How do you design a bridge around the loading block and eyebolt?
Why the load attachment shapes the whole design: leaving a gap for the eye and chain, loading at the top or bottom, and keeping the load point at mid-span.
Updated · 1 min read
Short answer: the way the contest attaches the load decides where your bridge needs a gap, a flat bearing area and a stiff middle. Design the middle of the bridge first.
What to find in the rules
- Where the load goes: on top, or hanging below, and how close to the exact middle it must be.
- The size of the loading block or plate, and which face of it touches your bridge.
- What hangs below it, such as an eyebolt, a chain and a hook, and how wide a passage they need.
- The minimum height of the load above the supports, if the rules set one.
Design consequences
- Leave a clear opening for the eye and chain, and make it a little larger than the minimum, since small errors happen.
- Give the block a flat, wide bearing. Balsa crushes across the grain, so spread the load over more wood. See why bridges fail.
- Stiffen the middle. The forces are greatest where the load goes.
- Keep the load centered between the two trusses so one doesn't carry more.
- Mind the height. The structure may have to stay above the support line, or the block may have to sit at a set height.
Draw the loading block, eye and chain on your plan at full size, and check them against the design. See how contests load a bridge and deck or through truss.