An 8-foot-long crack, described as "potentially hazardous," was discovered yesterday in one of the four giant steel girders that support the roof of the Filene Center at Wolf Trap. The new theater opened only six months ago after the original building was destroyed by fire.

The area around the $18 million building, famed as the centerpiece of this country's first national park for the performing arts, was closed yesterday as construction experts swarmed inside to study the fissure. It ran along three of the four sides of the hollow 130-foot-long girder.

The crack was the first indication of any structural problem with the new center that rose from the ashes of the 1982 blaze after an outpouring of support from around the nation and congressional passage of a $17 million package of grants and loans.

The cause of the crack, which appeared to be about two inches wide, was not immediately known, and experts cited a variety of possibilities, including contraction caused by the recent record cold spell.

While a crane was being rigged last night to bolster the beam, which supports one-third of the weight of the center's copper-sheathed wooden roof, a structural engineer rejected the possibility that collapse might be imminent.

Nonetheless the situation is "potentially hazardous," said John C. Welch of Dewberry & Davis, the Fairfax County firm that designed the new theater.

"There's no structural incapacity to the building," Welch said. "We're not trying to dismiss it and say there is absolutely no problem, but there's no present danger," he said.

The beam, which slopes upward from the rear of the auditorium to the stage, is not overloaded despite the crack, he said, " . . . or the thing would be lying on the ground out there."

However, the engineer said, "You've got to think that people are going to be sitting under this beam next summer when the park is open for concerts . And that's why we're so concerned about it."

Sandra Alley, a spokeswoman for the National Park Service, which owns the park, said authorities brought in the crane last night "to take precautions so nothing else will happen.

"We're worried about the safety of it," she said. The park's 15th season is scheduled to open June 8.

The beam is shaped like an elongated rectangular box, 130 feet long, 6 feet deep and 16 inches wide, welded together at the corners.

Running perpendicular to the 130-foot length of the beam, like a slice through the trunk of a tree, the crack cuts through the three-inch thickness of one of the beam's sides for a distance of four feet.

It continues through the 3/4-inch-thick steel plate that makes up the beam's 16-inch underside, and then runs back up the 6-foot-wide third face of the beam for another three feet.

Authorities said that late Tuesday night a park service employe making a routine inspection discovered a square piece of the roof's copper plating dangling from the beam rather than resting flush against it.

That discovery prompted further inspections and at 9 a.m. yesterday the crack in the beam was spotted. The segment of the beam where the crack was detected is above the balcony seats in the theater, which holds about 3,500 persons.

Park service spokeswoman Alley expressed doubt that the crack in the girder had existed for more than a day or two, asserting that if it had happened earlier, it would have been found during routine maintenance.

While such damage in a structural member is not unprecedented, engineer Welch said it is "not usual at all."

"We're still trying to figure out what happened," said Sidney Dewberry, a founding partner of Dewberry & Davis, in an interview at Wolf Trap.

The performing arts park in a bucolic section of Fairfax County near Vienna was founded with donations by octogenarian heiress Catherine Filene Shouse, who gave 100 acres and $2.3 million to build the original auditorium. It opened to the public in 1971.

On the night of April 4, 1982, a spectacular blaze destroyed the theater. A rebuilding campaign began almost immediately.