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Time Line - 1965: The First Full Size Captive
Column... |
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| Held On One Finger (21k) |
Loaded With Cement (30k) |
This early (first full size) Captive Column beam was built in
1965 by Mr. Bosch and had the following properties:
| Length: |
10 feet. |
| Depth: |
10 inches. |
| Cross Section: |
Equilateral triangle. |
| Materials: |
Balsa wood core with fiberglass columns and skin. |
| Weight: |
17.5 pounds (1.75 pounds per foot). Beam was light
enough to be balanced on one finger. |
| Load: |
Photo shows a load of 3,120 pounds (52 Mortar Mix bags @ 60 pounds each).
Old records show that the beam was tested to 4,500 pounds without failure. |
| Deflection: |
3 inches (linear) for the 3,120 pound load. Recovery was
100% due to composite construction. |
Mr. Bosch made quite a few beams using balsa wood since it has an excellent
strength-to-weight ratio if used properly. Balsa wood may be expensive if you buy it in a
hobby store, but it's amazingly cheap if you get the rejects not suitable for model
building.
This beam had a strength-to-weight ratio of 257 with a 4,500 pound load applied.
Recovery was always 100 percent so it was subjected to a variety of tests, including
parking trucks on it and dropping it from buildings onto concrete (it bounced).
Over the years Mr. Bosch built Captive Columns out of a variety of materials:
fiberglass, twine, thread, graphite, kevlar, steel, cardboard, tape, and so on. Three of
the more interesting models were:
- A Captive Column that used spaghetti noodles for its column elements. It was
approximately 6-inches long and supported over 60 pounds on end.
- A Captive Column with rubber bands for the skin. Although this was not a practical
load-carrying beam, it made a good teaching aid. With this model it was very easy to see
how the skin elements responded to different types of loads.
- A Captive Column that used 1/8 inch steel cable for its column elements. We're talking
about the very flexible braided steel cable that's used with small winches. This model
demonstrated how a flexible element could be used to handle compression loads (like pushing
on a string instead of pulling on it). This model was made with the cables extending about
6 inches beyond the end of the Captive Column structure. It was fascinating to see how
these cables (which could be tied in knots) could be used to make a very strong and stiff
Captive Column.
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