Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

This page supports the lesson note with a companion video and a short classroom-ready summary.

For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.

Performance objectives

• Identify and classify different types of structures.
• Explain the characteristics of frame structures.
• Describe the function of struts and ties.
• Understand how triangles make frames strong.
• Identify examples of frame structures in South Africa.

Lesson summary

This week, we're diving into the fascinating world of structures! Structures are all around us, from the buildings we live in to the bridges we cross. Understanding how structures work is essential for anyone interested in building, designing, or even just appreciating the built environment. In South Africa, where we are constantly striving to improve infrastructure and housing, understanding structures is especially important. Think about the RDP houses built after apartheid, or the Gautrain project – all rely on sound structural engineering principles.

Lesson notes

What is a Structure? A structure is anything that supports a load. The load can be its own weight (called self-weight or dead load), people, furniture, wind, or anything else that applies a force. A good structure must be strong and stable enough to withstand these loads without collapsing or deforming excessively.

Types of Structures: There are three main types of structures: Solid Structures: These structures are made entirely of a solid material. They resist loads because of the strength of the material itself. Examples include brick walls, dams made of concrete, and sculptures carved from stone. A disadvantage is that they use a lot of material.

Shell Structures: These structures are hollow, but their shape provides strength. They are often curved to distribute the load evenly. Examples include eggshells, domes (like the one at the Union Buildings in Pretoria), and car bodies. Shell structures can be very strong for their weight. Think about how thin an eggshell is, yet it can support a chick inside!

Frame Structures: These structures are made up of individual members (like beams, columns, and struts) that are joined together to form a framework. They are often used when it is necessary to span a large distance or to support a heavy load. Examples include bridges, buildings (the skeleton), and cranes. This is what we'll be focusing on.

Frame Structures in Detail: Frame structures are made of individual components that work together to support loads.

The key components include: Beams: Horizontal members that resist bending forces. Imagine a beam supporting the roof of a house.

Columns: Vertical members that support compressive forces. The pillars holding up a balcony are columns.

Struts: Members that resist compressive forces; often angled to provide extra support and prevent buckling. Think of the diagonal supports in a roof truss.

Ties: Members that resist tensile (pulling) forces; often cables or ropes. Imagine the cables supporting a suspension bridge.

Forces in Frame Structures: Understanding the forces acting on a frame structure is crucial to designing a stable structure.

The main forces are: Compression: A squeezing force that tends to shorten or crush a member. Columns and struts are designed to withstand compression.

Tension: A pulling force that tends to stretch a member. Ties are designed to withstand tension.

Bending: A combination of compression and tension caused by a load applied perpendicular to the member's length. Beams are designed to withstand bending.

Shear: A force that causes one part of a member to slide past another. Triangles are Key! Triangles are incredibly strong shapes in structures. This is because they are rigid – they cannot be deformed without changing the length of their sides. That's why you see triangles used extensively in frame structures like bridges and roof trusses. Structures that rely on squares or rectangles are much weaker because these shapes can easily deform into parallelograms. Adding a diagonal member (a strut or a tie) to a rectangle turns it into two triangles, significantly increasing its strength and stability.

Worked example

Example 1: Analyzing a simple bridge frame

Imagine a small bridge made of wooden beams crossing a stream. The bridge deck (the surface you walk on) is supported by two main beams running along the sides. Vertical columns support these beams from below.

Beams: The deck beams are subject to bending forces as people walk across the bridge. The top surface of the beam experiences compression, while the bottom surface experiences tension.

Columns: The columns are subject to compressive forces from the weight of the deck, the beams, and the people walking across.

Now, imagine adding diagonal wooden members from the columns to the beams, forming triangles. These are struts. The struts help distribute the load and prevent the beams from bending excessively, making the bridge much stronger.

Example 2: Comparing material strengths