Static and dynamic forces, even and uneven loads
A static force stays constant over time — like the weight of a roof pressing down permanently on its walls. A dynamic force changes over time — like wind gusting against a billboard, or traffic driving over a bridge. Structures must be designed to survive both.
A load is even (uniformly distributed) when it is spread evenly across a surface or member, such as a floor loaded with evenly spaced furniture. A load is uneven (point load) when it acts at one specific spot, such as a single heavy safe standing on one part of a floor. Uneven loads concentrate stress and are often more likely to cause failure at that point.
How materials resist forces
Look at the cross-section of any metal structural member and you can understand what force it is resisting:
- Tension — a pulling force stretching the member along its length. Cables and ties are in tension.
- Compression — a pushing force squeezing the member along its length. Columns and struts are in compression.
- Bending — a combination of both: when a beam bends under a load, the side curving outward stretches (tension), while the side curving inward is squeezed (compression). This is why beams are often shaped like an "I" — most material is placed at the top and bottom edges (furthest from the centre), where tension and compression are greatest.
- Torsion — a twisting force. Internal cross-bracing (diagonal members forming triangles inside a frame) is used to resist a structure twisting out of shape, converting the twisting force into tension and compression in the diagonal braces instead.
Beam bending in one picture: imagine bending a ruler — the outer curved surface stretches (tension) while the inner curved surface is squashed (compression). Exactly the same happens inside a loaded beam.
Properties of construction materials
Choosing the right material for a job means understanding several different properties, which do not always go together:
- Mass / density — how much matter is packed into a given volume. A dense material (like steel) is heavy for its size; a low-density material (like balsa wood or foam) is light for its size.
- Hardness — resistance to scratching, denting or being worn away at the surface.
- Stiffness — resistance to bending or flexing out of shape under load (a stiff material keeps its shape).
- Flexibility — the opposite of stiffness: the ability to bend or flex without breaking (useful in springs, cables, and fishing rods).
- Corrosion resistance — resistance to being chemically attacked and worn away, most commonly by oxygen and moisture in the air (rusting, in the case of iron and steel).
Preventing corrosion
Because ferrous metals (those containing iron) are especially prone to rusting, several methods are used to protect them:
- Painting — a coat of paint forms a barrier that keeps oxygen and moisture away from the metal surface. Cheap and easy, but the coating can scratch or wear off, exposing the metal again.
- Galvanising — coating steel with a layer of zinc (often by dipping it in molten zinc). Zinc corrodes in preference to the steel underneath (it "sacrifices" itself), so the steel stays protected even if the zinc coating is scratched.
- Electroplating — using an electric current to deposit a thin protective (and often decorative) layer of another metal, such as chromium or zinc, onto the surface.
Denser is not always stronger, and stiffer is not always tougher — always match the property to the job the material must do.