Lesson Notes By Weeks and Term v5 - Grade 9

Lesson Video

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Performance objectives

• Identify and describe advanced structural systems like trusses, arches, and suspension bridges.
• Explain the forces that act on structures: tension, compression, shear, torsion, and bending.
• Analyse how loads are distributed within different structures.
• Predict the effects of forces on a structure's performance.

Lesson summary

This week, we delve deeper into the fascinating world of structures, moving beyond basic frame structures to explore more advanced systems and the forces that act upon them. We will investigate how engineers use different structural designs and materials to create strong, stable, and efficient structures, considering real-world applications relevant to South Africa, such as bridges, high-rise buildings in cities like Johannesburg and Cape Town, and even sports stadiums. Understanding these principles is crucial for designing safe and effective infrastructure and for appreciating the engineering marvels around us.

Lesson notes

2. 1.

Advanced Structural Systems Trusses: A truss is a structural system made up of interconnected members (usually straight) that form a rigid framework. Triangles are the fundamental building block of a truss because they are inherently stable shapes. Trusses are incredibly efficient for spanning long distances with minimal material, making them ideal for bridges and roofs.

Types of Trusses: Pratt truss, Warren truss, Howe truss. Each type distributes forces differently, influencing their suitability for specific applications. South African

Example: Many railway bridges throughout South Africa use truss systems to efficiently span rivers and valleys. The old railway bridge in the Drakensberg is a good example of a truss bridge.

Arches: An arch is a curved structural element that primarily supports loads through compression. The curve directs the load downwards and outwards towards the supports (abutments). Arches are strong and can span considerable distances.

Types of Arches: Roman arch, Gothic arch, segmental arch. The shape affects load distribution and stability. South African

Example: The Zeerust Arch is a historical example of an arch in South Africa. Modern examples exist in some bridge designs and architectural details in buildings.

Suspension Bridges: Suspension bridges use cables suspended between towers to support the bridge deck. The cables are anchored to the ground at each end. This design is excellent for spanning extremely long distances.

Key Components: Towers, main cables, suspender cables, deck. South African

Example: While South Africa doesn't have suspension bridges on the scale of the Golden Gate Bridge, smaller suspension bridges are used in pedestrian crossings in some areas, especially in mountainous regions.

Reinforced Concrete: Concrete is strong in compression but weak in tension. To overcome this limitation, steel reinforcing bars (rebar) are embedded within the concrete. The steel provides tensile strength, creating a composite material that is strong in both compression and tension. This makes reinforced concrete one of the most versatile and widely used structural materials.

Key Concept: The steel rebar resists tensile forces, while the concrete resists compressive forces. South African

Example: Reinforced concrete is used extensively in the construction of buildings, bridges, and dams throughout South Africa, including high-rise buildings in Johannesburg, apartment blocks in Durban, and dams like the Gariep Dam. 2.

2. Forces Acting on Structures Tension: A pulling force that stretches or elongates a material.

Examples: Cables in a suspension bridge, ropes pulling a load.

Compression: A pushing force that squeezes or shortens a material.

Examples: Columns supporting a roof, the weight of a building pressing down on its foundation.

Shear: A force that causes one part of a material to slide past another.

Examples: Scissors cutting paper, forces acting on a bolt connecting two plates.

Torsion: A twisting force.

Examples: Twisting a screwdriver, the forces acting on a rotating axle.

Bending: A force that causes a material to curve or deflect. Bending is a combination of tension and compression. One side of the material is in tension, while the other side is in compression.

Examples: A beam supporting a load, a diving board. 2.

3. Load Distribution Understanding how loads are distributed within a structure is crucial for ensuring its stability and safety.

Trusses: In a truss, the load is distributed through the members, primarily as tension or compression. The joints (where the members connect) are designed to transfer these forces efficiently.

Example: Imagine a simple Pratt truss bridge. The load on the deck is transferred to the vertical members (typically in compression) and then to the diagonal members (some in tension, some in compression). The bottom chord is typically in tension, while the top chord is typically in compression.

Arches: The load on an arch is primarily distributed as compression along the curve of the arch. The abutments at the base of the arch must be strong enough to resist the outward thrust caused by this compressive force.

Example: A stone arch bridge distributes the weight of traffic and the bridge itself down along the curve of the arch to the supporting abutments on either side of the river.

Reinforced Concrete Beams: A reinforced concrete beam subjected to bending will experience tensile forces on the bottom and compressive forces on the top. The steel rebar is placed in the bottom of the beam to resist the tensile forces, while the concrete resists the compressive forces.

Example: In a concrete beam supporting a floor, the weight of the floor and any load on it causes the beam to bend. The steel rebar prevents the concrete from cracking due to the tensile forces. Guided Practice (With Solutions)

Question 1: Identify the primary type of force acting on the following structural elements: a) The cable of a suspension bridge.