MYP Design Forces in Structures Notes

Every Structure Is a Path for Force

1. A structure is anything that carries a load and passes it safely down to the ground, from a folded card phone stand to a motorway bridge.
2. The dead load is the weight of the structure itself, and the live load is everything a user puts on it afterwards.
3. Force is measured in newtons (N), and on Earth a mass of 1 kg weighs about 9.8 N, so rounding to 10 N per kilogram is close enough for a first sketch.
4. A 5 kg stack of textbooks on a shelf presses down with roughly 50 N, shared between however many brackets you fitted.
5. Follow the path that force takes: books to shelf board, board to brackets, brackets to screws, screws to wall plug, plug to wall.
6. The structure fails at whichever link in that chain is weaker than the force arriving at it, which in school projects is usually a joint rather than a panel.

Tension and Compression: Pull Apart, Push Together

1. Tension stretches a member by pulling its two ends away from each other, which is what happens to a bike brake cable and to the chains of a swing.
2. Compression squashes a member and tries to make it shorter, which is what happens to the legs under a stool and the bricks at the bottom of a wall.
3. Materials are rarely equally good at both, and a 2 mm steel cable that holds 200 kg in tension folds up the moment you push its ends together.
4. Concrete is the mirror image, strong in compression and weak in tension, so builders cast steel reinforcing bars into the stretched face of a beam.
5. Long thin parts in compression fail by buckling, bowing out sideways well before the material itself is crushed.
6. Stand a 300 mm strip of 3 mm MDF on end and a hand push bows it into a curve, while a 50 mm off-cut of the same strip takes far more before anything moves.

Shear, Torsion and Bending: The Other Three

1. Shear happens when two forces slide past each other in opposite directions either side of one line, the way scissor blades cut paper.
2. Every screw holding a shelf bracket to a wall is in shear, because the board drags down on one side of the screw while the wall holds the other side still.
3. Torsion is twisting, which is what a spanner applies to a bolt and what a bike frame feels when you stand up and stamp on one pedal.
4. Bending is two forces at once, because the top face of a loaded shelf is squeezed in compression while the bottom face is stretched in tension.
5. Between those faces sits the neutral axis, a layer carrying almost no force, which is why you can drill a cable hole through the middle of a beam and lose very little strength.
6. Pushing material away from that neutral axis is what makes an I section stiff, and the article on stability and strength deals with how to do it.

Four Products, Four Force Maps

1. The same part can carry different forces depending on how a user treats it, so map the forces before you pick a material or a section.
2. Chairs nearly always break at the back legs, because tipping back turns a leg that was only in compression into one that is being bent.
3. Bike tubes are round because a round tube resists twisting equally whichever way the twist comes from.
4. Drawing one arrow per force on your sketch is the quickest way to find the part of the design you have not thought about yet.

Static Loads Are the Easy Ones

1. A static load stays the same size and stays in one place, like a monitor sitting on a stand all day.
2. A dynamic load changes with time because it moves, arrives suddenly or repeats, like a queue of people walking across a footbridge.
3. Drop a 2 kg bag onto a shelf from 300 mm and it presses far harder than the 20 N it weighs standing still, because the shelf has to stop it in a few milliseconds.
4. Loading and unloading over and over causes fatigue, where a part finally cracks at a force it survived thousands of times before.
5. Bend a paperclip back and forth twenty times and it snaps, even though the first bend did no visible damage.
6. Wind on a tall stand, vibration from a motor and a door being slammed are all dynamic loads that a slow bench test never shows you.

Failure Has a Shape, and It Tells You What Went Wrong

1. Buckling leaves a slender compression member bowed into a curve, and the cures are a fatter section, a shorter span or a brace part way along.
2. Brittle fracture snaps acrylic cleanly with no warning bend, and the crack usually starts at a sharp internal corner or a drilled hole.
3. Rounding an internal corner to a 5 mm radius spreads the force out and delays that crack considerably.
   1. Aluminium and mild steel instead show plastic deformation, bending and staying bent, so the part survives but no longer fits.
4. Joint failure is the commonest result in a school workshop: glue peels off, a screw strips out of MDF end grain, or a 3D printed bracket splits along its layer lines.
5. Printing that bracket so the layers run across the load rather than along it makes a large difference to what it takes before it splits.
6. Photograph every failed test piece, because the shape of the break is the evidence you use when you judge your solution against your specification for criterion D.