Lesson Notes By Weeks and Term v5 - Grade 5

Lesson Video

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Performance objectives

• Define a frame structure and find examples.
• Explain forces as pushes and pulls.
• Identify four types of forces: tension, compression, shear, and torsion.
• Describe how materials make structures stronger.
• Design and build a simple frame structure.

Lesson summary

This week, we're diving into the world of structures, specifically frame structures. Think about the bridge you might cross on your way to school, the tower holding up a cell phone signal, or even the goalposts on a soccer field. All of these are examples of frame structures! Frame structures are all around us, and understanding how they work is crucial for building safe and strong things. In South Africa, we need strong structures for everything from houses to roads to hospitals. Learning about frame structures helps us understand how engineers design these important things to withstand forces and keep us safe.

Lesson notes

What is a Frame Structure? A frame structure is a skeleton-like structure made of separate parts joined together to support a load. The "skeleton" is the frame, and it's designed to resist forces and keep the whole structure stable. Think of the bones in your body – they form a frame that supports your weight and allows you to move. The parts of a frame structure can be made of different materials like wood, metal, or even bamboo.

Examples of Frame Structures: Bridges: Bridges are a vital part of our infrastructure, connecting communities and allowing for transport. Frame structures are frequently used in bridge designs, using materials like steel and concrete.

Buildings: The skeleton of a building is often a frame structure, especially in taller buildings. It provides the strength and stability needed to withstand wind, rain, and the weight of the building materials themselves.

Cell Phone Towers: These towers are essential for communication. Their frame structure allows them to be tall and withstand strong winds.

Goalposts (Soccer/Rugby): Even something as seemingly simple as goalposts are frame structures! They need to be strong enough to withstand players running into them and the impact of the ball.

Roof Trusses: These are the frame structures that support the roof of a house or building.

Forces: Pushes and Pulls A force is simply a push or a pull on an object. Forces can make things move, speed up, slow down, change direction, or even change shape. We measure force in Newtons (N).

Types of Forces Acting on Structures: Frame structures need to be designed to withstand different types of forces.

Tension: Tension is a pulling force. Imagine pulling on a rope – the rope is in tension. In a frame structure, tension might be present in cables or wires supporting a bridge. An example of tension in everyday life is a tug-of-war. The rope is under tension.

Compression: Compression is a pushing or squeezing force. Imagine pushing down on a spring – the spring is in compression. In a frame structure, columns or pillars are often under compression. Think of the legs of a table supporting the weight of the tabletop. The legs are under compression.

Shear: Shear is a force that causes one part of a material to slide past another part. Imagine cutting paper with scissors – the scissors are applying a shear force to the paper. In a frame structure, shear can occur at joints or connections. Consider a bolt holding two pieces of wood together. If a force is applied parallel to the bolt head, it experiences a shear force.

Torsion: Torsion is a twisting force. Imagine twisting a wet cloth to squeeze out water – you are applying torsion. In a frame structure, torsion might be present in a bridge deck when a heavy vehicle turns sharply. Think of turning a doorknob. You are applying a torsional force.

Materials and Strength: The materials used to build a frame structure are very important. Some materials are better at resisting tension, while others are better at resisting compression.

Steel: Steel is very strong and can resist both tension and compression, making it a popular choice for bridges and tall buildings.

Wood: Wood is strong in compression but weaker in tension. It's often used in smaller structures like houses and roof trusses.

Concrete: Concrete is very strong in compression but weak in tension. That's why it's often reinforced with steel (reinforced concrete).

Worked example

Example 1: A simple wooden bridge supports a person walking across.

Forces Present:* The supporting posts of the bridge are under compression due to the person's weight pushing down. If there are cables supporting the bridge, they are under tension pulling upwards. Shear can be present at joints where the wooden planks are connected.

Example 2: A cell phone tower in a windy location.

Forces Present:* The main structure of the tower experiences compression due to its own weight and the weight of the antennas. The wind can exert a shear force on the tower, trying to push it sideways. The guy wires (cables attached to the ground) are under tension, preventing the tower from falling over.