Over the opposition of building owners, City Council yesterday approved a bill designed to provide greater job security for janitors and other service workers in the city's large office buildings.

The law would require that if a building owner changes cleaning contractors, the newly hired company must offer six months' employment to workers of the departing company.

"A piece of legislation like this that protects the low-wage workers is good for the city," said Tom Hoffman, spokesman for Service Employees International Union Local 3, which represents janitors.

But local building owners are asking Mayor Tom Murphy to veto the legislation, contending that it violates part of the city's charter that prevents government from interfering with decisions made by private businesses.

In a letter to the mayor, the Building Owners and Managers Association of Pittsburgh said that the bill's passage means that "the city and private enterprise will need to spend more of their limited resources on illogical legal confrontations."

The law's passage comes nearly a year after the owners of Centre City Tower replaced a unionized cleaning company with a nonunion competitor, causing nine unionized janitors to lose their jobs at the Downtown building. The switch was made a month after the service employees union struck an agreement with most local building owners providing the janitors with new health-care benefits that extended coverage to families.