Building a small-scale bridge can teach you a lot about the strength and support needed for a life-size bridge to function safely. Sometimes teachers assign students the task of building their own small-scale bridge using only a small number of materials. Materials like straws, paper and tape may be weak materials by themselves, but put them together correctly and you may find the design can withstand more weight than you realise. The design of your bridge greatly affects how much weight it can support.
Gather your materials together. Work on a flat surface in a well-lit area.
Lay out the butcher paper. Decide how long a bridge you want to create. A good size to shoot for is around 30 cm to 36 cm (12 to 14 inches), but you can make it as long as you wish. Keep in mind that the longer you make the bridge, the more supplies you'll need.
Measure the length of the bridge you want to create. Stretch out the tape measure and mark the beginning and the ending of the length you want. Lay the tape measure to the side.
Plan the trusses for one side of your bridge. If your bridge is 30 to 36 cm (12 to 14 inches) long, you will need four equilateral triangles, two slightly larger than the other two. The larger two will be at the middle of the bridge, while the smaller two will be at either end.
Create the triangles by bending a straw to create a triangle. You will have to cut off a portion of two of the straws to create the two smaller triangles. Lay the triangles out side-by-side on the butcher paper to make sure they are going to fit within the measurement you created. Make the other side of the bridge by repeating this same process. You should end up with a total of eight triangles, depending on the length of your bridge.
Construct the top frame of either side of the bridge using straws. This frame will arch along the length of your bridge. Cut the straws in order to form an appropriate arch. Hold the ends together using tape. Create two of these frames, one for either side of the bridge.
Begin putting together one side of the bridge. Connect four of the triangles to the arch frame you just constructed. Wrap two to three strips of tape around the top angle of each triangle and around the frame. Repeat this process with the other side of the bridge.
Set up both sides of the bridge. Use pieces of straw to connect the two sides of the bridge together at anywhere from two to six points along the frame. Use tape to secure a hold.
Secure the ends of the triangles to a straight base or long connection of straws. This will be the bottom of the bridge. Make sure each triangle end is touching one other for maximum support. Cut a straw at an appropriate length and connect it in between each triangle as well as inside each triangle. This will create added strength to your bridge.
Use heavy paper to create triangular gusset plates at your connections along the bridge. Tiny triangles taped onto the connections are all that's required.