IGCSE Design and Technology - Systems and Control: Structures

Introduction

This section focuses on the structural considerations crucial in design and technology. Understanding how structures respond to forces, maintain stability, achieve strength, and utilize frameworks and reinforcement is fundamental to creating functional and durable products.

Forces on Structures

Structures are constantly subjected to various forces. These forces can be categorized into:

• Tensile Forces: Forces that tend to pull or stretch a material.
• Compressive Forces: Forces that tend to push or squeeze a material.
• Shear Forces: Forces that act parallel to a surface, causing one part of the material to slide relative to another.
• Torsional Forces: Forces that cause twisting.

The magnitude and direction of these forces determine the stresses acting on the structure.

Stability

Stability refers to a structure's ability to resist being toppled or overturned. Factors affecting stability include:

• Centre of Gravity (CG): The point where the weight of the structure is considered to be concentrated. A lower CG generally increases stability.
• Base Area: A wider base provides greater stability.
• Height: Taller structures are more susceptible to instability.
• Shape: Certain shapes are inherently more stable than others (e.g., triangles).

The relationship between the CG and the base of the structure is critical for stability. If the CG is outside the base, the structure will be unstable.

Strength

Strength is a material's ability to withstand applied forces without breaking or deforming permanently. Different types of strength include:

• Tensile Strength: The maximum stress a material can withstand before it starts to break under tension.
• Compressive Strength: The maximum stress a material can withstand before it starts to deform permanently under compression.
• Shear Strength: The maximum stress a material can withstand before it starts to deform permanently under shear.
• Yield Strength: The point at which a material begins to deform permanently.
• Ultimate Tensile Strength: The maximum stress a material can withstand before it breaks.

The choice of material is a key factor in determining the strength of a structure.

Frameworks

A framework is a structural system consisting of interconnected members that work together to support loads. Common types of frameworks include:

• Trusses: Structures made up of interconnected triangular units. Trusses are efficient at distributing loads and are commonly used in bridges and roofs.
• Beams and Columns: A simple and common framework consisting of horizontal beams supported by vertical columns.
• Arches: Curved structures that transfer loads downwards and outwards.
• Frames: More complex frameworks that combine beams, columns, and other elements.

The design of a framework involves selecting appropriate members and connections to ensure efficient load distribution.

Reinforcement

Reinforcement is the addition of materials to a structure to increase its strength and stability. Common types of reinforcement include:

• Rivets: Metal fasteners used to join structural members.
• Bolts: Metal fasteners used to join structural members.
• Welds: A process of joining materials by melting and fusing them together.
• Internal Bracing: Additional structural members placed within a structure to provide support and prevent deformation.
• Composite Materials: Combining different materials to achieve enhanced strength and stiffness (e.g., carbon fibre reinforced polymer - CFRP).

The type and placement of reinforcement are crucial for ensuring the structural integrity of a product.

Table: Material Properties and Structural Applications

Material	Tensile Strength	Compressive Strength	Typical Structural Application
Steel	High	High	Bridges, buildings, frameworks
Timber	Moderate	Moderate	Roofs, floors, frames
Aluminum	Moderate	Moderate	Lightweight frameworks, vehicle bodies
Concrete	Low	High	Foundations, walls, arches
Composite Materials (e.g., CFRP)	Very High	High	High-performance applications, aerospace

Understanding the interplay of forces, stability, strength, frameworks, and reinforcement is essential for designing safe and effective structures in design and technology.