Structures: members and design

A member is named by the force it carries, so identify it correctly: tension pulls apart (tie), compression pushes together (strut), bending makes a beam sag, shear slides one part past another, torsion twists. Examiners see a bending beam read as compression, torsion missed, and shear missed at glued joints. Look at how the load acts, then name the force.

Asked to define a factor of safety, weaker candidates say something vague like the materials, and score nothing. It is a deliberate design allowance: the member is made several times stronger than its working stress, covering variation in load, material and workmanship. Numerically it is strength / working stress -- a designed margin, not a material property.

A moment is force times distance, so a distance in millimetres must be converted to metres (400 mm is 0.4 m) before multiplying, or the answer is a thousand times wrong. Keep every length in the same unit. Examiners insist the answer carries its unit (Nm, N/mm2) and that working is shown, since method marks are awarded stage by stage. Convert, substitute, state the unit.

Stress is force divided by cross-sectional area, so work the area out correctly first. For a round bar or wire the area is pi times radius squared, and the commonest error is using the diameter instead of the radius, giving an area four times too big. Halve the diameter to get the radius first. For a rectangular section the area is width times depth.

Thinking a strut fails at the same load whatever its length. It does not. A long, thin strut does not crush, it buckles -- bends sideways and folds -- at a load far below the crushing strength, and the more slender it is, the lower the buckling load. Doubling its length sharply cuts the safe load even with the same section. Keep struts short, brace them, or use a tube.

Writing stress as force multiplied by area, instead of force divided by area. Stress = force / cross-sectional area: a bigger area spreads the force and gives LESS stress, so multiplying makes a thick member look more stressed than a thin one -- nonsense. The unit is a clue: N/mm2 is a force over an area. Always divide, and check a thicker member gives a lower stress.

Assuming concrete is equally strong pushed or pulled. It is strong in compression but weak in tension -- it cracks when pulled apart. In a beam the top compresses and the bottom stretches into tension, so plain concrete cracks along its underside before it is crushed. The fix is steel reinforcing bars in the tension zone (the lower face), where steel is strong.