Captive Column . Com
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arrow1.gif (893 bytes) How It Works...

Most people (especially engineers) find it surprising that such a simple and elegant geometry was overlooked for all of human history. The information presented here comes from the inventors copyrighted brochure.


Design principle...

The working principle of the Captive Column structure is to support the main load-carrying column elements totally along their entire length so they cannot buckle or move in any direction relative to each other. This is accomplished by holding them captive between core elements inside and skin elements outside. These three basic elements then work together to resist any type of loading applied to the structure.

Captive Column Design Concept


Functions and fabrication of the Skin Elements...

Function Of The Skin

The skin elements undergo either tension or compression. However, since they are relatively flexible due to their high length-to-thickness ratio, they function only in tension and merely relax under compression. When a structure is under load, as in a beam, these windings share the tension loads resulting from the shear between column elements. In the case of a torsion load applied to the structure, only the windings spiraling in the direction that opposes the load will go into tension. The opposite windings relax. The skin elements are in filament form (fiberglass wire, steel strap, etc.) and oriented in spiral fashion with approximately half of the filaments spiraling in one direction and the other half spiraling in the other direction. The spiral angle, or pitch, will vary along a structure of uneven contour but could be held constant along a straight structure. A pitch angle of approximately 45 degrees is suitable for most applications. Enough filaments need to be wound in each direction to cover the column elements completely along their entire length so as to restrain them from buckling in compression or sliding in shear. A good bond between skin and columns is essential.


Function and fabrication of the Core Elements...

Function Of The Core

The compression cores provide continuous support for the columns to prevent them from buckling inward toward the center of the structure. They undergo compression loads only, with shear forces too minor to consider in most applications (the skin elements absorb the shear loads). Whatever the material, the cross-section of the core elements should be thickest in the middle and taper toward the edges that abut the structure center and the column elements. This prevents bending of the core elements and permits better utilization of the compression strength of the material used. If made of wood, the grain would be aligned perpendicular to the center of the structure for maximum efficiency due to the comparatively high compression strength of wood parallel to the grain. Other compression core element designs include tubes laid side by side perpendicular to the structure center, sandwich panels with slightly thicker mid-section to prevent bending, and corrugated sheet with the corrugations running perpendicular to the structure center. The most efficient core anticipated would be individual captive column structures laid side by side and acting as columns.


Functions and fabrication of the Column Elements...

Function Of The Column

The column elements illustrated undergo tension only, tension and compression both, or compression only, depending on how the structure is loaded. The full strength of the material is utilized both in tension and in compression since the column elements are prevented from buckling by the internal core element and the external skin element. The column elements may then be likened to pilings driven into the ground, where the earth prevents bending of the pilot to permit efficient loading in compression. Column element materials and cross-sectional design are determined by the anticipated loads, impact resistance, attachment requirements, etc.

Some Loading Examples
Loaded As Simple Beam

Simple Beam
(Loaded In Middle)

  • Column Elements: Columns on top go into compression. Bottom one goes into tension.
  • Core Elements: All go into compression.
  • Skin Elements: Half of the filaments on each side go into tension due to shear forces between the column elements. The other half go into a relaxed state. Top filaments all go into tension to keep the top column elements from spreading apart.
Loaded As Column

Column

  • Column Elements: All go into compression
  • Core Elements: All go into compression.
  • Skin Elements: Tension takes place only at those points where the column elements try to bend outward or flex sideways.
Torsion Loads

Torsion

  • Column Elements: All go into compression
  • Core Elements: All go into compression.
  • Skin Elements: Only the filaments that spiral in the direction opposing the load go into tension. Filaments spiraling in the reverse direction simply relax.
Cantilever Load

Cantilever Beam
(Loaded Both Ends)

  • Column Elements: Column on top goes into tension. Bottom two are in compression.
  • Core Elements: All go into compression.
  • Skin Elements: Half of the filaments on each side go into tension due to shear forces between the column elements. The other half go into a relaxed state. The filaments on the bottom side all go into tension to keep the bottom column elements from spreading apart.
Cantilever Load

Cantilever Beam
(Wall Mounted & Loaded One End)

  • Column Elements: Column on top goes into tension. Bottom two are in compression.
  • Core Elements: All go into compression.
  • Skin Elements: Half of the filaments on each side go into tension due to shear forces between the column elements. The other half go into a relaxed state. The filaments on the bottom side all go into tension to keep the bottom column elements from spreading apart.

 

Some Shapes And Cross Sections

Straight Straight
Curved Curved
Circular Circular
Twisted Twisted
Variable Variable

 


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Copyright 1998-2004 by Lawrence R. Bosch.