The Fairfax County firms that designed and built the Wolf Trap Foundation's Filene Center accused each other yesterday of causing the structural defects at the open-air theater that prompted the foundation's $3 million lawsuit against them.

In court papers filed in Fairfax County Circuit Court, the G&C Construction Corp., which built the center, and Dewberry & Davis, the architectural and engineering firm that designed it, each denied responsibility for an eight-foot crack in a steel support beam that the foundation said could trigger the collapse of the roof.

Each firm sued the other for the costs incurred in repairing the roof. G&C is seeking $150,000 from Dewberry & Davis and four subcontractors; Dewberry & Davis is seeking $225,000 from G&C.

Those amounts reflect the contributions of each firm to a fund, established last March, that covered the initial $610,000 cost of repairing the defective beam. The four subcontractors named in G&C's suit supplied materials and inspected the construction work.

G&C was "in no way responsible" for the defect, the company argued in its response to the lawsuit filed last month by the foundation. G&C said it "justifiably relied" on the design and specifications furnished by Dewberry & Davis and attributed the problem to the "reckless [and] defective" work by that firm.

But James C. Gregg, an attorney for Dewberry and Davis, said: "As far as we know, there is no evidence to support that. As far as we know, the design was appropriate."

Sidney O. Dewberry, managing partner of the firm, said yesterday that his concern, a prominent area engineering and architectural company, "will be totally vindicated" by the courts. "I feel strongly that that was not our error," he said.

In its $3 million lawsuit, Wolf Trap contended that Dewberry & Davis and G&C were responsible for the gaping eight-foot crack discovered in January 1985 in a steel beam supporting the roof of the Filene Center, the country's first national park for the performing arts.

Rebuilt after a 1982 fire, the large open-air amphitheater had been open for only six months when the crack was discovered. The $3 million sought in the suit represents the amount spent by the foundation to repair the defects and projected costs for future related repair work.

In its suit, the foundation cited "errors and inadequacies" in Dewberry & Davis' structural design, including miscalculations of the loads and stresses placed on the roof's box girders and the effects of cold temperatures on the support beam.

The foundation contends that Dewberry & Davis failed to specify adequate testing and to detect errors in G&C shop drawings and construction. The suit also charges that the general contractor and its subcontractors "employed and allowed defective work and materials."