Firstly, respect and tributes are paid to all the victims and families, heroes and innocent civilians who have lost their lives at the hands of any terrorist event.

This is a skyscraper collapse simulation made with a rudimentary replica of the 1WTC or the North Tower. Due to system limitations (CPU melting) the structure is limited to 64 floors. The structure is made of ~12,000 objects. White blocks represent the outer skin columns, black are the floor trusses, and yellow represents the core structure. Impact event has been approximated based on analysis of 1WTC images and video.

This simulation forms a rough approximation of collapse stages but can not be presented as an accurate simulation of events due to a number of limitations with the software as follows:
Heat/Fire. There is no simulated heat or fire and as a result no weakening of any blocks/structure.
Deformation. All of the blocks are 'solid'. They do not bend, deform or snap. This means there is no compressive or tensile damage being simulated, and as a result occasional physics glitches can be seen .
Connections. There are no connections between blocks, meaning the structure is held up only by gravity so has no strength other than the weight of blocks and friction.