How to Build a Folding Bridge

The supporting sides of a folding bridge hold most of the weight.

When built across rivers, folding bridges are designed to have movable surfaces that lift to allow tall river traffic to pass underneath the structure. Therefore, when constructing a model of a folding bridge, similar rules apply to building the structure as they do for practical use. The bridge must be strong enough to withstand a scaled-down weight appropriate to its size, and its design must fulfill the structure's proposed design. When building a folding bridge, you can use many different types of materials, from toothpicks to slats of balsa wood.

Things You'll Need

• Drafting paper
• Wood
• Wood glue
• Scissors
• Miniature pulley
• Cord
• Marbles

Instructions

1. Draft your bridge design, choosing a bridge layout that suits your needs. Folding bridges have their weight balanced between two outside support beams, or load points. These beams hold the sides of the bridge steady and contain the internal rigging for lifting the folding bridge. Additionally, when drafting, design the trusses, which are the V-shaped spokes on the sides of the bridge. They help to support the sides of the bridge when the structure is built on unstable ground.

2. Line up your wood materials to represent the drafted design. Cut your wood down to size, if necessary. When working with Popsicle sticks or toothpicks, however you may not need to cut the materials at all.

3. Build your supporting sides, then two flat halves of the bridge. These halves must be identical to one another for the bridge to work correctly.

4. Install a pulley and line system within the side supports. This will allow you to control the up and down movement of the folding bridge. Attach the loose end of the cords to the central corners to allow for the greatest amount of leverage.

5. Fill in the rest of the bridge's structure, including the floor and trusses. Hold the structure together with wood glue and allow it to dry completely.

6. Test the weight of your bridge. Though it will be stronger at the sides, as the supports are closer, the maximum weight that the bridge can stand will be judged by its weakest point. Therefore, the points furthest from the supporting sides on each half of the folding bridge will probably be your weakest points. You can test the weight with marbles, ensuring that you leave yourself enough time to fix the bridge afterwards.

References

• Andrew Triumf: Building a Popsicle-Stick bridge
• Garrett's Bridges: Model Bridge Design