Construction crews finished the 20th floor of One World Trade Center in mid-May. The goal now is to build a new floor every 10 days and complete construction in time for the 10th anniversary of the Sept. 11, 2001, terrorist attacks, writes R.M. Schneiderman. There was an initial lull in construction because of financial concerns and disagreements between developer Larry Silverstein and the Port Authority of New York and New Jersey. The sides reached an agreement in March.