ERC International Inc., increasing its share of the market in logistical support and facilities management, has signed an agreement to acquire D-K Associates of Rockville for between $3.5 million and $4.1 million, the Vienna firm announced.

A joint statement by the companies said that the price, to be paid over a four-year period, will depend on D-K Associates' performance. The initial payment is expected to include 50,000 shares of ERC common stock, currently trading at $8.25 a share, plus an unspecified amount of cash.

The acquisition will add about $6 million in annual revenue to ERC's expected $60 million sales this year.

The addition of D-K Associates, a six-year-old Rockville firm specializing in technical training and logistical support, will add a "new dimension" to ERC's business, said Chairman and Chief Executive Officer Jack Aalseth.

Irvin Feld and Kenneth Feld Productions Inc., producers of the Ringling Bros. and Barnum & Bailey Circus, last week announced the formation of a new subsidiary to create, produce and manage major civic and city-sponsored festivals in cities across the U.S.

Production plans for the new Washington-based company, Festival Events Inc., include festivals featuring top musical artists, cultural arts festivals, regional food-tasting festivals and ethnic festivals.

Allen Flexer, vice president of the parent comany, will manage the new business.

Irvin Feld and Kenneth Feld Productions Inc., based in Washington, also produces Walt Disney's on-ice entertainment shows "World On Ice" and "Magic Kingdom On Ice" and the Las Vegas show "Beyond Belief."

"Festival Events Inc. is the first of its kind launched on a national basis in the United States," said Feld, who added that the company's national scope gives it a good basis for launching the new business.

BDM International Inc., a professional and technical services firm based in McLean, last week said it has won three contracts totaling $13.4 million from the U.S. Air Force to provide support to the Royal Saudi Air Force.

The contracts, which are through June 30, 1987, include: a $5.9 million contract for general support to the RSAF logistics planning section; a $2.6 million contract to assist the RSAF aeronautical engineering section; and a $4.9 million contract to help in data tracking.