struts) at the corners and along the sides. These struts will support the roof.

Roof: The roof could be a simple sloping frame, also made of wooden planks, supported by the vertical struts.

Joints: The wooden planks would be joined together using nails or screws to create strong connections. In this example, the wooden planks act as both struts (resisting compression from the roof) and ties (resisting tension if the frame tries to spread apart). The nails or screws act as the joints, holding everything together. Guided Practice (With Solutions)

Question 1: Identify the type of structure used in a typical shack (informal dwelling) found in many South African townships. Explain your reasoning.

Solution: The structure is most likely a frame structure. Shacks are usually built with a wooden or metal frame as the primary support. The frame provides the basic shape and stability of the structure. The walls and roof are then attached to this frame using materials like corrugated iron sheets, plastic sheeting, or reclaimed materials. The frame, therefore, bears the load.

Question 2: Draw a simple sketch of a rectangular frame structure, labeling the struts, ties, and joints.

Solution: \[Sketch: A rectangle. Label the vertical sides "Struts". Label the horizontal sides "Ties". Label each corner where the sides meet "Joints".] The vertical members (struts) resist compression, while the horizontal members (ties) resist tension.