Captive Column . Com

arrow1.gif (893 bytes) Time Line - 1965: The First Full Size Captive Column...
Held On One Finger Loaded With Cement
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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Copyright 1998-2004 by Lawrence R. Bosch.