internal forces Β· structural elements Β· materials Β· bridge types
5 Internal Forces
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Tension
A pulling force that stretches or elongates a material. The material is being pulled apart from both ends. Cables, ropes, and suspension bridge hangers work in tension β they are strong because materials resist being pulled apart.
A pushing force that squashes or shortens a material. The material is being pushed together from both ends. Columns, arches and foundations work in compression. Concrete is strong in compression but weak in tension.
Examples: columns Β· arch bridges Β· human spine Β· walls Β· dam foundations
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Shear
Two forces acting parallel to each other in opposite directions, causing layers to slide past each other. Bolts, rivets and welds typically resist shear forces. Scissors work by applying shear to cut material.
A twisting force that causes one part of a material to rotate relative to another. Shafts, axles and drive shafts experience torsion. Circular cross-sections are most efficient at resisting torsion.
Examples: car drive shaft Β· screwdriver Β· propeller shaft Β· doorknob Β· drill bit
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Bending
A force that causes a structural element to curve. One side is in tension (stretches) and the other in compression (squashes). Beams experience bending when loaded. I-beams are efficient because material is placed where stress is greatest.
Examples: floor beams Β· bridge decks Β· diving board Β· tree branch under load
Structural Elements
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Beam
BendingTensionCompression
Horizontal structural element that carries loads by bending. I-beams and box beams place material at the top (compression) and bottom (tension) flanges where stress is highest. Used in floors, bridges, and cranes.
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Column
CompressionBending
Vertical structural element that transfers loads from above down to the foundation. Primarily in compression. Slender columns can buckle β cross-sectional shape and length affect resistance to buckling.
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Arch
Compression
A curved structure that converts vertical loads into compressive forces along its length, pushing outward at the base (abutments). Pure arches have no tension β ideal for masonry. Romans built arches without any mortar.
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Truss
TensionCompression
A framework of triangles. Each member carries only tension or compression β not bending. The triangle is the only rigid polygon. Very strong and lightweight β used in roof structures, bridges, and tower cranes.
Material Properties Comparison
Material
Strong in
Weak in
Approx. Tensile Strength
Best use
Concrete
Compression
Tension
~3β5 MPa (tension)
Foundations, columns, dams
Steel
Tension & Compression
Corrosion
250β1000 MPa
Beams, cables, frames
Timber (wood)
Tension & Compression
Shear across grain
~40β120 MPa (along grain)
Floors, roofs, trusses
Reinforced Concrete
Both (steel + concrete)
Cost, weight
Steel bars carry tension
Beams, slabs, bridges
Aluminium
Tension, lightweight
Fatigue
~150β500 MPa
Aerospace, window frames
Carbon Fibre
Tension, very light
Compression, cost
~3,500+ MPa
Aerospace, sports, F1 cars
Masonry (brick/stone)
Compression
Tension
~0.5β5 MPa (tension)
Walls, arches, chimneys
Bridge Types
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Suspension Bridge
Main cables draped between tall towers carry the deck in tension. Extremely efficient for very long spans. The cables are in tension; the towers in compression.
Examples: Golden Gate Β· Humber Β· Akashi KaikyΕ (world longest: 1,991 m)
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Arch Bridge
Weight transferred to abutments at each end via compression in the arch. Strong and elegant β can use materials weak in tension (stone, brick, concrete).
Examples: Sydney Harbour Bridge Β· Pont du Gard (Roman, 19 BCE)
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Truss Bridge
A framework of triangles distributes loads efficiently. Members in tension or compression only. Common for railway bridges and medium spans.
Examples: Forth Rail Bridge Β· most railway bridges worldwide
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Cable-Stayed Bridge
Cables run directly from towers to the bridge deck. Similar appearance to suspension but different load path β cables in tension, deck in compression.
Examples: Millau Viaduct Β· Queensferry Crossing Β· many modern bridges
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Beam Bridge
Simplest type β a horizontal beam supported at each end. Top of beam in compression, bottom in tension. Limited span but cheap and quick to build.
Structural engineering studies how shapes and materials carry loads. Bridges, buildings, and towers distribute force safely using tension, compression, and geometric efficiency.
Why does it matter?
Without structural engineering, nothing would stand. It shapes every building, vehicle, aircraft, and piece of furniture β keeping people safe under enormous forces every day.
Key terms
Tension β a force that stretches or pulls a material apartCompression β a force that squeezes or pushes a material togetherLoad β the weight or force a structure must support safelyTruss β a framework of triangles used to distribute loads efficientlyArch β a curved structure that converts loads into compression along its curveFactor of safety β how much stronger a structure is beyond the minimum required
π― Try this challenge
Which shape is structurally stronger β a triangle or a square? Look at the bridge designs above. Why do trusses use triangles everywhere instead of squares?
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