Technologies K–10 · Years 5–6

Triangles or squares: bracing a straw tower

Design and Production (NSW Science and Technology K–6, 2017), design and production outcome only: the Stage 3 Physical World content has no point on structures, and the point on how forces and materials interact in a product or system (ACTDEK011) sits in Stage 2; Design and Technologies: Knowledge and understanding, Technologies context: Materials and technologies specialisations (ACARA v9)

PracticalLow risk

The idea

A square frame with pinned corners folds into a parallelogram under a sideways push, while a triangle cannot change shape without a side changing length, so a diagonal brace makes a frame rigid.

What you need

75 paper drinking straws per group, each at least 18 cm long, and one piece cut from each straw: either a 12.0 cm frame member or a 17.0 cm diagonal, because the diagonal of a square with 12.0 cm sides measures 12.0 × √2 = 17.0 cm and no straw yields both pieces (5 straws for the square in steps 1 and 2, 28 for the unbraced tower, 40 for the braced tower, 2 spare)
Pipe cleaners cut into 4 cm pieces as corner joints
Masking tape
1 spring balance (newton meter) reading to 0.1 N
Scissors and a ruler, for cutting the pieces to length and measuring how far the tower top moves

How to do it

Cut 4 frame members 12.0 cm long, join them at the corners into a square frame, then push one top corner sideways and watch the shape.
Cut one 17.0 cm diagonal, fit it corner to corner across the square and push again.
Build two 3-storey towers on a square base ring of 4 members, all 12.0 cm; each storey adds 4 uprights and a square ring of 4 members above them. Put a 17.0 cm diagonal in every side face of one tower and none in the other.
Pull the top of each tower sideways with the spring balance until the top moves 2 cm. Record the force.
Repeat 3 times per tower.
Point to real structures that use triangles (roof trusses, bridges, power pylons, bike frames).

What you should see

The unbraced square folds into a parallelogram under a light push because its corners can turn; with one diagonal it becomes two triangles and holds its shape. The braced tower needs a much larger force to move its top 2 cm than the unbraced tower, which leans at the slightest pull. The learner knows it worked when the braced reading is clearly larger in all 3 trials and they can explain why the triangle cannot fold.

What changes

What you change: bracing (none or a diagonal in every face)
What you measure: sideways force to move the top 2 cm (N)
What you keep the same: the same straws and joints
the same tower height
the pull applied at the same point

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

A square is as strong as a triangle because it has more sides (its corners can turn, so it can fold).
Thicker straws alone make a tower rigid (without bracing a thick-strawed frame still folds).
Triangles are used in bridges only for looks (they keep the shape fixed under load).

Safety card

Low riskLearners carry it out

Hazards

Pipe-cleaner wire ends

Controls

Fold the ends over

Note

No chemicals or heat.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science and Technology K–6 Syllabus (2017), NESA. The syllabus taught in 2026; the 2024 syllabus replaces it from 2027. Code read from the official syllabus document (DOCX) on 2026-09-22.ST3-2DP-T
Science and Technology K–6 Syllabus (2024), NESA. Implementation from 2027, so this code describes the future syllabus. Code read from the outcomes page on 2026-09-22.ST3-DDT-01
Australian Curriculum v9AC9TDE6K05AC9TDE6P03

Sources

The pages the author read to write this activity.

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