Balsa Wood Bridge

This instruction set will teach you how to design and construct your own bridge made out of balsa wood. This is a fun, intriguing project that will entertain an individual for hours.  Many high school and college science and engineering classes will have projects similar to this one. Keep in mind for those individuals building high school projects that if this design does not meet the particular project specifications, for example truss height and bridge length that is ok. For these individuals the main concept to take from this project is the truss design and how to put the bridge together.

.The supplies required to complete this project include the following

   • ¼ X ¼ inch balsa wood. This project used five 3ft long sticks
   • Hot glue/hot glue gun or wood glue
           o Hot glue gun is highly recommended as it dries much faster
   • One 18 inch ruler
   • One xacto knife
   • A cutting board
   • One pen or pencil to mark measurements

Measure and cut a 14 inch piece of balsa wood and label it the base. (Tip: when cutting balsa wood it has a tendency to fray at the ends leaving an uneven finish. To prevent this, cut the balsa wood by pulling the knife toward you similar to cutting a steak with a knife. This will leave a clean cut.)

Warning: keep your fingers away from the blade of the xacto knife as can cut the skin.

Use a ruler to mark one inch in from the left end of the base. Then, mark every two inches after that mark. This will indicate the location of the vertical truss supports. (Note: If a ruler is placed next to the base the marks should be read at the 1,3,5,7,9,11 and 13 inch marks.)

Measure and cut 7 two inch pieces of balsa wood. These will serve as the vertical supports for the trusses.

Glue the seven vertical supports to the base. Glue one support at every one of the 7 marks that have been placed at the base. Wait about 30 seconds to allow the glue to dry.

Warning: Avoid contact with hot glue as it can leave serious burns.

Now it is time to add the diagonal supports to the structure. The best approach to doing this is to first line up the desired piece of balsa wood over the area you wish to connect. Then using a pen/pencil, mark the angles and location at which make your cuts at . Then cut on the marks to get the desired angles.

Next line up the newly cut diagonal piece with the area you wish to connect it to if the piece lines up continue to step 8 if not then go back to step 5 and 6 and repeat them until it fits appropriately. The diagonal should create a triangle between the two supports. (This step can take a little time trying to get the correct fit. Don’t get discouraged as it does not have to be perfect.) Do this for all six of the bridge sections.

Glue the diagonal supports to the appropriate vertical support. Then wait about 30 seconds to let the glue dry. For the best results make sure the diagonals are positioned so that the lower connection is moving out from the center of the bridge. This will help distribute the load from the center of the bridge which is where it will be the most concentrated. As in the picture, the diagonals on the right side of the bridge will form an N shape with the vertical supports, while the left side of the bridge will form a reverse N shape with the vertical supports.

If desired add extra diagonal supports to the center of the bridge. As stated in steps 5 through seven. cut the diagonal to fit in the desired location then glue it to the vertical and the diagonal support. This should form an X shape between the sections as seen in the center of the image.
(Note: this can be done for all sections of the truss to provide extra strength to the truss.)

Measure and cut 12 1/2 inches of balsa wood and label it Top

Glue the top piece to the vertical supports. Apply pressure for 30 seconds to allow for the glue to set properly. (Note you may need to trim some of the vertical supports down to allow for a uniform fit.) Then trim off any hangover on the top piece make sure it is flush against the two end vertical supports. 

Connect the exterior vertical supports to the end of the base using a diagonal support as shown in the photo.

Repeat steps 1 through 13 for the second truss

Now it is time to connect the two trusses to each other. Start by cutting six 2 1/2 inch pieces of balsa wood. These will serve as the bottom bridge connection.

Space the 2 trusses approximately 2 inches apart from each other and glue the bottom supports to the base of the truss as shown in the photo. Wait about 30 seconds to allow the glue to dry.

Cut the top supports to a length of 2 inches.

Connect the top of the trusses by gluing the top supports to them. Wait 30 seconds to let the glue dry.

Now your bridge is complete and is ready to be tested.