(Portland, OR)  --  Workers at four Portland Burgerville locations are on strike over failed wage negotiations.  Employees walked off the job yesterday at the burger chain's locations at the Convention Center, Hawthorne, Montavilla and Southeast 92nd Avenue and Powell Boulevard.  The Burgerville Workers Union says they plan to continue striking until they've "sufficiently demonstrated their strength to the company."  Employees say they're fighting to earn a living wage.