Contractors have adopted a unique system in constructing the $7.5 billion City Center project on the Las Vegas Strip — the largest commercial development in U.S. history. Rather than building the entire structure first and adding finishes later, contractors are swooping through the building at the same time. As the structure on lower floors is completed, finishing touches are being added right away, even before construction of higher floors has begun. Construction is also taking place 24 hours a day, 7 days a week to get the mega-complex up and running as quickly as possible.