On August 19, 1994, at approximately 1250 central daylight time, a Beech C90A, N1553N, operated by the University of North Dakota, and piloted by commercial pilots, encountered severe turbulence while deviating around weather at 20,000 feet mean sea level, in the vicinity of Janesville, Wisconsin. The airplane experienced an uncontrolled altitude deviation, descending 3,000 feet before recovery by the pilots. During the recovery the airframe sustained substantial damage. The two pilot crew and one non- flying instructor reported no injuries. The CFR 14 Part 91 instructional flight was on an instrument flight plan. Thunderstorms were reported in the area. The flight departed Detroit, Michigan, at 1230. After the event the flight continued to the intended destination of Grand Forks, North Dakota. The instructor pilot in a detailed statement indicated that in the vicinity of Janesville, Wisconsin, while cruising at 20,000 feet mean sea level, and maneuvering around build-ups the flight entered instrument meteorological conditions. Shortly after entering the clouds the airplane encountered severe turbulence and some of the flight instruments became unreliable (tumbled). She said that the power was reduced and the vertical speed indicator was observed to indicate a 3,000 feet per minute altitude loss and the airspeed indicator was at the "barber pole." Positive control of the airplane was regained between 16,800 and 17,000 feet. The flight then continued to the destination airport without further incident. The instructor stated that weather was checked prior to departure from Detroit, Michigan, and the possibility of thunderstorm activity along the proposed route of flight was forecast. The instructor pilot stated that an examination of the airframe after the event revealed that the airframe had sustained substantial damage as a result of the weather encounter; however, she stated that she does "not feel that the control inputs during the recovery were in such a manner to over-stress the aircraft."