Key Stage 3 design and technology · Technical knowledge

KS3 design and technology: beams, frames and forces

In Years 7 to 9, pupils learn why some structures stand up and others sag, buckle or snap. They study the performance of structural elements such as beams, frames, struts and ties, and the forces that act on them. They then test their own models, judge them against a specification, take account of what users and others think, and improve the design.

Beams, frames and forces, explained

A structure is anything that has to carry a load and keep its shape, from a shelf bracket to a footbridge. Pupils break a structure down into structural elements. A beam spans a gap and is loaded across its length. A strut is a part pushed along its length, a tie is a part pulled along its length, and a frame is a set of such parts joined to make a rigid shape. A truss is a frame built from triangles, as seen in many roofs, pylons and bridges. A shell structure, such as a bicycle helmet or a drinks can, gets its strength from a thin outer skin. Children usually meet these ideas by building and breaking things, then naming what happened.

Forces are named precisely. Tension stretches a part, compression squashes it, bending combines the two (when a beam is loaded in the middle, the top edge is squashed and the bottom edge is stretched), torsion twists it, and shear tries to slide one layer past another, as scissors do. Pupils learn that a part can fail in several ways. A long, thin strut under compression usually fails by buckling, bowing sideways, long before the material itself is crushed. Seeing which force acts on which part is the first step in deciding how to make it stronger.

Several reinforcing methods are taught. Triangulation turns a wobbly rectangle into a rigid frame, because a triangle cannot change shape unless one of its sides changes length. Beams are made deeper, or shaped as an I, a channel or a hollow box, so that most of the material sits at the top and bottom, where bending forces are greatest. Laminating glues thin layers together, corrugating adds folds, and gussets, the small corner pieces, stiffen joints. For a solid rectangular beam, doubling the depth makes it about eight times stiffer, while doubling the width only doubles its stiffness. Depth matters far more than width, and many teachers show this with a ruler laid flat and then on its edge.

This topic also covers evaluation. The curriculum asks pupils to test, evaluate and refine their ideas and products against a specification, taking into account the views of intended users and other interested groups. In practice, a pupil builds a model, tests it to a stated target, records what happens, and suggests specific improvements. A bridge might need to span a set gap and carry a set load, but it should also be pleasant to look at for the people who will walk across it and not too expensive for the town. Judging a result against both measurable and human requirements is a mark of a strong project.

Words your child will hear

Beam: A horizontal part that spans a gap and carries a load across its length.
Tension: A pulling force that tries to stretch a part. A part working in tension is a tie.
Compression: A pushing force that tries to squash a part. A part working in compression is a strut.
Triangulation: Adding diagonal members to make triangles, so that a frame cannot be pushed out of shape.
Buckling: A long, thin part bowing sideways under a compressive force.
Specification: A list of requirements a product must meet, such as size, load and cost, that is used to judge the finished result.
Deflection: How far a structure bends under a load. Less deflection means a stiffer structure.

What the National Curriculum says

The programme of study for Key Stage 3 says:

When designing and making, pupils should be taught to understand and use the properties of materials and the performance of structural elements to achieve functioning solutions
When designing and making, pupils should be taught to test, evaluate and refine their ideas and products against a specification, taking into account the views of intended users and other interested groups

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

Try it: testing and refining a card bridge

A pupil must make a bridge from two sheets of A4 card that spans a 20 cm gap between two piles of books. The specification says it must carry a 500 g load at the centre without bending more than 2 cm, and it should look attractive to the people who would use it. How do they test and improve it?

Make a first prototype. A flat strip of card laid across the gap is the simplest design. Hang a 500 g load at the centre and measure how far it sags. The flat strip sags so far that it folds and touches the table, far beyond the 2 cm limit. (The measurements in this example are illustrations; real results depend on the card.)
Diagnose the problem. The bridge is acting as a beam. The top surface is being compressed and the bottom surface is being stretched, and a thin flat strip has very little depth to resist this. The material has not failed, but the shape is wrong.
Refine the design. Fold the card into a box section about 4 cm deep, and glue the seam. Most of the card now sits at the top and bottom, and the walls between them stop the sections sliding past one another. Add small card triangles inside the box at intervals to stop the thin walls buckling.
Retest under exactly the same conditions: same span, same load, same measuring method. This time the bridge sags less than 1 cm, which meets the specification. Keeping the test the same is what makes the comparison fair.
Evaluate against the whole specification, including the users. The bridge now works, but the box section looks plain, and an adult asked for a rail for safety. The pupil decides to add a lightweight rail and to use coloured card on the outside, then retests to make sure the extra weight does not spoil the result.

The bridge improved because the depth of the beam increased, not because more card was used. A good evaluation states the measured result against the target (under 1 cm against a 2 cm limit), says what changed and why, and responds to the views of the people who would use the bridge.

Common mistakes and how to help

Using more material always makes a structure stronger

It seems obvious that extra card or wood must help. But where the material goes matters more than how much there is: loose layers simply slide over one another, and a single sheet folded into a deep shape can beat them.

What helps: Let your child test one sheet of paper laid flat and the same sheet folded into a channel or tube. Same amount of material, very different result.

Triangles are strong because they are solid

A triangle looks sturdy, and children assume the shape itself is strong.

What helps: The point is that a triangle’s shape cannot change without a side changing length, so the sides only have to push or pull. A rectangle made from four strips can fold flat. Build one from strips and paper fasteners, then add a diagonal.

Tension and compression are the same thing

Both are forces along a part, and in a quick sketch the arrows look similar.

What helps: Compression pushes inward, tension pulls outward. Use a rubber band for tension and a drinks straw pushed from both ends for compression. Notice that the straw bows and buckles, but the band does not.

A test that fails means the project has gone wrong

Pupils often see a collapse as a bad result, and tend to hide it or start again without recording anything.

What helps: In D&T a failed test is useful evidence. Ask where it broke, which force was acting and what change would help. Writing that down is exactly what the evaluation asks for.

How to help at home

Ruler on edge. Lay a ruler flat across two piles of books and press gently in the middle, then stand it on its edge and press again. Ask your child to explain the difference using the word depth. Try the same with a strip of thick card.
Build and test with newspaper. Roll newspaper into tubes, tape them into frames and test them with a small load such as a tin of beans. Compare a square frame with one that has a diagonal brace. Use a table or the floor, and keep fingers clear of anything that might fall.
Spot the structural elements. On a walk, look for beams, frames and trusses in bridges, pylons, bus shelters and playground equipment. Ask which parts are being pulled and which are being pushed, and why the shapes were chosen.
Practise fair testing and writing it up. Set a target, such as holding a tin of beans without sagging more than 2 cm, and let your child change one thing at a time. Ask them to describe what they changed, what they measured and what they would try next.

Builds on

Key Stage 2: Strengthening and reinforcing structures
Key Stage 2: Gears and pulleys
Key Stage 2: Cams, levers and linkages
Key Stage 2: Series circuits in products
Key Stage 2: Programming and controlling products