Wednesday, December 13, 2006

Construction Details

The basic construction unit was a steel wire grid composed of 1/2 inch squares. The steel wire was approximately 0.65 mm thick. At 1:200 scale, this would translate to a 5 inch thick solid steel column-- smaller than a WTC outer column but more steel.

My 12 inch model wall had 24 columns. This translates to about 1/3 of the columns on a WTC wall (62). However my columns WERE about 2.4 times thicker in terms of steel than a WTC wall at the 80 floor (2.1 inches of steel-- 13/16 inch x2 plus 1/4 inch x2), and since I had only 1/3 the outer columns, my outer walls were overall WEAKER than a WTC wall-- at least at the 80th floor.

The outer wall was somewhat WEAKER but the inner core in my model was made up of about 96 x 1 mm columns. It is hard to know how this compares to the real WTC core, as I don't have the dimensions of the WTC core columns. I did have more columns overall than in the WTC core, and I could make the assumption that my core was somewhat stronger than the real WTC core.

The core was roughly the same dimensions as the real WTC core-- in my model it was 4 inches on one side versus 8 inches on the long side. This translates to 67 feet by 133 feet, which is very close to the WTC core dimensions.

The floors were one sheet of the steel wire grid, and my floors did not extend into the core area. The floors only spanned the distance between the outer walls and the core. The floors did not have concrete or any stiffener or truss system. Overall, the floors were probably comparable in strength to a WTC floor. Each floor was attached to each wall in two places by 0.5 mm wire, and to the core in two places by 0.5 mm wire. Thus, the floors were not anchored tightly to the structure, and this favored a collapse mechanism.

There were only 22 floors, which makes the tower weaker compared to the WTC with 110 floors. Each floor adds significant strength and rigidity to the structure.

My WTC model was divided into three sections, at 2.2 foot intervals, similar to the real WTC which had mechanical and elevator floors dividing the tower into thirds. The lowest section had an outer wall of three sheets of steel grid, the middle section had an outer wall of two sheets of steel grid and the top third had an outer wall of one sheet grid. Between the sections, I had a triple-ply steel grid floor which was to mimic the solid steel beams of the mechanical and elevator floors. My core was similar in the middle and top sections, but had an extra set of "columns" in the lowest base section.

In terms of overall load, the weight of my tower was approximately 105 pounds. Since my tower was a 1:200 scale model, we can multiply that weight by 200 in 3 dimensions-- 115 X 200 x 200 x 200 = 8.4 x 10^8 pounds = 420,000 tons. This is actually very similar to the estimated weight of one WTC tower (between 250K and 500K tons).

My tower was 19% structural steel, which is very similar to the figure I've heard for a WTC tower (20% structural steel).

SUMMARY: My model was similar in overall structure to the WTC, but had many fewer floors. Fewer floors with more space between should have favored a rapid collapse. The floors were not tightly and strongly anchored to the vertical elements, and this should have favored collapse.

The outer walls of my model were somewhat weaker in proportion to the WTC. Possibly the inner core section of my model was relatively stronger than the real WTC, though my model core did not have a well-ordered inner structure. My core was therefore not as structurally sound as the real WTC should have been.

2 Comments:

Blogger Rob said...

In making your 1:200 scale model you have fallen into a classic trap with engineering scale models: volume and thus mass & weight scale as a cube of length, but area only scales as the square of length.

The real building - as you mention - weighed approximately 8,000,000 (200 x 200 x 200) times more than your model, but the cross-sectional area of the steel columns and beams supporting the real building's weight was only 40,000 (200 x 200) times greater.

When you say "My tower was 19% structural steel, which is very similar to the figure I've heard for a WTC tower (20% structural steel). SUMMARY: My model was similar in overall structure to the WTC" you draw the wrong conclusion. Precisely because your 1:200 model contained the same proportion of steel as the real WTC, the steel in your model was approximately only 1/200th as stressed as the steel in the real building.

1:38 PM  
Blogger valachus per skorilo said...

pwned :)

3:43 PM  

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