Originally Posted by BenBurch
We do know that. Within a reasonably well-known margin of error, the towers were as strong as they were supposed to be. Different grades of WTC steel and their connections were tested for yield strength and thermal response by NIST, which also took into account the structural modifications to the building (primarily strengthening to meet specific tenant needs) over its lifetime. Those modifications were extensively documented. Constant inspections were made of mill and on-site fabrications. Periodic structural inspections were also made throughout the life of the buildings. Data about fire spread and the structural thermal response were collected during several fires, most notably during the big 1975 fire. The 1993 bombing caused engineers to closely scrutinize the response to a catastrophic blast event (the main concern being the loss of lateral support to columns when floors are removed).
NIST's ability to predict the structures' response to wind and gravity loads far exceeded the capabilities of engineers in 1964. For instance, NIST determined that the designers' calculations had underestimated the wind load on the towers. However, the designers had prudently planned for errors in their calculations and built in enough reserve capacity for sustaining wind load (NIST determined actual reserve strength to be 1.9-2.7x for overturning and 11x for shear, depending on the tower and wind direction). About 5% of the building columns, particularly the corner columns, were found to have slightly exceeded their designed demand-to-capacity ratios, but again, enough redundancy was built in to redistribute loads. Additionally, NIST had a good deal of data from photos and videos to determine if the south tower's response to the impact of flight 175 matched their models.
Independent engineering investigations by Weidlinger (Silverstein) and Exponent (insurance companies) confirmed that the construction was not substandard.
The big unknowns were the extent of core damage, the extent of removal of fireproofing materials and the exact thickness of the SFRM, the inflammable fuel load per square foot on the fire floors, and the exact office layout of some floors (and therefore the extent of destruction of office contents). Some knowledgeable people also think that the thermal expansion of steel over multiple floors could have played a major role in the collapses, which NIST did not factor in.
So, while in the early 21st century we can fairly accurately predict a building's response to events like high winds and earthquakes, the ability to model extremely complex events like 9/11 is in its infancy. But when the towers were designed, engineers had no such ability. Even if the engineers of the 1960s had wanted to design a skyscraper to withstand more extreme events, they wouldn't have had many options besides adding a lot of weight in steel and fireproofing, as was done in skyscrapers in the first half of the 20th century. The advent of technology such as high-strength concrete for the cores of tall buildings has given engineers tools that weren't available in the 1960s.