US Gulf Coast · Balsa: 20 supplied 1/8 × 1/8 in sticks
12 in gap · a loading block on top at mid-span · 100 g max
Official rules (PDF)(opens in a new tab)The rules don't mention AI tools. Check with your teacher or the organizer before you enter a design from this site.
Read it in the rulebook(opens in a new tab)Soon this page will show ready-made designs for this contest, each checked against every rule below, loaded until it breaks, and buildable with a hobby knife, with a printable build pack. For now, here are the rules in plain words.
Our summary of the official rules. If anything here disagrees with the rulebook or the organizer, they win.
Where a rule can be read two ways we use the reading the organizer most likely meant and show the other. Where ours is the looser one, ask your organizer before you rely on it.
The block's 7.5 in side runs across the bridge (as drawn) and rests on both top chords, each seated at least 1/4 in inside its ends.
The other reading: 7.5 in along the span, resting on a transverse beam.
The top chords peak at mid-span, so the block seats at the centre rather than rocking on an off-centre high point.
The other reading: Any top profile the block can sit on.
The tunnel floor is the deck surface (top of the floor system), and the deck is level.
“span a clear gap of 12 inches ... maximum overall length of 15 inches”
“The maximum height of the bridge is 5 inches.”
“block 1.5 inches in height and 2 inches in width to pass through the bridge deck”
“loading block 7.5 inches (length) x 2.5 inches (width) x 3/4 inch”
“A half-inch diameter hole must be provided through the center”
“total mass of the bridge must not exceed 100 grams”
“20 pieces of balsa wood, each 1/8 x 1/8 x 36”
“No other materials or reinforcements are permitted”
“must not be coated, painted, or laminated with any substance”
The other reading: The tunnel floor is the very lowest point of the bridge, which would forbid floor beams there.
'Laminated with any substance' means a surface coating; members built from several sticks glued together are allowed.
The other reading: No member may be several sticks glued together.
Gussets are allowed if cut from the supplied sticks (edge-glued 1/8 in plates). The rules set no size limit; we cap them at 2 in.
The other reading: No gussets at all.
The bridge reacts at the gap edges (x = +-6 in), the conservative span, and bears at least 1/4 in onto each abutment.
The other reading: Reaction spread over the bearing length.