Load-Bearing Fillings

Structural question — which layers support the assembly, and which only add load?

A load path is the continuous route a force takes from where it is applied to whatever finally resists it. Trace one and every layer falls into one of two categories: it is on the path, or it is riding on it.

Support and payload#

A layer is on the path when three conditions hold together. It is continuous across the span. It is in contact with the layers above and below it. And it is stiff enough relative to its neighbours to take a share of the force rather than compressing out of the way. Fail any one of the three and the layer is payload — mass that must itself be carried, by something else.

Discontinuity is the condition most often misread. A layer made of discrete pieces transmits compression wherever two pieces happen to be touching, but it transmits nothing across the gaps between them, so it cannot stiffen the assembly in bending. It can still feel entirely firm under a fingertip. Firmness under a point load and continuity across a span are different properties, and only the second puts a layer on the path.

One arrangement is worth naming because the whole course is standing on it. In a structural sandwich panel, two stiff outer faces are separated by a light core, and the arrangement is efficient precisely because the core does almost nothing except hold the faces apart and carry shear between them. The core is neither beam nor payload; it is a spacer doing structural work. Assemblies on this bench rarely achieve that condition, but it is the arrangement your specimen should be measured against, and the distance from it is the finding.

Investigation#

At the exploded load-path station, separate a specimen into named layers and record the stack exploded, in sequence. Reassemble it, apply a centre load, and classify each layer as support or payload with one observation attached to each classification. Focus each layer in turn and mark where the illuminated support path breaks.

A classification with no observation behind it is a guess, and guesses are recorded as guesses.

Failure condition#

The assembly deflects further under a given load than its outer layers did alone under the same load in Week 2. Everything between them is then demonstrably payload, and the record should state by how much.

What leaves the session#

A Load-Path Annotation: one exploded drawing or photographic sequence with every layer named, each marked support or payload, the continuous path drawn from the loaded region to a support, and the first break in that path circled. Carry it into the Component Test if your declared variable is layer position or stiffness — the annotation is where you show the variable was structural rather than merely present.