SpaceX’s Big Rocket, the Falcon Heavy, Finally Reaches the Launchpad - The New York Times

Also see Falcon Heavy Expands the SpaceX Fleet (NYT)

"“We were pretty naïve about that,” Mr. Musk said in July at a conference in Washington, D.C. “At first, it sounds really easy. Just stick two first stages on as strap-on boosters. How hard can that be? But then everything changes. All the loads change. Aerodynamics totally change. You’ve tripled the vibration and acoustics.”

The central core was redesigned and reinforced to handle the stresses, one of the key reasons that the Heavy is more than three years behind schedule. While the two side boosters are reused from earlier Falcon 9 launches, the core is all new, as is the second stage."

SpaceX’s Big Rocket, the Falcon Heavy, Finally Reaches the Launchpad - The New York Times