As they do every morning before beginning their shifts, Mortenson Construction workers and subcontract employees who are taking down the Metrodome stretched Wednesday to avoid soft-tissue injuries. About 100 workers are on the job now, with more than 1,000 expected at the peak of the project building a new Vikings stadium in the footprint of the Dome. Mortenson is the general contractor for the $975 million, multipurpose stadium, which is scheduled to open in July 2016.