Engineers often refer to this as a solid "steel tube" design, and rightly so compared to most buildings, which may have vertical support columns spaced as far as 20 feet apart. Thirdly, the outer columns helped the buildings withstand a huge wind load, but another smaller group of steel beams shot through the centers of the buildings and bore the main load of supporting the towers. This "core", which housed elevator shafts and stairwells, was connected to the outer steel columns by still more steel running through each concrete floor. The “double support" may have been what ultimately kept the buildings standing for so long. What actually caused the twin towers to fall? All the same, no mount of reinforcement could have prevented the towers' eventual collapse. Immediately after September 11, most engineers thought the fires, and not the planes' impacts, were to blame.