How to make a Whipple Truss Bridge using cardboard and duct tape:
In 1841 Squire Whipple patented a bridge which would be more stable than the bridges of that time; his secret was to use diagonal rods and to make the bridge wider near its ends. He analyzed his design and found that the weight on the bridge put the vertical pieces in compression and the diagonals only under tension. This is perfect for cardboard and duct tape.
Duct Tape is extremely strong under tension, so it will be used for the diagonal pieces.
Cardboard is made many ways; corrugated cardboard usually is stronger under compression along one axis, and is relatively easy to bend otherwise. This design will use the cardboard so that it is aligned to handle the compression.
Components of the Whipple Truss Bridge:
Arch Truss pieces (a)
Vertical pieces (v)
Diagonal pieces (d)
Horizontal pieces (called ties by Whipple) (T)
Flooring (not shown)
Knife (box cutter, kitchen knife, X-acto knife, etc.)
Scissors (for cutting duct tape cleanly)
Pen or pencil (the pen didn't draw on greasy cardboard well)
Step 1: Arch Pieces
The Arch supports the flooring, and all of the vertical and horizontal pieces are made of cardboard.
Step 2: Drawing the Arch Pieces
Using the patent drawing by Whipple as a guide, a scaled model for a 80 foot bridge was drawn.
These pieces were drawn onto cardboard:
(2) 11.25 x 2 inch rectangles (a)
(4) 10.75 x 3.4 inch rectangles (b)
(4) 8.75 x 4.5 inch rectangles (c)
(4) 11.5 x 6 inch rectangles (d)
A lot of 1.5 inch wide cardboard for the U parts of the arch
A lot of 1 inch wide cardboard for the vertical pieces
Step 3: Making the Sides
The Arch has interlocking sides, this is what they look like.
After making the first side of the bridge without them, and seeing how unstable it looked, I made the second side like Whipple designed it. How should I know about mechanical engineering; I'm an electrical engineer.
Step 4: Assembled Arch Segment
This is what the arch segments look like assembled with duct tape holding the tops and the sides together.
Step 5: Assembled Arch 1
This is all of the arch segments assembled together.
Note that the vertical pieces are just straight pieces of cardboard, shoved through holes in the top arch segments. In this picture they have not been cut to a reasonable size to make an arch shape.
Step 6: Assembling Arch 2
This is a picture of the other arch of the bridge.
Step 7: Tape Diagonals
Tape the diagonals of all the arch segments. This will dramatically increase the rigidity of the bridge. Whipple's design had these diagonals be adjustable with threaded rods and nuts, but I think that the duct tape holds quite well.
Notice also that the arch segments have been placed on some horizontal ties, which have been duct taped together.
Flooring has also been placed between the two arches.
Step 8: Finished Bridge
This is what the finished bridge looked like.
Many thanks to Gene, Nick, and Gary for their assistance cutting out all the pieces of cardboard.