A3.2 / Introduction to Structural Systems

How are structures present in everyday products?

Structures are ubiquitous, appearing in both the natural world and the human-made environment. In product design, a structure is the framework or component that supports weight and withstands forces to maintain the product's integrity. Designers must be able to analyse and interpret these structures to ensure products are safe, stable, and durable

Natural beehive honeycomb structure

LongBien Bridge, Hanoi

3.2.1 - Structures in Nature and the Built Environment

Giant's Causeway, Antrim, Ireland

Structures in Nature

Structures in nature are self-assembling, load-bearing forms created without human intervention, often optimized for stability and efficiency using available materials. They are generally categorized into animal, vegetable, and geological groups. Examples include geological formations like mountains and canyons, alongside biological structures such as nests, honeycombs, termite mounds, shells, and tree branches.

Tree-Structure canopy, WestendGate Tower, Frankfurt, Germany

Structures in the built environment

In the built environment, structures are man-made forms designed to support human activity and withstand environmental forces like gravity, wind, and seismic activity. Unlike natural structures, these are engineered using diverse materials like steel, reinforced concrete, and glass.

Biomimicry

In the context of design technology, biomimicry is the practice of analysing and interpreting natural structures to inform the development, strength, and functionality of human-made structures. It involves studying how nature supports weight and withstands forces to create products that are stable, durable, and efficient

3.2.2 - Structure classification

Products and components are generally categorized into three primary structural types: Frame, Shell and Solid

Frame

A frame is comprised of discrete parts joined together to form a skeleton, such as the structure of a large building.

Shell

A shell is a thin outer layer that provides both shape and strength, like a molded plastic lunch container

Solid

A structure consisting entirely of a single piece of material without internal voids, such as an anvil

3.2.3 - Structural components

"Lunch atop a Skyscraper", Rockerfeller center, NewYork, USA

Beams

Beams are horizontal structural members that carry loads. Designers must understand how different beam configurations behave under stress

Types of Beams:

Simply Supported: Supported at both ends.

Fixed: Anchored firmly at both ends.

Cantilever: Anchored at one end & free at the other, such as an aircraft wing.

Continuously Supported: Supported at multiple points along its length.

Ancient greek column, Athens, Greece

Columns

Columns are vertical members designed to resist compressive forces such as gravity and the weight of beams.

Internal structure of an Aircraft Wing

3.2.4 - Forces

Taccoma Narrows Bridge collapse

Structures must cope with two types of loads:

static forces (stationary weight)
dynamic forces (moving or changing loads, like wind or movement).

These loads manifest as five primary forces:

Compression: Pushing or squeezing together.
Tension: Pulling apart or stretching.
Torsion: A twisting force.
Bending: Applying a load that causes a structure to curve.
Shear: Parallel forces acting in opposite directions across a material.

Static forces: Loads

Dead load: The weight of the structure (walls, floors, roof, fittings)

Structural elements shown in the drawing as heavy black lines
Fittings shown as light lines

Live load: Contents and occupants

Not normally shown in technical drawings (in the drawing, on the right, some furniture is shown). Live load is accounted for in load calculations

Environmental load: Wind, water, snow, etc

Not shown in technical drawings but accounted for in load calculations

Structural Load Urban Myth:
The Indiana University Library building suffered subsidence (sinking) because the architects and engineers failed to account for the weight of books (Live Load)

Chulalongkorn University Library

Dynamic Forces

Loads create Forces that act on beams and coloumns creating stresses on the structure.

Compression

Compression force calculation:

Total Vertical load ÷ area it acts on

Tension

Tension force (Stress) calculation:

Total load ÷ Cross-sectional area

ie: Stress = Force ÷ Area

Torsion

Torsion force (Torque) calculation:

Force x Distance from the axis of rotation

Bending

Bending (Moment) calculation:

Moment = Force x Distance

Shear

Shear force calculation:

Force ÷ Area

Q: What forces are at play when arm wrestleing?

3.2.5 - Stress & Strain

Simplified Stree-strain Diagram

Stress-Strain Relationship:

A critical part of structural design is knowing when a material will stretch, bend, or break

Designers test materials, plotting results on graphs to identify three points about a material

The basic principle is that a material undergoes elastic deformation when it is compressed or extended, returning to its original shape when the load is removed. More deformation occurs in a flexible material compared to a stiff material.

In designing structures and products, it's crucial to understand how materials behave when forces are applied to them. Materials can stretch, bend, or even break depending on the type and amount of force they experience.

This can be simply explained using candy:

3.2.6 - Young's modulus

Young’s modulus - also known as tensile modulus or elastic modulus- is a measure of the stiffness of a solid material, and is a quantity used to characterise different materials. It is defined as the ratio of stress along the axis to the strain.

Young’s Modulus (Elastic Region).

- The slope (gradient) of the initial straight part of the graph.
- Shows how stiff or flexible a material is.
- Steeper slope = stiffer material.

Yield Strength (Elastic Limit).

- The point where the material starts to deform permanently.
- Up to here, the material will return to its original shape when the force is removed.

Ultimate Strength (Tensile Strength).

- The maximum stress the material can withstand before starting to neck or weaken.
- Peak of the graph.

Fracture Point.

- Where the material finally breaks after being stretched beyond its limits.

Stree-strain Diagram including elastic regions

Calculating Young's Modulus

It's not required in our context to make complex calculations about the stiffness or flexibility of a particular material, but it is useful to understand the process.

