Thousands of building employees in the Bronx and the rest of the city are threatening to walk off the job.The union contract for the workers expires at the end of the year and they say their new contract offer has to be fair. The Local 94 International Union of Operating Engineers wants better health benefits and a raise. The union says landlords are making more money while vacancy rates are going down, so there is no reason for workers to pay more for health insurance.The workers are in charge of almost all the day-to-day operations at commercial buildings. Those operations include security, heating and air conditioning. They say they're also the first responders for building emergencies.Local 94 represents about 5,000 workers in the Bronx and the rest of the city.