WTC STRUCTURAL MATERIALS (kg) 388,700,000 NOTES
Text Box: One of the unusual features of the 911 WTC event was the paucity of rubble remaining following the collapse of Buildings 1, 2 and 7. My goal in making this estimate was to have some means of comparing the mass of WTC dust with the combined structural mass of WTC 1, 2 and 7, with a view to answering the question: "Where did the buildings go?" The estimate is based on ground-level photographs and video combined with satellite imagery, from which I estimated the extent of dust distribution and dust depth. It was also necessary to make an assumption about the density of the uncompressed dust. So far as the assumptions used are approximately correct and the model is in principle valid, the estimate suggests it is plausible that substantially the entire structural mass of WTC buildings 1, 2 and 7 was reduced to dust. This appears to be somewhat new. It appears that existing estimates – which so far as I’ve seen don’t include a statement of method – are at least an order of magnitude lower. My estimate is at the rough outline stage: it's a concept for a study. I’d like to suggest that others may – perhaps as a piece of original research for the award of a Masters or PhD – wish to refine the basic model and work it up into a formal study for publication. The concept involves application of a normal distribution to model both dust depth distribution and dust density distribution. That means that most dust volume - and also the denser dust particle types - are regarded as falling close to the origin and least near the circular periphery within which all the dust is assumed to fall in accordance with the classic bell curve form. Based on a US Geological Survey satellite image, I set the periphery at 4000 m, and considered the enclosed circular area of about 50 square kilometres to be divided into 10 concentric rings, each with a width of 400 m. The innermost "ring" is thus a circle with r = 400 m. The model is very sensitive to variation in the value of sigma. To illustrate, setting sigma < 1 broadens the distribution peak, placing relatively more dust depth in the larger outer concentric rings, which in turn implies more total dust volume and hence more dust mass. Setting sigma > 1 does the opposite. It is difficult to define a value for sigma based on the photographic record at street level because it is not clear what proportion of the dust in the streets had originally fallen on buildings and simply blew down. With allowance made for denser particles falling closer to the origin than lighter particles, I set sigma such that 90% of the dust mass falls within a one mile radius. The model's uncertainty is probably about +/- 50%. Even at its lower error limit, the model returns a value for total dust mass much higher than earlier estimates. Another goal was to check how RJ Lee Group's analysis showing 6% iron microspheres in WTC dust (which is reportedly 150 higher than in the control building dust) corresponded to the weight percentage of steel in the WTC buildings. At first sight, it does not. Steel comprises more than half the total structural mass in steel-framed buildings, while the iron content of the dust as analysed by RJ Lee Group is about an order of magnitude less than that. But it is to be expected that the iron microspheres, being among the densest of all the dust particle types, would also show the highest distribution density. As the RJ Lee Group's report identifies the sampling sites, which lay at considerable distance from the WTC origin, it provides important - though not sufficient - source data for setting distribution density parameters for the various dust particle types. A workable approach would be to set up a layered analysis applying the method proposed here to each main particle type. Michael Lusk
  WTC1 WTC1 WTC1 WTC_1,2,7
TOTAL 169,000,000 91,000 100.0% 388,700,000
  Mass (kg) Mass (kg) Mass (%) Mass (kg)
Structural steel 96,000,000 91000 56.8% 220,800,000
Concrete 48,000,000   28.4% 110,400,000
Aluminium cladding 2,000,000   1.2% 4,600,000
Wallboard 8,000,000   4.7% 18,400,000
Plumbing fixtures 5,000,000   3.0% 11,500,000
Air conditioning 5,000,000   3.0% 11,500,000
Electrical & telecom wiring 5,000,000   3.0% 11,500,000
"Live weight" - occupants, furniture, etc. (EXCLUDED)   (EXCLUDED) (EXCLUDED)
Source PhysForum J911S    
Factor - my rough estimate       2.3
WTC DUST (kg) 250,000,000
  Mass (kg)    
Total dust ("high estimate") 30,000,000      
Source RockCreek      
No explanation of estimation method - perhaps limited to dust falling on Manhattan Island?
Rough estimate_MAL 250,000,000 ± 130,000,000  
 
ASSUMPTIONS_MAL
Dust falls in a normal distribution arround WTC site    
Standard deviation       0.775
Uncompressed dust depth at d = 0 m (mm)     120.000
Uncompressed dust depth at d = 4000 m (mm)   0.066
Dust beyond d = 4000 m disregarded      
Average density of uncompressed dust (kg/m3)   300.000