Following a brief strike threat, the union representing 5,000 employes at Safeway's 110 supermarkets in the Washington area agreed yesterday to a three-year contract that is virtually identical to a recent settlement reached with Giant Food employes.

Like the Giant settlement, the Safeway contract sets up a new two-tier pay scale, under which current employes will receive 4 to 5 percent annual wage increases, but all new employes will work under a substantially lower pay and benefit scale.

"We made a trade-off," said Thomas McNutt, president of Local 400 of the United Food and Commercial Workers, following a ratification vote at the D.C. Armory. "We traded the new hires to preserve things for our existing people."

McNutt, who heads the region's largest local union with 30,000 members, said supermarket chains, like other companies around the country, have demanded "give backs" in the form of reduced pay or benefits for existing workers. He said Local 400, by agreeing to lower pay and benefits for future employes, preserved all benefits for existing workers, including a generous plan of company-paid health insurance, pensions and legal services.

"We think the most important thing is not to give away existing benefits," McNutt said.

Safeway is the second-largest supermarket chain in the metropolitan area. Giant has 137 stores.

At the same time yesterday, Local 400's A&P division, representing 700 union members and 22 A&Ps, also approved the same settlement already adopted by the union's 8,000 members at Giant, 5,000 at Safeway and 800 at Grand Union.

Currently, most supermarket employes earn a maximum of $11.50 hourly, which will increase to about $13 after three years. But newly hired employes for all four chains will be on a lower scale with a $9.35 maximum. In addition, existing employes receive health and welfare benefits averaging $328 a month and a pension contribution of $160 a month. But the new employes' health plan costs only $120 monthly and the pension contribution is only $24.

A Safeway spokesman, who asked not to be identified, said yesterday the company was pleased with the settlement and with the avoidance of the strike, which the union threatened over a Safeway demand to eliminate a "manning clause" that requires a meat department worker to be on duty at all times the store is open.

Safeway wanted to eliminate that requirement to give the company "more flexibility" in scheduling workers, the spokesman said.

Despite the intervention of a federal mediator last week, negotiations broke down over the manning issue on Friday, with McNutt predicting a strike. The issue was resolved with an agreement Saturday that the manning requirement would apply only until 8 p.m. daily, with Safeway free to leave positions unfilled in later hours.

"We felt this was very important," the Safeway spokesman said. "We felt we were being forced to run our stores by the terms of a bargaining agreement and not by the needs of management." With the 8 p.m. agreement, he said, "we didn't get what we wanted but it's a beginning."

McNutt said the union would have been willing to consider a strike because changes in staffing are "a very sensitive area" to union members.