Well first off have you ever looked at pictures of a wreckage site from other large planes? There is usually a trench from the impact and the engines are buried deep in the earth. A a Boeing 767-200ER is 395,000 pounds, traveling at 530mph converted to feet per second 777fps. about the velocity of a 230 gr 45acp. Any vertical structure within the direct line of the fuselage and engines was removed on impact weakening the building. If you watch videos of the first building collapsing you can see it clearly shifts to one side first and the debris falls well to the side. After that the remaining structural parts are so weak and now shifted out of alignment they can’t carry the weight from above. The combined weight of the above floors falling collapsed each subsequent floor. Just like a house of cards. Whatever the shear strength of the horizontal components were designed to withstand, they were not designed for that type of sudden energy loading. Buildings are designed for static loads. If you want to understand why there are few papers and such, just look at the video of the dust. Virtually everything was squashed and ground into fragments and dust.
Immediately following the attacks, a building performance study (BPS) team of engineering specialists was formed by the Structural Engineering Institute of the American Society of Civil Engineers (SEI/ASCE) and Federal Emergency Management Agency (FEMA). The BPS team issued its report in May 2002, finding that the aircraft impacts caused “extensive structural damage, including localized collapse” and that the resulting fires “further weakened the steel-framed structures, eventually leading to total collapse.” They also presented recommendations for more detailed engineering studies of the disaster.
“The BPS team investigation was later followed by a more detailed investigation conducted by the National Institute of Standards and Technology (NIST), which also consulted outside engineering entities. This investigation was completed in September 2005. The NIST investigators did not find anything substandard in the design of the WTC towers, noting that the severity of the attacks and the magnitude of the destruction was beyond anything experienced in U.S. cities in the past. They also emphasized the role of the fires and found that sagging floors pulled inward on the perimeter columns: “This led to the inward bowing of the perimeter columns and failure of the south face of WTC 1 and the east face of WTC 2, initiating the collapse of each of the towers.”http://en.wikipedia.org/wiki/Collapse_of_the_World_Trade_Center