Lever arm
Answering one question without solving the rest
A truss of fifty members can be interrogated about one of them. Cut through three, take moments about the point where two of them meet, and the third falls out in a single line.
Depth is the cheapest strength there is
Doubling the depth of a truss halves its chord forces without adding a gram of material to the chords. Nothing else in structural design is that cheap, and almost every structure has already spent it.
Bending is a pair of forces, pushing and pulling
A bending moment is not a mysterious twisting. It is a push near the top of a section and a pull near the bottom, separated by a lever arm — a couple, made out of stress.
When half the section has given up
Bending theory puts the neutral axis through the centroid. That is a consequence, not a rule — and when the tension side cracks, the same reasoning moves the axis somewhere else entirely.
Named alongside it
The objects these essays reach for when they reach for this one.
Chord forceNeutral axisSecond moment of areaStress blockBucklingCentroidCracked sectionEfficiencyElastic limitFirst moment of areaFree body diagramMethod of sections