In this city where the skyscraper was born, it is thriving like never before. Luxury condominium towers and office buildings that climb 600 feet and more are sprouting up all over downtown. Along the Chicago River, the Trump International Hotel and Tower is inching its way up to a planned 92 stories. Plans are in the works for a nearby 124-story skyscraper, the Fordham Spire, that would knock the Sears Tower from its perch as the tallest building in the United States. Since 2000, no fewer than 40 buildings at least 50 stories high have been built, are under construction...