Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Identify and classify different types of structures (natural and man-made).
• Define a frame structure and explain its purpose.
• Identify struts and ties in a simple frame structure.
• Explain how struts and ties contribute to strength and stability.
• Explain what a load is and different types of loads.

Lesson summary

This week, we're diving into the fascinating world of structures! Structures are all around us – from the houses we live in to the bridges we cross. Understanding how they work is crucial for building safe and efficient things. Think about the informal settlements that are being built in South Africa; are they well-structured? Why or why not? How could better structural design improve living conditions? This knowledge isn't just for engineers; it's valuable for everyone to understand the world around them and make informed decisions. In South Africa, with our diverse landscape and growing infrastructure needs, understanding structures is more important than ever.

Lesson notes

What is a Structure? A structure is something that is built or made from different parts that are put together in a particular way. Its main purpose is to support a load. A load is any force that acts upon a structure. This force can be its own weight (called self-weight or dead load), the weight of things placed on it (like furniture in a house – live load), or external forces like wind and water pressure. Structures can be classified into two main categories: Natural Structures: These are structures that occur naturally in the environment and were not made by humans.

Examples include: Mountains: Provide support and stability, formed over millions of years by geological processes.

Trees: Support their own weight and withstand wind forces through their trunk, branches, and root system.

Spider webs: Amazingly strong and efficient structures designed to trap insects.

Termite mounds: Complex structures built by termites to provide shelter and regulate temperature.

Man-made Structures: These structures are built by humans for various purposes.

Examples include: Buildings: Provide shelter and support for people and activities.

Bridges: Allow passage over obstacles like rivers or valleys.

Dams: Hold back water for irrigation, power generation, and flood control.

Towers: Used for communication, observation, or support. Types of Structures There are different types of man-made structures.

Shell Structures: These structures get their strength from the shape they are. They're hollow, think of an eggshell or an empty plastic bottle.

Solid Structures: These structures get their strength from the amount of materials used, and are solid, think of a brick wall, a statue, or a concrete dam.

Frame Structures: These structures get their strength from the way the materials are arranged, with separate parts joined together, think of the skeleton in your body, or a bicycle frame.

Cable structures: These structures make use of tension only, think of rope bridges, cables supporting bridges, and ziplines.

Frame Structures: Our Focus This week, we are focusing on frame structures. A frame structure is a type of structure that is made up of individual members (or parts) joined together to form a framework. The framework is designed to distribute the load and provide strength and stability. Think of the Eiffel Tower or the steel framework of a building under construction.

Components of a Simple Frame Structure: Struts and Ties Two fundamental components are essential for understanding how a frame structure works: Struts: These are members that are designed to resist compression forces. Compression is a pushing or squeezing force. Struts are typically thicker and stronger to prevent them from buckling (bending) under pressure. Think of the vertical supports in a bridge or the legs of a table.

Example: Imagine a stack of books. The bottom books are acting as struts, supporting the weight of the books above them through compression.

Ties: These are members designed to resist tension forces. Tension is a pulling or stretching force. Ties are typically made of materials that are strong in tension, such as cables or ropes. Think of the cables suspending a bridge or the rope used to pull a load.

Example: Imagine pulling a rope in a tug-of-war. The rope is acting as a tie, resisting the tension force you are applying. How Struts and Ties Work Together In a frame structure, struts and ties work together to distribute loads and maintain stability. Struts resist compressive forces, preventing the structure from collapsing under its own weight or external loads. Ties resist tensile forces, preventing the structure from being pulled apart. The arrangement of struts and ties in a frame structure is crucial for its overall strength and stability.

Example: A Simple Triangular Frame Imagine a simple triangular frame made of three wooden sticks joined together at their ends. If you place a weight on the top of the triangle, the two sticks at the base will act as struts, resisting compression. The top stick will act as a tie, resisting tension. The triangle's shape distributes the load evenly, making it a strong and stable structure. Triangles are often used in frame structures because of their inherent strength and stability.

Types of Loads: Dead Load: The weight of the structure itself (e.g., the weight of the roof, walls, and floors of a building).

Live Load: The weight of the objects and people that the structure supports (e.g., furniture, occupants, and snow on a roof).

Environmental Load: Forces caused by natural phenomena such as wind, rain, snow, and earthquakes. Guided Practice (With Solutions)

Question 1: Identify whether the following structural members are acting as struts or ties: a) The supporting columns of a classroom. b) The cables suspending a suspension bridge. c) The roof rafters in a house. d) The legs of a chair.

Solution: a)

Struts: The supporting columns are resisting compression. b)

Ties: The cables are resisting tension.