Up to 3,600 building employees may walk off the job on January 1 if their contract demands are not met, union representatives said Friday.The end of the year marks the contract expiration date for Local 94 International Union of Operating Engineers members. The workers oversee all day?to-day operations at up to 20 buildings across Brooklyn and the Bronx, performing duties that range from heating and air conditioning maintenance to first response during emergencies.Union representatives say landlords are making money, but vacancies are up. They complain that landlords are forcing them to cover losses by having workers pay for their own health insurance. The union says if members don?t get raises and better health care plans they will walk out as soon as the ball drops in Times Square.