Lesson Notes By Weeks and Term v5 - Grade 9

Lesson Video

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Performance objectives

• Identify three advanced structural systems.
• Explain the five main forces: tension, compression, shear, bending, and torsion.
• Analyze forces acting on simple structures.
• Describe key structural materials like steel and reinforced concrete.
• Suggest ways to strengthen a simple structure.

Lesson summary

This week, we delve into advanced structural systems and the forces that act upon them. Buildings, bridges, and even everyday objects like cell phone towers rely on clever structural designs to withstand loads and maintain their integrity. Understanding these advanced systems is crucial because it allows us to design safer, more efficient, and more sustainable structures. From the RDP houses being built across the country to the skyscrapers of Johannesburg, structural integrity is paramount. Moreover, knowledge of forces helps us understand how things break, leading to improved safety in design and construction.

Lesson notes

Advanced Structural Systems: Suspension Structures: These systems use cables or ropes to support a deck or platform from above. The cables are typically anchored to towers or pylons. The key advantage is the ability to span very long distances with relatively little material. A classic example is a suspension bridge.

Real-life example in South Africa:* While large-scale suspension bridges are not very common in rural areas, many smaller pedestrian bridges in mountainous regions or over rivers use suspension principles, often with simpler materials like treated timber and wire ropes. Consider a rural community needing a river crossing. A suspension bridge might be the most viable solution.

Geodesic Structures: These are dome-shaped structures made up of interconnected triangular or polygonal elements. The triangular shape distributes stress evenly, making them very strong and lightweight. Buckminster Fuller popularized geodesic domes, but they have roots extending far back in architectural history.

Example:* Imagine a large, temporary shelter needed after a natural disaster. A geodesic dome could be rapidly assembled and provide a strong, weatherproof space. Think of large events or markets too; geodesic domes can offer expansive, covered spaces.

Shell Structures: These structures are thin, curved surfaces that derive their strength from their shape. Think of an eggshell – it’s strong when force is distributed across its surface, but easily broken when focused on one point. Materials like reinforced concrete are often used to create shell structures.

Example:* Consider the roofs of some modern buildings or stadiums. The curved shape provides strength and allows for large, column-free spaces. Think about the architectural features of some shopping malls, which might incorporate elements of shell structures in their roof designs.

Forces Acting on Structures: Tension: A pulling force that stretches or elongates a material. Imagine pulling on a rope – the rope is in tension.

Example:* The cables in a suspension bridge are under tension.

Compression: A pushing force that squeezes or shortens a material. Imagine stacking bricks – the bricks are under compression.

Example:* The columns in a building are under compression, supporting the weight of the floors above.

Shear: A force that causes one part of a material to slide past another part. Imagine using scissors – the blades exert a shear force on the paper.

Example:* The force acting on a bolt connecting two pieces of wood when they are pulled in opposite directions is a shear force.

Bending: A force that causes a material to curve or deform. Imagine placing a weight in the middle of a plank supported at both ends – the plank bends. Bending involves a combination of tension and compression.

Example:* A beam supporting a roof is subject to bending forces.

Torsion: A twisting force that causes a material to rotate. Imagine twisting a towel – the towel is under torsion.

Example:* The drive shaft in a car is subject to torsion.

Structural Materials: Steel: A strong and versatile material widely used in construction due to its high tensile and compressive strength. Steel structures require protection from corrosion (rust).

Reinforced Concrete: Concrete is strong in compression but weak in tension. Steel reinforcing bars (rebar) are embedded in the concrete to provide tensile strength, creating a composite material that is strong in both tension and compression. This is commonly used in buildings and bridges across South Africa.

Composites: Materials made from two or more different materials that are combined to create a material with enhanced properties. Examples include fiberglass (glass fibers in a resin matrix) and carbon fiber (carbon fibers in a resin matrix). These are becoming increasingly important in high-performance applications. Wood is also a naturally occuring composite (cellulose fibres bound with lignin).

Worked example

Scenario: A simple wooden beam is supporting a load in the middle. What types of forces are acting on the beam?

Solution: The beam is subject to bending. The top surface of the beam is under compression (being squeezed), while the bottom surface is under tension (being stretched). There is also a shear force near the supports of the beam.

Scenario: A concrete column is supporting a section of a building. What type of force is predominantly acting on the column?

Solution: The column is primarily under compression, supporting the weight of the structure above. The reinforcing steel inside the concrete helps to resist any tensile forces that might develop due to imperfections or uneven loading.

Scenario: The cables of a suspension bridge are connected to large concrete blocks (anchorages) at each end. What forces are acting on these concrete anchorages?

Solution: The anchorages are primarily under tension, resisting the pulling force of the suspension cables. The large mass of the concrete helps to provide the necessary resistance to prevent the cables from pulling free.

Guided Practice (With Solutions)