In fact I have seen the opposite from the NIST (and it is still posted). The NIST dismisses first the idea that the WTC1 and WTC2 were designed to sustain a crash of a Boeing 707 by saying it had no documentation to confirm that point
Buildings are not specifically designed to withstand the impact of fuel-laden commercial airliners. While documents from the Port Authority of New York and New Jersey (PANYNJ) indicate that the impact of a Boeing 707 flying at 600 miles per hour, possibly crashing into the 80th floor, was analyzed during the design of the WTC towers in February/March 1964, the effect of subsequent fires was not considered. Building codes do not require building designs to consider aircraft impact.
То есть все, как расчитали: после столкновения башни устояли.
ЗЫ. Хотя еще раз - я не специалист и глубоко не разбирался. Первый раз копаюсь в инете по данному вопросу. Вполне могу где-нибудь грубо лажануть.