A3.2 Introduction to Structural Systems

Structure

An arrangement of parts designed to support loads, withstand forces and keep its form and stability.

Three Families of Structures

Frame (strength from braced skeleton), Shell (strength from thin curved surface), and Solid (strength from dense mass).

Three Demands of Every Structure

Load-bearing (carries weight, wind, pressure), Stable (holds shape as conditions change), and Efficient (uses as little material as possible).

Built Environment

The collective name for constructed structures, features and facilities where people live and work.

Structural System

The parts of a product that resist loads, such as the shell, framework, or solid structure, helping it maintain strength and stability.

Natural Structures

Structures built by nature that are light, strong, and efficient (e.g., honeycomb, tree trunk, bird bones, seashells).

Human-Made Structures

Structures that resist forces and often copy nature (e.g., bridges, skyscrapers, corrugated cardboard, domes, and arches).

Solid Structure

A structure with few or no internal spaces that resists shape change under load through a single dense mass.

Shell Structure (Geodesic Frame)

A 3D hollow structure with thin walls relative to its overall size where curve shape spreads the load across the whole surface.

Frame Structure

A structure made of beams and columns that can support its own weight without needing an outer shell.

Triangulation

The method where a diagonal brace splits a square frame into rigid triangles that hold their shape under load.

Strengthening Existing Structures

Methods such as adding ribs and fillets, triangulation, bracing and cross-beams, folded or corrugated surfaces, material choice, and reinforcement.

Causes of Structural Failure

Overloading, dynamic impacts, material choice, material failure, inadequate size, and poor shape

Member

An individual load-bearing part of a structure (beam, column, truss).

Load Path

The route a load follows, from where it is applied, through the components, and into the foundations.

Beam

A horizontal member that spans a gap and resists bending as loads try to sag it.

Column

An upright pillar that supports the beams above and carries their combined compressive load straight down to the foundations.

Simply Supported Beam

A beam that rests on supports at each end (typically a pin and a roller) that allow the ends to rotate freely as the beam bends.

Fixed Beam

A member in a structure that is fixed rigidly on both ends to supports (cast or welded) so the ends cannot rotate or shift.

Cantilever Beam

A rigid structure that is fixed rigidly at one end and extends horizontally into open space without support on the other.

Continuously Supported Beam

A beam that runs across more than two supports along its length, sharing loads and cutting deflection.

I-Beam

A beam cross-section shaped like a capital letter I that places the largest amount of material as far away from the neutral axis as possible.

Box Beam

A beam with 2 webs and 2 flanges that encloses its interior, giving greater resistance to twisting or torsional forces.

Strut

A beam forming part of a framework designed to resist compression and support other beams.

Tie (Tension Member)

A member forming part of a framework that resists tension (pulling forces).

Trusses

Frameworks created by adding extra members to beams in interconnected triangles to distribute loads efficiently and maintain geometric stability.

Short Column Failure

Failure by crushing when compressive stress finally exceeds the material's strength.

Slender Column Failure

Failure by buckling, bowing suddenly sideways long before it could be crushed.

Static Force

A stationary force acting on a structure, including its own weight and steady loads, staying constant or changing slowly.

Dynamic Force

Moving forces exerted on structures (such as wind, vehicles, earthquakes) that change over time, often suddenly.

Bending (Curving) Force

A force that causes a material to curve, involving both tension (on one side) and compression (on the opposite side).

Bending Moment Formula

M = F x d (Force multiplied by the perpendicular distance from the force to the point of interest).

Compression (Squeezing) Force

A force that squeezes a material and tends to shorten it.

Shear (Sliding) Force

Caused by two equal and opposite forces that are not in line, sliding one layer of material past another with a cutting effect.

Tension (Pulling) Force

A pulling force that stretches a material and tends to extend it.

Torsion (Twisting) Force

A force applied when a material is turned about its axis.

Moment (Torque)

The measure of a force's tendency to cause an object to rotate about a specific pivot point, axis, or center.

Buckling

Lateral deformation where a long, thin structural element under excessive compressive load becomes unstable, deflects sideways, and collapses.

Dynamic Amplification

Occurs when a moving or changing load causes a structure to experience greater forces, stresses, or movement than the same load applied statically.

Stability

The ability of a structure to maintain its shape and position under load.

Equilibrium

A state of balance where equal and opposite forces act on a system.

Translational Equilibrium

Forces sum to zero (Sigma F = 0), so there is no net push or pull and nothing accelerates.

Rotational Equilibrium

Moments

Structural Equilibrium

A state where all forces and moments acting on a structure are balanced (Sigma Vertical Forces = 0 and Moments = 0).

Factors Affecting Stability

Base width and security, center of mass height, overloading, uneven load distribution, foundation instability, dynamic impacts, and material failure.

Structural Components Working Together

Vertical elements provide height and transfer loads, horizontal members extend reach, and counterbalancing elements reduce overturning moments.

Lamination

Covering a material's surface with a thin sheet of a different material using adhesion to improve toughness, flexibility, and combined properties.

Composite Materials

A material comprised of two or more materials to improve physical, mechanical, and chemical properties or aesthetics (e.g., high strength-to-weight ratio).

Safety Factor (SF)

The ratio of the maximum force a structure can take before failure (ultimate load) divided by the everyday working load.

Non-Critical Safety Factor Range

SF of 1.5 to 2 (used for benches and furniture).

Buildings and Bridges Safety Factor Range

SF of 2 to 3.

Aircraft or Medical Devices Safety Factor Range

SF of 3 to 5+.

Reasons for a Safety Factor

Ensures safety, satisfies regulations, and covers unpredictability in loads, material variability, ageing, and human error.

Stress (Sigma)

Force (F) applied to a material divided by the cross-sectional area (A) over which it is applied (Sigma = F

Strain (Epsilon)

The response of a material to stress, defined as the change in length divided by the original length (Epsilon = Delta L

Young's Modulus (E)

A measure of a material's stiffness, defined as stress divided by strain (E = Sigma

Low Young's Modulus Materials

Flexible, elastic materials that deform noticeably under stress and absorb energy well (e.g., rubber, polyethylene, nylon).

High Young's Modulus Materials

Stiff materials resistant to deformation that efficiently support loads and transfer forces (e.g., steel, carbon-fibre-reinforced polymer, ceramics).

Stress-Strain Graph

A visual representation plotting stress against strain showing the elastic region, plastic region, yield strength, ultimate strength, and fracture point.

Yield Strength

The maximum amount of stress a material can withstand before undergoing permanent plastic deformation.

Ultimate Strength

The peak of the stress-strain curve representing the maximum stress a material can withstand before breaking.

Fracture Point

The point where a material breaks after reaching its stress and strain limits.

Brittle Failure

Failure where a material snaps suddenly with almost no stretch first (as in glass and ceramics).

Ductile Failure

Failure where a material stretches and deforms visibly before breaking (as in steel and aluminium).

Fatigue Failure

Failure where repeated loading well below the ultimate strength grows tiny cracks over time until the part gives way.

Stiffness Against Weight

Comparing a material's stiffness-to-weight ratio rather than raw modulus to select materials for specific applications like aerospace versus buildings.