A dispute between the Realty Advisory Board and the building maintenance union Local 32BJ over an upcoming contract renewal could end up in a strike, which would affect more than 90 percent of the city's Class A office buildings and 2,000 buildings total. Union members--including janitors, doormen and elevator operators--seek higher wages to reflect office buildings' increasing rents and property values. The union's contract is due to expire on New Year's Eve.