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Bridge Building Basics:

What is a model bridge?

This Model Bridge Competition seeks to challenge an individual to build a bridge that is the most structurally efficient.

  • Structural efficiency basically means the lightest bridge that can hold the most weight

  • The bridges in this competition are to be constructed of strips of basswood and held together with any commonly available glue

  • The model bridge must be built to the dimensions shown in the Rules

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The bridge type that is most commonly designed and constructed is a truss type. More advanced designers can attempt to build bridges that are arched, bowstring or other configuration as long as it complies with the rules. (Advanced designs are not necessarily more structurally efficient but they are more challenging to build)

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Some examples of bridge types that can be seen in this competition are as follows:

  • Truss Bridges (common):

    • A-Truss

    • Howe Truss​

    • Pratt Truss

    • Warren Truss

    • Camel Back Truss

  • Truss Bridges ( less common):

    • Whipple Truss​

    • Baltimore Truss

    • K-Truss

    • Petit Truss

  • Other Bridge Types:

    • Bowstring​

    • Arched

Truss Bridge Examples

How are the bridges tested?

The bridges are tested using one or both of the following techniques:

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  • Hanging a weight (usually a container that can be filled with various smaller weights, steel shot or sand) from the midpoint on the bridge span. The container is filled with known weights until the bridge breaks. The container is then measured to determine the final weight held by the bridge.

  • Using a mechanical or pneumatic testing device that pushes down on the bridge until the bridge breaks. Some devices of this type can also exert predetermined forces or patterns of forces on the bridge. They can also save the bridge from being destroyed by sensing exactly when the bridge starts to buckle and executing immediate reduction of testing forces.

Bridge Building Tips

Below are a few tips to building a good bridge.

  • You can use graph paper to draw the shape of your bridge. Then you can use this template to cut out both sides of the bridge and keep it symmetrical. And don't forget to measure twice and cut once!

  • Make sure to add plenty of lateral bracing between the two sides of the bridge to keep it from twisting during the competition (Lateral bracing refers to the cross members that hold the 2 halves of the trusses together)

  • Although any commonly available glue can be used, make sure to investigate the properties of the glue before deciding on which one to use

  • Don't get disqualified. Be careful with your measurements. Most disqualifications are a result of the model being just slightly the wrong size

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