Structural systems · GCSE Engineering

Forces in structures

Tension: a pulling force that stretches a member. Compression: a pushing force that squashes it.
Torsion: a twisting force, as in a drive shaft. Shear: forces acting in opposite directions that make parts slide apart, as on a bolt. Bending: a combination of tension and compression, as in a loaded beam.
Static loads don't move (the weight of a bridge). Dynamic loads move or change (traffic crossing a bridge, or wind).

Frame structures are made of members joined together, such as bridges, cranes and pylons. Shell structures are hollow and get strength from their shape, such as car bodies and drinks cans. Solid (mass) structures rely on their weight, such as dams.
Triangulation makes frames rigid, because a triangle can't change shape without changing a side length.
Struts are members in compression; ties are members in tension.
I-beams resist bending using less material, by placing most material at the top and bottom where stresses are greatest.

Stress = force ÷ area. A force of 5,000 N on an area of 25 mm² gives a stress of 200 N/mm².
Strain = extension ÷ original length. An extension of 0.5 mm on 250 mm gives a strain of 0.002 (no units).
Factor of safety = failure load ÷ working load. A part that fails at 12 kN with a working load of 3 kN has a factor of safety of 4. Higher factors are used where failure would be dangerous.