The North Tower stands alone almost 30 minutes. This photo is important because it gives a good idea of just how much of the North Tower remained intact below the point of impact. These buildings were designed specifically with airliner impacts in mind.
In their day, the Twin Towers were the tallest buildings ever constructed. The revolutionary, award-winning, 'tube-in-tube' design they first made use of remains the standard used in steel hi-rise construction to this day. Because of their unique, tight and full cross-braced outer column assembly arrangement they were among the most redundant (strongest) buildings ever constructed.