Paper Bridge Strength Test
Same paper, same coins — but the folded bridge holds ten times the load. Shape is the secret engineers never stop exploiting.
No hazards — just paper and coins. Keep coins away from small children who might swallow them.
Materials
- Plain paper sheets (several)
- Two equal stacks of books (the supports)
- Coins (pennies work well)
- A ruler
- Tape
Procedure
Test the flat sheet
Set the book stacks about 20 cm apart. Lay a flat sheet across the gap and add coins one at a time in the center until it collapses. Record the number.
Test the accordion fold
Fold a fresh sheet into a tight accordion (1 cm folds). Lay it across the same gap and load coins the same way. Record the new record.
Test the tube
Roll a sheet into a tight triangular tube and tape it. Test it against the same gap and load.
Compare fold sizes
Try a wide flat fold (3 cm folds) versus a narrow one (1 cm). Which holds more, and what fails first — the paper or the shape?
Score efficiency
Divide each bridge's coin count by the sheet's mass for a strength-to-weight score. Crown the most efficient design.
What to observe
A flat sheet collapses under just a few coins.
An accordion-folded sheet of the same paper holds dozens.
Taller folds hold more — until the folds buckle sideways.
Why it happens
A flat sheet fails because its thin cross-section gives almost no resistance to bending — stiffness scales with the cube of depth. Folding the same sheet into corrugations or a triangular tube moves material away from the neutral axis, multiplying the second moment of area without adding a gram. Real bridges do exactly this: I-beams, box girders, and trusses put steel where bending stresses are highest and leave the middle nearly empty.
Try variations
Span a wider gap (30 cm). Which design degrades least?
Glue two accordion sheets together. Does the strength double, or better?