Did you realize how many different types of bridges we have in the Quad Cities?
Since designing the Exploratorium Building Bridges program, I keep looking at bridges, explaining to my very patient husband that the footbridge he just crossed over was a beam bridge and the strengths and weaknesses of that type of bridge. All this time I just thought bridges were “pretty” or “ugly” because of their design. I never thought there was a reason for the style of bridge or to really think about the amazing amount of weight these bridges have to carry. It’s been an eye opener.
It all started with the model of the new I-74 bridge that is currently located in our lobby. I thought this would be a great time to do a STEAM program on bridges and how they are built.
At the program we observed what made a design strong with 4 different type of bridges. At the end, everyone designed their own bridge that would be strong enough to hold an apple. If you want to try these at home, they are very simple.
Supplies: 2 even stacks of books, 2 pieces of card stock, scissors, tape and weights. Weights can be anything the same size like small blocks or other toys that are the same size and weight that will not roll away from you or break.
Place a piece of cardstock on the two stacks of books so it looks like a bridge. Now begin adding weights, one at a time. How many weights did you add before the bridge collapsed?
Now create 2 piers from the 2nd sheet of card stock by rolling up the cardstock into a tube and taping it. Cut the tube into 2, sizing them so it fits under the “road” of your bridge. Place the 2 tubes or piers under the bridge and start adding weight again. Did it hold more weight?
Beam bridges are simple to create and are ideal for short distances unless you have other ways to support the bridge.
Supplies: 2 even stacks of books, 2 full pieces of card stock, weights.
Recreate your beam bridge without the piers. Form the second piece of cardstock into an arch and place it under the road of the bridge. How many weights does the bridge hold? Was this more than the beam bridge without piers? With piers?
If you were to create an arch from stones or blocks, this would need tension to hold the arch together. An example of the tension in an arch is to stand facing someone about the same height as you. Both of you hold up your hands and grasp them to form an arch. Now lean forward. The force you feel where your hands meet is the same force that would hold stones together in an arch. The arch shape is able to disperse the weight to the ground or abutment of the bridge.
Supplies: 2 even stacks of books, 1 full piece of card stock, 1 piece of paper, weights.
Truss bridges get their strength from a framework made of triangles. Triangles are much stronger than squares or rectangles because they can move the pressure of the road load from a single point to a much wider area.
Recreate your beam bridge. Remember how many weights it would hold?
Now fold your piece of paper like you would a fan. Spread it out and place it under your road so it spans the bridge. Now try adding weight. Does it hold more?
Why do you think the fanned paper was helpful? Would it work if it were a flat sheet of paper?
Supplies: a long piece of cardboard about 4 ft long for the road. It may be bent. A hole punch, 2 chairs with an open back, pipe cleaners, 2 long pieces of cord about 12-14 ft long and some weights – we used 4 Lego blocks.
I found this bridge fascinating. It seems impossible that two pieces of cord can hold up a bridge!
Place the chairs back to back. Place the strings parallel to each other over the chairs so that the ends go over the front of each chair and reach the floor. If it helps for set up, tape the cords to the top of the chairs. Just be certain to take off the tape when the bridge is assembled. Tie the ends of your string to your weights and pull the weights away from the chairs to make the cords taught. Punch holes on either side of the cardboard, 3-4 on each side, evenly spaced out. You do not need to have holes on the far ends of the cardboard. Attach a pipe cleaner to each hole by threading it through the hole and twisting the end to the pipe cleaner. Place the ends of the cardboard on the seats of the chairs. You may need to support the cardboard until you have the pipe cleaners in place. Next, hook the pipe cleaners onto the cords. Now you can pull the road support away and take off the tape.
Try putting on a toy car on the bridge. Does it hold?
What happens if you move the weights closer to the bridge?
What if you have fewer pipe cleaners? No pipe cleaners?
Can you determine what is supporting the weight?
When I started designing this program, I didn’t know the first thing about bridges, however, through the books and kits we have at the library plus the Internet, I developed a great respect for the bridges I see every day. Which is a lot of bridges!
It turns out the Quad Cities has many examples of the different types of bridges. When you cross a bridge, see if you can determine what kind it is: Beam, Arch – Tied Arch, Suspension, Truss, Swing, or Cable-Stayed.
And the next time you come into the library, be certain to check out the new I-74 bridge model in the lobby. It’s awesome!