The final steel beam was lifted by crane Monday to the top of SunTrust Park as Braves executives, dignitaries and media members watched from what will become the playing field of the new Cobb County stadium. The 33-foot-long, 1,422-pound beam was bolted into position 155 feet above field level, completing a “topping-out” ceremony that Braves chairman and CEO Terry McGuirk called a “major milestone” in the building of the ballpark. In keeping with construction-industry tradition, the final beam had a flag and a tree attached to it — in this case, an American flag and an ash tree.