"For a few months in 2001, architects worried that the era of the super- skyscraper was over.
Even as ash from the World Trade Center still swirled, it was clear that the high floors of the twin towers had been a deadly trap. Experts wondered if anyone would ever build tall again.
The answer was quickly revealed to be an emphatic 'yes.'
Skyscraper construction has surged globally since the terrorist attacks, prompting architects and engineers to ponder a new question: what should be done to make new towers safer? That question has been harder to answer."