Best Strength to Weight Ratio

OBJECTIVE Design and build a bridge using toothpicks to hold as much mass as possible over a span of at least 8 inches. Awards will be given for: Best Looking Bridge Strongest Bridge Best Strength to Weight Ratio Each student will research bridge designs from class, literature, the Internet or actual structures noting bracing points and reinforcements. Gravitational and load forces will also need to be taken into account as well as tension and compression forces. Each student will then design and build a bridge using wooden toothpicks and Elmer’s white glue. Testing will be conducted on all bridges.

CONSTRUCTION 1. The overall length of the bridge may not exceed 18 inches and must be able to span at least 8 inches. 2. The overall width of the bridge may not exceed 3 inches. 3. The bridge shall allow a 1-inch X ¾ inch board to pass through over the roadbed. 4. The bridge shall have no structures below the abutments that support the bridge. In other words, no portion of the bridge shall extend below the level of the support surfaces. 5. The bridge shall have an opening to allow a 3/8 inch bolt to pass through the center of the bridge when tested. 6. The bridge shall be constructed entirely of toothpicks. No additional materials may be used. 7. Any common adhesive/white glue such as Elmer’s may be used at the joints of the wood members. No Gorilla Glue, Tacky glue or Craft/Construction glues may be used. 8. Bridges may not be covered with glue.

MATERIALS

Toothpicks – flat or round, white or colored. 2 boxes to start with. Elmer’s glue – one smaller bottle – will only be used at joints Card board/poster board – about 18X24 inches – to build on Nail clippers – optional – to trim toothpicks Masking or clear tape – will be supplied- to hold sections together while drying. TOOTHPICK BRIDGE BUILDING CONSTRUCTION TIPS

1. Start early, plan thoroughly, and work steadily. Do not put your work off until the day before. You will receive a daily participation grade. If you do not have your supplies or choose not to work on your bridge, I will provide an instructive assignment for you to work on during class. 2. There are many different versions of bridges to model yours after. There are some examples provided in this packet. Go on the Internet to look at many different types of “Toothpick Bridges”. You will need to work from some type of design that needs to be submitted at the time of the Bridge Testing. 3. Be creative when building your bridge, but keep track of time. You may have to consider letting go of an idea so that your final design can be thoroughly tested. 4. Keep track of the mass of your bridge during construction. Wet glue weighs more than dry glue and makes weaker joints. Be sure to have your bridge completed several days before testing so it will be in the strongest condition possible. 5. The spacing between the intersections of toothpicks greatly affects the strength of your toothpick bridge. 6. Test each toothpick before you use it. When using flat toothpicks to build your bridge, roll the toothpick between your fingers to make sure they are truly flat and will not twist when pressure is applied. If using round toothpicks, apply a little pressure to bend it to see if it will hold up. 7. Good glue joints make for good strength. 8. Your bridge strength is also dependent on its vertical height. However, this is not to say “the higher the better”, as there are limits. 9. Consider the strengths and weaknesses of a single toothpick under: COMPRESSION, TENSION… Design your bridge to use the strengths of each toothpick as much as possible such as laying flat toothpicks on its side when designing for tension/compression. 10. Watch out for STRESS RISERS. Before failing, your bridge will give and bend under the load of your test weight. If you have made some parts of your toothpick bridge so strong and hard that it will not give, your bridge will fail in another area sooner than if the whole bridge were able to adjust. 11. To reduce drying time, do initial gluing with SMALL drops of glue (applied with an extra toothpick). You can go back later and reinforce these small drops with more glue after everything dries.

To research designs and see other bridge projects, Google “toothpick bridge design plans” and go to any of the websites. Most are very helpful and have many pictures of past designs to draw ideas from.

Your grade will come from the effort put into this project and will be your participation grade. A grading sheet will be supplied the day of the testing in order to give your project grade. Adherence to design, neatness, being built to specifications as well as the ability of the bridge to hold weight will be aspects on which you can earn points.