In particular, it is understood that during the long lifetime of an expensive building, there will be fires. It is important to everyone involved, including the project's investors, that even a major fire, one that is truly catastrophic, not be able to destroy the building. First and foremost there is the human cost of such a catastrophe; if a gigantic building is destroyed by a fire, then anyone trapped inside the building dies. Beyond that, there is the liability issue of unnecessary deaths in the building if it collapses. The death or injury of people and the damage of property in neighboring structures are a liability issue, as well, in case of collapse. Finally, there is the investment; hopefully, everyone evacuates safely, the catastrophic fire is safely extinguished, and now the building itself can be restored to a money-making capacity. As a result of this, major new buildings, especially skyscrapers in America, are built such that a major fire, which sooner or later will occur, is not able to collapse the building.
This is more than just theory. For decades of American history, although there have been skyscrapers and fires, never had a skyscraper been collapsed by a fire. Then, on one day, Tuesday, September 11th, 2001, not just one, but three skyscrapers, all in the same complex, supposedly collapsed due to fires. Since then, it has never happened again.
Another thing one might take into consideration when building a skyscraper in America is the likelihood that an aircraft might crash into it. It has even been anticipated that the fuel on board an aircraft might cause a catastrophic fire after the crash.
From an article entitled February 27, 1993: WTC Engineer Says Building Would Survive Jumbo Jet Hitting It:
In the wake of the WTC bombing, the Seattle Times interviews John Skilling who was one of the two structural engineers responsible for designing the Trade Center. Skilling recounts his people having carried out an analysis which found the twin towers could withstand the impact of a Boeing 707. He says, "Our analysis indicated the biggest problem would be the fact that all the fuel (from the airplane) would dump into the building. There would be a horrendous fire. A lot of people would be killed." But, he says, "The building structure would still be there." [Seattle Times, 2/27/1993] The analysis Skilling is referring to is likely one done in early 1964, during the design phase of the towers. A three-page white paper, dated February 3, 1964, described its findings: "The buildings have been investigated and found to be safe in an assumed collision with a large jet airliner (Boeing 707—DC 8) traveling at 600 miles per hour. Analysis indicates that such collision would result in only local damage which could not cause collapse or substantial damage to the building and would not endanger the lives and safety of occupants not in the immediate area of impact."
In fact, it was believed that each of the Twin Towers was able to withstand more than one such impact simultaneously. From a statement made several months before 9/11, reported in Twin Towers' Designers Anticipated Jet Impacts Like September 11th's (numbers in brackets refer to footnotes; see the original):
Frank Demartini's Statement
Frank A. Demartini, on-site construction manager for the World Trade Center, spoke of the resilience of the towers in an interview recorded on January 25, 2001.The building was designed to have a fully loaded 707 crash into it. That was the largest plane at the time. I believe that the building probably could sustain multiple impacts of jetliners because this structure is like the mosquito netting on your screen door -- this intense grid -- and the jet plane is just a pencil puncturing that screen netting. It really does nothing to the screen netting.
Demartini, who had an office on the 88th floor of the North Tower, has been missing since the 9/11/01 attack, having remained in the North Tower to assist in the evacuation.  Demartini had first worked at World Trade Center when Leslie E. Robertson Associates hired him to assess damage from the truck bombing in 1993.
The Twin Towers of the World Trade Center didn't just "meet" standards; an incredibly important investment, the WTC towers exceeded standards by a significant margin.
Like All Skyscrapers, the Twin Towers Were Over-Engineered
One aspect of engineering that is not widely understood is that structures are over-engineered as a matter of standard practice. Steel structures like bridges and buildings are typically designed to withstand five times anticipated static loads and 3 times anticipated dynamic loads. The anticipated loads are the largest ones expected during the life of the structure, like the worst hurricane or earthquake occurring while the floors are packed with standing-room-only crowds. Given that September 11th was not a windy day, and that there were not throngs of people in the upper floors, the critical load ratio was probably well over 10, meaning that more than nine-tenths of the columns at the same level would have to fail before the weight of the top could have overcome the support capacity of the remaining columns.
There is evidence that the Twin Towers were designed with an even greater measure of reserve strength than typical large buildings. According to the calculations of engineers who worked on the Towers' design, all the columns on one side of a Tower could be cut, as well as the two corners and some of the columns on each adjacent side, and the building would still be strong enough to withstand a 100-mile-per-hour wind.  Also, John Skilling is cited by the Engineering News Record for the claim that "live loads on these [perimeter] columns can be increased more than 2000% before failure occurs." 
Here are excerpts from the February 27, 1993, Seattle Times article (cited as a source in the first quote) Twin Towers Engineered To Withstand Jet Collision; the article was written in the wake of the first terrorist attack on the WTC:
Engineers had to consider every peril they could imagine when they designed the World Trade Center three decades ago because, at the time, the twin towers were of unprecedented size for structures made of steel and glass.
"We looked at every possible thing we could think of that could happen to the buildings, even to the extent of an airplane hitting the side," said John Skilling, head structural engineer. "However, back in those days people didn't think about terrorists very much."
The article says "Skilling ... is among the world's top structural engineers." Because of a previous incident where an aircraft collided with the Empire State Building, the design team considered the possibility of an airliner crashing into the twin towers. As mentioned in a previous quote, it was believed the buildings would survive. In a comment about the February, 1993 WTC bombing, the article states:
Skilling - a recognized expert in tall buildings - doesn't think a single 200-pound car bomb would topple or do major structural damage to a Trade Center tower. The supporting columns are closely spaced and even if several were disabled, the others would carry the load.
"However," he added, "I'm not saying that properly applied explosives - shaped explosives - of that magnitude could not do a tremendous amount of damage."
Chillingly, the article concludes:
Although Skilling is not an explosives expert, he says there are people who do know enough about building demolition to bring a structure like the Trade Center down.
"I would imagine that if you took the top expert in that type of work and gave him the assignment of bringing these buildings down with explosives, I would bet that he could do it."
This series of posts will address the collapse of three World Trade Center skyscrapers, referred to here as WTC 1, WTC 2, and WTC 7, on Tuesday, September 11th, 2001, during kamikaze-style terrorist attacks by two hijacked airliners. This series is intended to be read in parallel with the initial posts of the series Jihad, Inc.; posts from both series, as well as other related material, can be found under the label The Killing. This material will lead us to an understanding of what really happened that day, how it happened, and why.