Week 2: Beams, Spans and Recovery

Week 2 — the outer layer as a structural member, and the difference between bending that comes back and bending that does not

1 March 2027

The first lecture with a measurement in it. It borrows one idea from structural engineering — the beam — and asks how far the borrowing survives contact with a porous, inconsistent, changing material.

The borrowing#

A beam under load carries compression along one surface and tension along the other, and its deflection under a central point load rises with the cube of its span. Halve the span and the deflection falls by a factor of eight.

That relation is exact for a uniform elastic beam. An outer layer is not one: its stiffness varies across its own width, and it is losing moisture to the air while the test runs. The lecture is explicit that the beam is a model being tried on for fit, not a fact about lunch.

Elastic and plastic#

The distinction that matters for the rest of the semester is between behaviour that reverses when the load is removed and behaviour that does not. A structure designed to stay in the first regime has a safety margin. Lunch routinely operates in the second.

So the Week 2 record is not “how much did it bend” but “at which load step did the behaviour change, and what came back afterwards”.

Before the Field Log#

Week 2 supplies the second of the Field Log’s four specimens, and the first one with a number attached to it. Bring a reading you are unsure of, marked as unsure.