A federal judge ruled here yesterday that a portion of the American Institute of Architects' ethics code violates the Sherman Antitrust Act because it limits competition among architects.

U.S. District Judge John J. Sirica said that as a result architect Aram H. Mardirosian was entitled to treble damages from the AIA, the principal U.S. professional organization for architects and a group that suspended Mardirosian from its memberships rolls for a year on grounds that he violated the ethics code. The judge did not rule on how much Mardirosian should receive.

Mardirosian, a former architectural adviser to the National Park Service, was censured by the AIA after the group found that he had taken over work at the National Visitor Center in 1975 that another architect, Seymour Auerbach, had already been awarded.

One of AIA's ethics regulations states that AIA members "shall not" seek a commission for work for which another architect has already been "selected or employed" until the second architect has been informed that the original architect is no longer working on a project, and told the original architect that he is seeking the work.

In the National Visitor Center case, the federal government decided to terminate the portion of Auerbach's contract for architecturall work related to the Visitor Center, paid him about $700,000 and hired Mardirosian.

The AIA, which claims a membership of 27,500 of the 60,000 registered architects in the country, found that Mardisosian failed to give Auerbach notice that his firm was negotiating with the National Park Service for the Visitor Center work.

But Sirica, in a 46-page opinion, said, "On the undisputed facts of this case, the conclusion that Mardirosian was suspended from membership in the AIA for in some manner competing with another architect is inescapable."

The judge said that Mardirosian was "professionally disciplined for attempting to obtain a commissionn 'belonging' to another architect, for obtaining that commission and for not giving the other architect notice that he was going to do either. Such restrictions, placed on commercial conduct of (AIA) member achitects, are facially anticompetitive."

Mardirosian's lawyer, Edward Greensfelder Jr., said the architect's business was "virtually decimated" by the AIA finding. Mardirosian is seeking $3 million in damages against the AIA and Auerbach, who filed the professional misconduct charges against Mardirosian with the AIA. Sirica has not ruled on Auerbach's liability in the lawsuit.

Greensfelder said the ruling could also effect other professional groups that maintain similar restrictions like that contained in the AIA code.