Young's Modulus Calculation: Steel Wire Example

Input Data:

Force (F) = 200 N

Original Length (L0) = 2.0 m

Cross-sectional Area (A) = 0.000001 m^2 (1.0 x 10^-6 m^2)

Change in Length (dL) = 0.002 m

Calculate Stress (Force / Area):

Stress = 200 / 0.000001

Stress = 200,000,000 Pa (or 200 MPa)

Calculate Strain (Change in Length / Original Length):

Strain = 0.002 / 2.0

Strain = 0.001 (dimensionless)

Calculate Young's Modulus (Stress / Strain):

E = 200,000,000 / 0.001

E = 200,000,000,000 Pa

Final Result:

Young's Modulus (E) = 200 GPa

Young's Modulus part-worked example

In practice, we can use online calculators to calculate Young's Modulus for applications such as IA material choice.

Young's Modulus Online calculator

Haas F1 car with Carbon Fiber Chassis and Steel/Titanium HALO device

High Young's modulus - Stiff

Steel: Used for bridges and car chassis because it resists changing shape under heavy loads.

Carbon Fibre: Used in racing cars and aircraft for high stiffness with very low weight.

Glass: Used in windows because it maintains its shape, though it is brittle and can break easily.

Stiff materials (high Young's Modulus) are chosen for applications requiring strength and stability.

Memory Foam Pillow

Low Young's modulus - Flexible

Rubber: Used for tires and shock absorbers because it stretches and returns to shape easily.

Foam: Used in packaging and cushions because it compresses easily to protect items.

Silicone: Used in kitchenware and molds because it is durable but highly elastic.

Flexible materials (low Young's Modulus) are chosen for applications requiring Movement and energy absorption.

3.2.7 - Structural Equilibrium

Tower cranes used in construction demonstrating equilibrium

Understanding how forces interact within a structure is essential to ensuring it remains safe and stable. When a structure is in equilibrium, all the forces and moments acting on it are perfectly balanced, meaning the structure stays at rest or moves in a controlled way.

If these forces become unbalanced, the structure can bend, twist, collapse or leading to failure. Designers must evaluate these forces carefully during the design process to ensure that buildings, bridges, furniture, and other structures can safely support the loads they are expected to carry.

A structure is in equilibrium when all the forces acting on it are balanced.

The sum of all vertical forces = 0
The sum of all horizontal forces = 0
The sum of all moments (turning forces) = 0

When these conditions are met, the structure stays stable and doesn't move, tip, or collapse.

Examples:

A bridge holding up equal loads on each side without bending.
A bookshelf standing upright with weight evenly distributed.
A crane lifting a load without tipping over.

These structures have balanced forces and moments, so they stay still and safe.

When Will a Structure Fail?

A structure will fail (go out of equilibrium) when:

Forces are unbalanced: One side is heavier or loaded more than the other.
Moments aren’t equal: Too much turning force on one side causes tipping or twisting.
Support reactions can’t counter the loads: Weak foundations or broken parts can’t hold the forces.

Weak Points in Design

Structural failure rarely happens in the middle of a large, flat surface; instead, it is most common at joints, corners, and connection points where materials are bolted, welded, or glued together.

Reasons for Weakness at Connections:

Stress Concentration: Instead of spreading out, forces like compression and tension gather at sharp angles or joints, creating "high-stress zones" that are more likely to crack or lose their shape.
Material Discontinuity: When two different parts meet at a junction, the small imperfections or changes in the material's structure (such as its grain or hardness) can make that specific spot weaker than the rest.
Movement and Flexing: Because structures naturally bend under weight, joints and corners often move at different rates than flat sections, leading to long-term strain and fatigue.
Manufacturing Limitations: The process of joining materials isn't always perfect—small gaps, poor weld penetration, or rust in rivets can cause these areas to break down much faster than solid sections.

Tower crane collapse, 2021, British Columbia, Canada

3.2.8 - Strengthening techniques

Designers use various methods to reinforce structures against compression, tension, and bending.

Common Truss structures created by combining struts into shapes

Struts

Structural members are designed to resist compression.

How they work: They transfer loads away from weak areas and distribute force evenly.

Example: In a bridge or roof truss, struts prevent sagging or buckling.

Shape

Geometry dramatically affects strength.

Curved shapes: Arches or domes distribute loads more effectively than flat shapes.

Triangles: These provide excellent stability in frameworks to prevent deformation.

Example: Bicycle frames use triangles to remain light but strong.

Lamination

Bonding multiple layers of material together.

How it works: This reduces weak points and increases resistance to bending or splitting.

Example: Plywood is stronger than a single wood plank.

Composite Materials

Composites are materials made by combining two or more different materials to get the best qualities of each.

How they help: They are often stronger, lighter, or more durable than traditional options.

Example: Carbon fiber composites used in areospace and sports equipment.

Place holder for G11 photo competition

3.2.9 - Safety factor

A Safety Factor is a design requirement that ensures a system is built significantly stronger than the maximum load it is expected to carry. It acts as a "buffer" to protect against failure and keep users safe.

Calculating Safety Factor

To calculate the Safety Factor, you divide the maximum load the object can handle by the actual load it will use in real life:

Safety Factor (SF) =

Maximum Load \ Working Load (Intended Load)

SF = 1: The structure will fail exactly at its intended load. There is zero margin for error.
SF > 1: The structure is stronger than required. For example, an SF of 4 means the structure is four times stronger than necessary.

Common safety factor guidleines

Applied Safety Factor Examples

Designers must account for "the human factor"—the reality that people often push limits or misuse products.

Beyond human error, safety factors protect against:

Unpredictable Loads: Unexpected forces like high winds or overcrowding.
Material Variability: Small defects or inconsistencies in manufacturing.
Wear and Degradation: Weakening over time due to rust, corrosion, or age.
User Error: People using a product in ways it wasn't originally intended for.

Designers must balance safety with cost and efficiency. If a safety factor is too high, the project becomes too heavy and expensive (overdesigned).

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