This is the Margaret McDermott Bridge on Interstate 30/Tom Landry Highway.
See the time lapse video at the Dallas Morning News Transportation blog
http://transportationblog.dallasnews.com/ (http://transportationblog.dallasnews.com/)
Completion of the arch was delayed because of the flooding at the construction site in May and June. There will be a second identical arch on the north side of the new vehicle bridge.