Fig 19 shows a flat-pack storage shelf unit made from medium density fibreboard (MDF). The structure consists of two upright side panels, four horizontal sh...

(a) The thin back panel prevents the shelf structure from collapsing sideways because it acts as a brace that resists the shear forces which would otherwise allow the rectangle to distort into a parallelogram. [1]

When fixed to all four edges of the rectangular frame, the back panel effectively triangulates the structure. Any sideways push creates diagonal tension and compression forces in the panel, and these forces resist the racking distortion. Without the panel, the pin-like joints between the shelves and uprights offer no resistance to sideways movement, so the rectangle can fold flat.

(b) The force acting in the upright side panels when the shelves are fully loaded is compression. [1]

The weight of the loaded shelves pushes down on the uprights, squeezing them along their vertical length. Each upright must support its share of the total load (up to 60 kg across four shelves) without crushing or buckling.

(c) One reason MDF is suitable rather than solid hardwood: [1]

MDF is cheaper and available in large uniform sheets without natural defects such as knots, splits, or grain irregularities that can weaken solid timber. Its consistent density throughout means it machines cleanly, holds screws well along its face, and provides a smooth surface that can be painted, veneered, or laminated without additional preparation. Solid hardwood would be significantly more expensive and more difficult to obtain in the wide, flat panels needed for shelf sides.