Slender Column Buckling - Braced vs. Unbraced

An important factor in understanding the collapses of tall building is the role of slender column buckling - or more specifically understanding just how strong lateral bracing can make a column.
Here I demonstrate using a steel bar. When it's evenly braced along its length by seven "floors" it's incredibly strong and can support a wildly swinging dynamic load of well over 150 pounds. However when the column is removed from its supports it becomes significantly weaker, and is now only capable of supporting a few pounds.
In the collapse of World Trade Center 7 this applies in two ways. Firstly there's the initial collapse of Column 79 after the floors around it collapsed. Secondly there's the buckling of the exterior columns after they lost support from the interior.

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