We ask a lot out of our bridges. We build them across vast spaces, stack them into double decker’s and place bumper-to-bumper traffic on them. We expect that our bridges will support a great deal of weight and that they will last for a long time. Bridge designs have improved a lot in the past centuries and our understanding of the physical principles of design, along with the invention of new materials has made it possible for people to design and build bridges that our ancestors could never have dreamed of. Though bridges are safe, there are occasional accidents and errors in design that cause deaths. Engineers need to understand exactly what a bridge can do as well as what it cannot if they are to design a bridge that will serve the needs of the community while keeping the community members safe from harm.
The purpose of this experiment is to build models of various types of bridges and then place weights on them until they break to determine which bridge design is the strongest.
Are certain bridge designs are stronger than others?
- Popsicle sticks
- Kite string
- A bowl of water
- Using popsicle sticks, glue and string, construct three different types of bridges. It is important to use the same materials in each of your bridges so that you are testing the strength of the bridge design and not the strength of the bridge materials.
- Start by making a beam bridge: Glue three sticks together at the ends to form a triangle. Make 10 of these.
- Glue 4 sticks together in the shape of a square. Make 7 of these.
- Allow the glue on the 10 triangles and 6 squares to dry completely (check the label on the glue to find out how long to let them sit).
- Place one square down flat. Place two more squares upright on opposite sides and glue them to the flat square. Allow the top edges of the upright squares to come together and glue them there.
- Follow step 5 to make one more like this and allow the components to dry.
- Use 5 triangles to make each side. Place one triangle so that a point is facing away from you. Line up the next triangle so that a point is facing towards you. Glue these triangles together along one of the popsicle sticks (they will be side-by-side and slightly overlapping). Continue in this fashion until you have five triangles in a row.
- Repeat step 7 to make one more row of 5 triangles and allow the glue to dry completely.
- Place the last square in the center of your workspace. Place the two structures made of three squares each on opposite sides of this square. Make sure the edges are touching and glue the seams together. Allow the glue to dry completely.
- Turn the bridge so that the points on the three-square structures are facing towards you.
- Glue the first side (5 triangle piece) onto the bridge. Allow the glue to dry completely.
- Turn the bridge over. Glue the second side onto the bridge, making sure the points of the triangles are lined up with the points on the other side. Allow the glue to dry completely.
- Glue a popsicle stick between each set of triangle points (there are three). Allow the glue to dry completely.
- Construct an arch bridge: Cut the round edges off of 76 popsicle sticks. Place six of these in a bowl of warm water.
- Glue 16 popsicle sticks together into a stack. Use the popsicle sticks with the ends cut off.
- Repeat step 15 to create one more stack of 16.
- Glue 17 popsicle sticks together into a stack. Use the popsicle sticks with the ends cut off.
- Repeat step 17 to create one more stack of 1.
- Glue the stacks together in this order: 16, 17, 17, 16.
- Glue one of the wet sticks onto one side of the top of these stacks.
- Glue another one of the wet sticks onto the other side of these stacks. Both these sticks will bend to accommodate the different heights of the sticks.
- Glue one stick to the side of the bottom of the stacks.
- Glue another stick to the opposite side of the bottom of the stacks. These sticks will remain straight.
- Attach a popsicle stick to each end of both bottom sticks by overlapping them slightly.
- Use the other 4 wet sticks for the next step. Glue one end of each of these to the ends of the sticks on the top of the bridge. Connect these sticks to the sticks coming off the bottom of the bridge. (You may need to trim them or add additional sticks to make them line up. You should now have an arc from the top of the bridge to the base on each side.
- Glue popsicle sticks on top of the edges of the arc, flat side down.
- Make a suspension bridge: Place 50 popsicle sticks together so that the long sides are touching on a piece of wax paper. Glue the edges together and allow the glue to dry completely.
- Make 4 squares using 4 popsicle sticks each. Glue the popsicle sticks together at the corners. Allow the glue to dry completely.
- Glue 2 popsicle sticks together at the ends to create one long popsicle stick.
- Repeat step 29 until you have 16 of these. Allow the glue to dry completely.
- Take two of these long sticks and glue them together with the long edges touching at a 90 degree angle. You are making the sticks into a corner. Repeat until you have 8 corners. Allow the glue to dry completely.
- Glue four of these corners upright into the corners of one of the squares. Glue another square to the corners at the other edge. You should have a tower with a square at either end.
- Repeat steps 31 and 32 to make one more tower. Allow the glue to dry completely.
- Remove the bridge from the wax paper. Place one tower at either end. Run a string between the two towers so that it sags towards the bridge and almost touches it in the middle. Make sure this string is on the outer edge of the towers and the bridge. Repeat on the other side of the bridge.
- Tie shorter pieces of string from the long string onto the individual popsicle sticks on the bridge. Start in the middle and work your way out.
- Test the strength of each of your bridges: Place your beam bridge between two tables. Slowly add weight until the bridge collapses.
- Record how much weight it took to break the bridge.
- Repeat steps 37-38 for each bridge.
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