What is odd about the WTC debris is how few larger fragments of concrete it contains. Reports on the structure say reinforced concrete was used for floor slabs and customarily in demolition such material breaks apart in relatively large chunks, the broken concrete remaining attached by steel reinforcing bars or wire mesh. However, there is a structural design where plain concrete is placed on metal floor decking with little or no reinforcing. Also, lightweight concrete, whose course aggregate is cinder rather than stone, is often used to limit the weight of the structure, especially in high-rise building. This lightweight material does not usually have the strength of regular concrete and strength is provided by metal decking below it. The large amount of dust produced by the towers' collapse and the relative lack of larger concrete shards raise the possibility that the building disintegrated at least in part due to lesser strength of the type of concrete used in the floors. That remains to be examined. Observation of the towers' remains show that while main steel vertical structural members of the exterior wall and central core withstood the collapse, the horizontal floor-supporting trusses broke away from these vertical supports at the points of attachment (probably made by welding). One view of the North Tower (1 WTC) shows the exterior and core remnants and the ruptured points of floor structure attachment. Photos: http://cryptome.org/wtc/wtc035.jpg http://cryptome.org/wtc/wtc047.jpg What this suggests is the possibility that the structure of each floor collapsed due to load of collapsing floors above them, and that only afterwards did the vertical supports at the exterior and core collapse. Review of video of the collapse appears to confirm this sequence as well as the consequent supposition that the floor structure was the weakest part of the buildings -- which would not be uncommon for floor structure supports only a single floor while the vertical members support all floors above them. However, a slow motion examination of visual recording would be needed to confirm exactly what collapsed first. There has been speculation about the initial step in the collapse of the buildings, most commonly attributed to the intense heat of burning jet fuel softening structural steel, usually the steel of the core. However, it is possible that collapse of the core steel was not the initial phase, but instead it was the floor structure breaking away from vertical supports. The collapse in this scenario would be that of floors dropping one after the other onto floors below, the load of the upper floors overwhelming the relatively weak attachments of floors to vertical supports -- the attachments customarily being designed to support only a single floor load. For example, a single floor dropping onto the one below could have ruptured the next lower attachment, thus setting off a disastrous sequence. This could have occurred without fire initially weakening the vertical steel structure as has been speculated. The impact of the crash, and/or subsequent swaying of the buildings, could have ruptured floor structure attachments, and only one floor breaking away would have been enough to precipitate the collapse. Alternatively, the fuel fire, and flaming building contents, could have weakened floor structure and/or its attachment to vertical supports, in particular if the crash destroyed fire-protection materials of the floor structure. Thus, with heat weakening floor structure along with the attachments being ruptured by the crash, the collapse sequence commenced. One significance of these speculations is that weakening of the core steel by intense heat may not have been the initial cause of collapse. A New York Times report of October 6 describes an investigative engineer "finding what appears to be a few pieces of the south tower that were directly hit by the Boeing 767 jetliner, and the discovery poses a few new puzzles. While the impact sliced through half the column, the column did not buckle; each column is designed to support the weight even if half is missing. The column also exhbits no outward signs of smoke or heat damage." Another signficance is that more steel reinforcing in the concrete could have increased the strength of the floor structure and better withstood the initial step in the floor-by-floor collapse sequence. And, the immediate and long-term adverse affects of the huge dust clouds of the collapse might have been lessened. It is likely that some victims were suffocated by these clouds.