“…While this approach has also been frequently applied in the field of structural dynamics Thomson, 1974, 1977;Ibrahim and Mikulcik, 1973, 1977Ibrahim and Pappa, 1982;Ibrahim, 1977Ibrahim, , 1978Ibrahim, , 1983Junkins, 1978;Beck and Jennings, 1980;Yao, 1985;Torkamani and Hart, 1975;Shinozuka et al, 1982;Rajaram and Junkins, 1985;Hac and Spanos, 1987), there is a paucity of studies that are concerned with the problems encountered by this approach when applied to realistic problems arising in the vibration field. Consequently, the present section of this paper is devoted to presenting an indepth, unified, and efficient approach for using least-squares…”