Key content: structures — types and materials

Candidates call concrete simply strong without saying WHERE it is weak. It is strong in COMPRESSION but WEAK in TENSION (stretching or bending). A concrete beam whose lower face is stretched as it bends is cast around steel reinforcing bars (rebar) placed where the tension acts, so the steel carries the tension the concrete cannot. Always name the weakness (tension) AND the fix (steel).

A four-sided pin-jointed frame is NOT rigid — a sideways force shears it into a parallelogram, so it can collapse. Adding a DIAGONAL splits it into TRIANGLES, which cannot change shape without altering a side, so the frame becomes rigid. This is triangulation. Candidates miss this link, or wrongly credit triangulation with easier transport or storage; it adds only RIGIDITY and stability.

A key definition trap and a common examiner complaint about muddled force identification. A STRUT carries COMPRESSION: it is pushed and can buckle, so it is made stout. A TIE carries TENSION: it is pulled, so it can be thin (even a cable or rod). Label a stretched member a strut, or a squashed one a tie, and you lose the mark. Decide first: pushed (strut) or pulled (tie)?

Both use little material, so they get swapped. A FRAME is separate members joined together with gaps between them (a truss, a pylon, a bicycle frame). A SHELL is one continuous thin curved skin with no internal frame at all (an egg, a can, a car body panel, a dome). Ask: separate members with gaps (frame), or a single continuous skin (shell)?

A cable can only PULL, so thinking it could carry a compression (pushing) load is a common error. Being flexible, it works purely in TENSION and would go slack or buckle if pushed. That is why cables serve as TIES (suspension-bridge hangers, guy wires) and never as STRUTS. Always ask whether a member is pushed or pulled before choosing it — direction decides.

A frequent slip: calling a pylon SOLID because it looks big, or a can SOLID because it is metal. Judge a structure by HOW it carries its load, not by size or material. A SOLID (mass) structure resists load by its own bulk (a dam); a FRAME carries load through separate joined members (a pylon); a SHELL carries load in a thin skin (a can). Big and heavy does not mean solid.

A sharp, testable wrong belief: that a rectangle of four members joined at the corners is already rigid. It is not. With movement at the joints, a sideways push shears the rectangle into a PARALLELOGRAM, so it racks over and can collapse. Only a DIAGONAL brace, splitting it into TRIANGLES, makes it rigid — a triangle cannot change shape without altering a side.