Barbara C.Simko;81;of R.D.1;Fayette City;died Wednesday;Dec.21;1994 at 10:15 p.m.in Mon Valley Hospital after a lengthy illness.  She was born Jan.18;1913;in Monessen;a daughter of the late Anton and Mary Plaventich.  She was a member of the Holy Spirit Roman Catholic Church;Fayette City;and the CFU Lodge 194;Monessen.  Survivors include one daughter and son-in-law;Barbara Ann and Patsy DeRienzo of R.D.Belle Vernon;and two grandsons;Patrick and Matthew DeRienzo.  In addition to her parents;she was preceded in death by her husband;Edward S.Simko on Sept.18;1979;and several brothers and sisters.  Friends will be received Friday from 2 to 4 and 7 to 9 p.m.in the Howard W.McCrory Funeral Home;Fayette City.  A Prayer service will be held Saturday at 10:30 p.m.;followed by a Mass of Christian Burial at 10:45 a.m.in the Holy Spirit Roman Catholic Church;Fayette City;with the Rev.Richard E.Mackiewicz as celebrant.  Burial will be in Belle Vernon Cemetery.  The Howard W.McCrory Funeral Home is in charge of the arrangements.