Groundbreaking for a 1,300-foot, 46-story office skyscraper, designed by Kohn Pedersen Fox Associates, at the Hudson Yards site in New York City signals the beginning of the $15 billion, 26-acre mixed-use project. Other design firms, including Skidmore Owings & Merrill and Diller Scofidio + Renfro, are involved in the massive development, which includes apartments, parks, shopping centers and entertainment spaces, said Stephen Ross, chairman of Related Companies, the project's developer.