Building Lab, Part V: Cables and Suspension Bridges

When thinking about building, your student might not think too much about the role of cables, however they are vital to a suspension bridge. A suspension bridge's cables and towers transmit the dead load of the bridge deck and the live load of traffic to the massive anchor blocks at each end of the bridge. The tension in the cables leading up from the bridge deck is balanced by the tension in the cables leading to the anchor blocks, as well as the compression in the towers. The anchor blocks must be massive enough to resist the tension in the cables caused by the weight of the bridge deck. 

Cable Demonstration

In order to demonstrate how cables can work to use the mechanical advantage that pulleys provide, you need some rope, a smooth stick like a broom handle or walking stick and some students.

Tie one end of a rope on a smooth stick like a broom handle or walking stick. Next, loop it around another stick and have the person holding the first stick to hold onto the remaining rope. It will look like this picture. 

Now, stand about three feet apart. You will need to let out some rope.

Once it is set up, have both people pull back on each broom, and you use the end of the rope to try to pull them together. It is hard to pull them together.

But, if you loop the rope first around the stick twice, it is so much easier to pull the two together this time.

Even your youngest student can pull heavier students or family members together.

Discuss how this design can also work with cables and bridges, converting the pulling-down force into a force that pulls up on the weight. 

Now that we have already looked at beam bridges, we want to look at suspension bridges and the differences between the two.

Students should find that adding the cables to their straw bridge and anchoring the cables on both sides significantly increases the load that the bridge can support. 

Make a model of a beam bridge by taping two straws together at one end.  At the other end, tape the straws together with a small piece of straw as a spacer, making a tall triangle out of straws. 
Make two sets of these, These are the two towers of the bridge. Tape these to chairs and then place another straw on top of the spacers to form a beam bridge. 
Next make a load tester by hanging a cup from the beam, and then take begin putting coins in the cup. It should hold a fair amount (about 15 coins) before the beam bends, dropping its load.

Next make a suspension bridge to compare how much it can hold. Tie the center of a length of thread around the middle of a new straw and place the straw between the towers. Pass each end of the thread or "cable" over a tower and down the other side. Pull the cable tight and anchor it on the table. The cup of the suspension bridge should hold many more coins (our held over 25 coins) and the bridge should be able to hold even more weight once the cup is filled. The difference in the ability of the bridges to hold weight and the importance of cables in larger bridges (with much heavier loads) should be easy for your students to see.