It appears that i am in error.
There has apparently been made two studies regarding the safety of the WTC. One by Robertson and one by Skilling, both in the 60's. Robertsons study seem to be about a low-velocity impact and did not model the effect of the fuel, while Skillings study was regarding a high-velocity impact with roughly 90m^3 fuel - the planes contained 34m^3 in comparison. The details of the study took up 3 pages and the methology and detail in modelling is lost since the paper and copy was kept with the rest of the documentation in WTC1 and WTC7 (ju-bi-duh). NIST, ofcourse, conclude that Skilling did not model the fire properly which also appear difficult without an extensive model, infact i have read that NIST fire model has been critisized for the same.
See more here: http://en.wikipedia.org/wiki/Collapse_of_the_World_Trade_Center#cite_note-12
Sorry, i was unaware that multiple studies had been performed.
UPDATE: It appear you have allready found the study :-).