This Coal Mine Crib Beam used a Modified Captive Column design. A spaced core (a core with gaps) was used in this Captive Column, which the inventor rarely used because it is less efficient. However, the Captive Column Company made this particular beam, and their reasons for making this beam with a spaced core are vague. The Captive Column Company was one of the inventors first licensees and went out of business after this photo was taken. The history of the Captive Column Company is a chapter in itself and will be covered at a later date.
Mr. Bosch did not make this beam so the material used for the core elements is unknown, but it looks like aluminum in some old photos. Here are the recorded characteristics for the Coal Mine Crib Beam:
The strength-to-weight ratio of this beam was 5,000 (1,000,000 / 200). One of the reasons that this beam could take such a heavy load was that its slenderness ratio was relatively low (5 feet long and 18 inches deep) when compared to Captive Columns tested in the past. In addition, Captive Columns are most efficient when loaded on end.
This was a particularly heavy Captive Column due to the loading requirements. This beam was designed to replace the extremely heavy wood logs used to prop up coal mine tunnels.
Copyright © 1998-2004 by Lawrence R. Bosch.