On December 29, 1999, about 1230 Eastern Standard Time, a Beech C-23, N2116W, was substantially damaged while landing at the Manassas Regional/Harry P. Davis Field Airport (HEF), Manassas, Virginia. The certificated private pilot was not injured. Visual meteorological conditions prevailed and an instrument flight rules flight plan had been filed for the flight that departed Leesburg Executive Airport, Leesburg, Virginia, about 1200. The personal flight was conducted under 14 CFR Part 91. The airplane was landing on Runway 34R, a 5,700 foot long, 100 foot wide, asphalt runway. In a written statement, the pilot described the approach to the runway as normal; however, after touchdown the airplane began to "porpoise strongly." The pilot said he tried to stabilize the airplane's attitude and maintain directional control with the power at idle; however, the airplane's nose wheel collapsed and the propeller contacted the ground. The airplane then departed the right side of the runway, and came to rest in a grass area. The pilot did not report any mechanical problems with the airplane. The pilot reported he had accumulated 210 hours of total flight experience. He purchased the accident airplane about 1 month prior to the accident, and reported 14 total flight hours in make and model.