Building Big: Suspension Bridges

This week we looked at the differences between suspension and beam bridges.

The tension in the cables leading up from the bridge deck is balanced by the tension in the cables leading up to the anchor blocks as well as the compression in the towers. We made a model of a beam bridge by taping two straws together at one end and at the other end taping them together with a small piece of straw as a spacer, making a tall triangle out of straws. We made two sets of these and these were the two towers of the bridge. The towers were taped to chairs and another straw was placed on top of the spacers to form a beam bridge. We made a load tester by hanging a cup from the beam, and they took turns putting coins in the cup. It held a fair amount (about 15 coins) before the beam bent, dropping its load. The next step was to make a suspension bridge and compare how much it could hold. We tied the center of a length of thread around the middle of a new straw and placed the straw between the towers. We passed each end of the thread or "cable" over a tower and down the other side. The cable then was pulled tight and anchored on the table. The cup of the suspension bridge filled with over 25 coins, but the bridge could have held more weight if the cup could have held more. We need to devise a system with a larger cup! After we finished with this activity, Fourth and Fifth Students continued to use the materials to make structures to test.