Suspension Bridge

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Our Suspension Bridge was designed and built by Carroll Vogel of Sahale, LLC.  (They also designed and built most of our tail system, and re-engineered and assembled the Canopy Tower.)  Besides being fun, the bridge spans the narrowest part of ravine – other means of crossing the ravine would have eroded the banks and/or have been too costly.

A classic suspension bridge, the design includes two towers that support cables, anchored at each end.  The bridge deck (or trail in our case) is suspended from the cables by hangers or stringers.  On our bridge, the towers are just over 22 feet tall, and are set 189 feet apart.  At the center of the bridge, the ridge deck is approximately 58 feet above the stream.

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Instruction

Kids study different layers in the forest canopy; look for birds/animals; face challenges – like crossing the bridge.



About Suspension Bridges

What happens to a rope during a game of tug-of-war? It undergoes tension from the two sweaty opposing teams pulling on it. This force also acts on bridge structures, resulting in tensional stress. What happens when you push down on a spring and collapse it? That's right, you compress it, and by squishing it, you shorten its length. Compressional stress, therefore, is the opposite of tensional stress. Tension and compression are two forces every bridge knows well.

In a suspension bridge, the towers support the majority of the weight as compression pushes down on the suspension bridge's deck and then travels up the cables, ropes or chains to transfer compression to the towers. The towers then dissipate the compression directly into the earth.

The supporting cables, on the other hand, receive the bridge's tension forces. These cables run horizontally between the two far-flung anchorages. Bridge anchorages are essentially solid rock or massive concrete blocks in which the bridge is grounded. Tensional force passes to the anchorages and into the ground.

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The cables must be secured and anchored past the ends of the bridge. Why? Look at Figures 1a and 1b. In Figure 1a the bridge is not anchored to the land. What happens to the bridge?

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To feel the difference between a cable that ends at the tower top and one that goes over it and continues to the ground, grab your head with your right hand and gently pull (Figure 2a). If you try to keep your head straight, your neck will feel compressed, but also pulled to the right.

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Now interlace the fingers of your hands, put them over your head and pull with both arms (Figure 2b). Your head and neck will feel compressed, but your neck will not feel a pull to the right or left, since the force of your right arm is balanced by that of your left arm.

A good activity to help explore the ideas of tension and compression is the Yurt Circle.


Look at this video of the bridge being build ----> Bridge_Builders.mp4