If the building workers’ union fails to reach a deal on a new contract with management by tomorrow night, the city’s 30,000 doormen, porters, superintendents, elevator operators and handymen are prepared to walk out on the job for the first time in nearly two decades. Residents, co-op boards and building managers, meanwhile, are organizing a plan B. The Realty Advisory Board on Labor Relations, which represents building owners, has issued a 45-page manual with recommendations on keeping buildings in operation without their staff. Security guards are slated to arrive at buildings one hour before the deadline, 12:01 a.m. Wednesday, in order to take over the overnight shift in the case of a strike. In some buildings, residents have been asked to volunteer to watch the front doors, clean the hallways and take out the garbage. In others, service elevators, storage rooms and garages would be taken out of operation.