IB Design Technology Topic B3.2.1 Forces Revision…

Forces: Finding Them Inside Everyday Products

Types of Forces Acting on Structures

Tension

Compression

Shear

Torsion

Bending

Note

Understanding these forces is crucial for analyzing how products withstand stress and maintain their integrity.

Strengthening Existing Structures

Ribs and Fillets: Add internal ridges or curves to distribute force.
Triangulation: Use triangular shapes to prevent distortion (common in trusses and bike frames).
Bracing and Cross-Beams: Add angled supports to resist bending or twisting.
Folded or Corrugated Surfaces: Used in cardboard, sheet metal, etc. for increased strength.
Material Choice: Select materials with suitable strength-to-weight ratios.

Common Mistake

Don't assume that adding more material always strengthens a structure.
It's essential to consider how the material is distributed and how it interacts with forces.

Analysing a Real Product: Follow the Load Path

A3.2 defined these five forces. Here the skill is spotting them in a product you are holding.
Start at the load and trace it to the ground.
Every part it passes through is carrying something, and every part it does not pass through is decoration.
At each part ask one question: is this being pulled, squashed, twisted, bent, or sliced?
A shelf bracket is the clearest example: the shelf bends, the horizontal arm is in tension along its top and compression along its bottom, and the screws into the wall are in shear.

Example question

A folding camping chair has a fabric seat slung between two aluminium tubes, with crossed legs that pivot on a single bolt. A heavy user reports that the legs splay outwards and the chair feels unstable.

(a) Identify the force acting in the fabric seat, in the legs, and at the pivot bolt. (3 marks)

(b) Suggest two ways of strengthening the chair against the splaying, and justify each. (4 marks)

Solution

(a) Award [1] for each correctly identified force, up to [3 max].

The fabric seat is in tension: it is being pulled taut by the user's weight, and fabric can carry no other kind of load. [1]
The legs are in compression, carrying the weight down to the ground. [1]
The pivot bolt is in shear: the two crossed legs try to slide past each other across the bolt. [1]

(b) Award [1] for each method and [1] for each justification, up to [4 max].

Add a cross-brace or tie between the leg feet. [1] This triangulates the frame, converting the splaying action into tension in the tie, and a triangle cannot change shape without changing the length of a side. [1]
Change the tube section, for example to a larger-diameter or thicker-walled tube. [1] This resists bending without a large weight penalty, because stiffness depends heavily on the shape of the section, not just the amount of material. [1]
A stronger bolt or a double pivot is also creditworthy, since it addresses the shear at the joint rather than the splaying itself.

Active recall

Name the five forces that act on structures, and describe each in one phrase.
Which force can a rope or a fabric carry, and which can it not?
In a loaded shelf bracket, where is the tension and where is the compression?
Name four ways of strengthening a structure without changing its material.
Why does adding more material not always make a structure stronger?

Forces: Finding Them Inside Everyday Products

Types of Forces Acting on Structures

Tension

Compression

Shear

Torsion

Bending

Note

Understanding these forces is crucial for analyzing how products withstand stress and maintain their integrity.

Strengthening Existing Structures

Ribs and Fillets: Add internal ridges or curves to distribute force.
Triangulation: Use triangular shapes to prevent distortion (common in trusses and bike frames).
Bracing and Cross-Beams: Add angled supports to resist bending or twisting.
Folded or Corrugated Surfaces: Used in cardboard, sheet metal, etc. for increased strength.
Material Choice: Select materials with suitable strength-to-weight ratios.

Common Mistake

Don't assume that adding more material always strengthens a structure.
It's essential to consider how the material is distributed and how it interacts with forces.

Analysing a Real Product: Follow the Load Path

A3.2 defined these five forces. Here the skill is spotting them in a product you are holding.
Start at the load and trace it to the ground.
Every part it passes through is carrying something, and every part it does not pass through is decoration.
At each part ask one question: is this being pulled, squashed, twisted, bent, or sliced?
A shelf bracket is the clearest example: the shelf bends, the horizontal arm is in tension along its top and compression along its bottom, and the screws into the wall are in shear.

Example question

(a) Identify the force acting in the fabric seat, in the legs, and at the pivot bolt. (3 marks)

(b) Suggest two ways of strengthening the chair against the splaying, and justify each. (4 marks)

Solution

(a) Award [1] for each correctly identified force, up to [3 max].

The fabric seat is in tension: it is being pulled taut by the user's weight, and fabric can carry no other kind of load. [1]
The legs are in compression, carrying the weight down to the ground. [1]
The pivot bolt is in shear: the two crossed legs try to slide past each other across the bolt. [1]

(b) Award [1] for each method and [1] for each justification, up to [4 max].

Add a cross-brace or tie between the leg feet. [1] This triangulates the frame, converting the splaying action into tension in the tie, and a triangle cannot change shape without changing the length of a side. [1]
Change the tube section, for example to a larger-diameter or thicker-walled tube. [1] This resists bending without a large weight penalty, because stiffness depends heavily on the shape of the section, not just the amount of material. [1]
A stronger bolt or a double pivot is also creditworthy, since it addresses the shear at the joint rather than the splaying itself.

Active recall

Name the five forces that act on structures, and describe each in one phrase.
Which force can a rope or a fabric carry, and which can it not?
In a loaded shelf bracket, where is the tension and where is the compression?
Name four ways of strengthening a structure without changing its material.
Why does adding more material not always make a structure stronger?

IB Design Technology Topic B3.2.1: Forces Notes

IB Design Technology Topic B3.2.1: Forces Notes

Forces: Finding Them Inside Everyday Products

Types of Forces Acting on Structures

Tension

Compression

Shear

Torsion

Bending

Strengthening Existing Structures

Analysing a Real Product: Follow the Load Path

Forces: Finding Them Inside Everyday Products

Types of Forces Acting on Structures

Tension

Compression

Shear

Torsion

Bending

Strengthening Existing Structures

Analysing a Real Product: Follow the Load Path

A. Design in theory

B. Design in practice

C. Design in context

IB Design Technology Topic B3.2.1: Forces Notes

A. Design in theory

B. Design in practice

C. Design in context

IB Design Technology Topic B3.2.1: Forces Notes

Forces: Finding Them Inside Everyday Products

Types of Forces Acting on Structures

Tension

Compression

Shear

Torsion

Bending

Strengthening Existing Structures

Analysing a Real Product: Follow the Load Path

Forces: Finding Them Inside Everyday Products

Types of Forces Acting on Structures

Tension

Compression

Shear

Torsion

Bending

Strengthening Existing Structures

Analysing a Real Product: Follow the Load Path