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arrow1.gif (893 bytes) Time Line - 1980: UND Tests NATO Tower Components...

The following photos show the results of an impromptu test of two Captive Column NATO Tower components. These tests were performed at the University Of North Dakota (UND) Engineering Experiment Station. UND is located at Grand Forks, North Dakota. Higher quality photos may be made available at a later date.

With UND's test equipment down for repairs during summer vacation, a fixture with load-cell and read-out was assembled especially for this test. However, due to the impromptu nature of the test, provisions were not made for deflection measurements or containment of the structures.

Due to the high strength-to-weight ratio and energy absorption characteristics of Captive Columns, great care must be taken when testing them to destruction. Therefore, some portions of this impromptu test were abandoned since the required protective fixtures were not available.

(22002 bytes) A 5 foot Captive Column, consisting of four 1/8 inch x 5 foot fiberglass columns was easily loaded to over 500 pounds (1,000 times its own weight) with no discernable effect. Although weighing only 8 ounces (approximately twice the weight of the columns alone), the Captive Column geometry supported the columns along their entire length and prevented them from buckling under load. (22k)
(20645 bytes) The 8-ounce Captive Column began to bow at a load of about 700 pounds, but there was no sign of weakness or failure in any of its components. While each individual column element had a slenderness ratio of 480 to 1, using them in a Captive Column shifted their total slenderness ratio to 30 to 1. Thus, with only a doubling of weight, slenderness was reduced by a factor of 16. This accounts for much of the great increase in compressive strength. (20k)
(15645 bytes) Structure bow at about 950 pounds, when first failure occurred. Strength dropped off to about 800 pounds and more weight was required to fail it further. As strength continued to drop, additional load was required to propagate the failure. (15k)
(18829 bytes) Although the Captive Column severely bowed in the middle, the structure broke at the bottom. The Captive Column returned to its normal straightness (except for the damaged end) when the load was removed. (19k)
(41446 bytes) This 10 foot double-taper composite Captive Column weighed only 2.4 pounds (0.24 pounds per foot) but easily held a load of 1,300 pounds (over 500 times its own weight) with no signs of stress. While this structure was designed to hold over 2,000 pounds, the test was abandoned for fear that it would escape the press. (41k)

 


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