A3.2 Introduction to Structural Systems

Structure

An arrangement of parts designed to support loads

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

Structural System

The parts of a product that resist loads

Natural Structures

Structures built by nature that are light

Human-Made Structures

Structures that resist forces and often copy nature (e.g.

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

Causes of Structural Failure

Overloading

Member

An individual load-bearing part of a structure (beam

Load Path

The route a load follows

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

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

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

Static Force

A stationary force acting on a structure

Dynamic Force

Moving forces exerted on structures (such as wind

Bending (Curving) Force

A force that causes a material to curve

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

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

Buckling

Lateral deformation where a long

Dynamic Amplification

Occurs when a moving or changing load causes a structure to experience greater forces

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)

Rotational Equilibrium

Moments/torques sum to zero (Sigma M = 0)

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

Structural Components Working Together

Vertical elements provide height and transfer loads

Lamination

Covering a material's surface with a thin sheet of a different material using adhesion to improve toughness

Composite Materials

A material comprised of two or more materials to improve physical

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

Stress (Sigma)

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

Strain (Epsilon)

The response of a material to stress

Young's Modulus (E)

A measure of a material's stiffness

Low Young's Modulus Materials

Flexible

High Young's Modulus Materials

Stiff materials resistant to deformation that efficiently support loads and transfer forces (e.g.

Stress-Strain Graph

A visual representation plotting stress against strain showing the elastic region

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.