To make the sauce, combine the celery, onion, garlic, ketchup, barbeque sauce, mustard, beer, cider vinegar, Worcestershire sauce, brown sugar, chili powder, salt, and pepper in a bowl; stir until well blended.
Place the chuck roast in a slow cooker. Pour the sauce evenly over the meat. Cover and cook the roast on HIGH for 3 hours. Reduce heat to LOW, and continue cooking until very tender, about 4 hours more.
About 1/2 hour before serving, remove the chuck roast from the slow cooker and shred the meat with a large fork. Return the meat to the slow cooker, and cook uncovered so the sauce thickens, on LOW for 20 minutes.
To serve, dip the flat sides of hamburger buns into the sauce and top with meat.