The true behavior of structural systems, mainly essential in seismic analyses, is nonlinear and dynamic. In analysis of nonlinear dynamic behaviors of structural systems, a versatile tool is direct time integration. In spite of the versatility, the responses obtained from time integration are approximations and the computational costs are considerable. For analyses subjected to excitation available as digitized records, a technique to reduce the computational cost with negligible sacrifice of

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... curacy is proposed in 2008. The technique is successfully implemented in seismic analysis of frames, buildings structures, bridges, reservoirs, power stations, silos, etc., and undergone theoretical studies from different points of view. This is a complementary study that, with the aim of reducing the small errors because of the technique and reducing the computational costs even more, numerically investigates the frequency content of the errors originated in the computational cost reduction technique. As the conclusion, more errors are expectable in lower modes and/or modes with more contribution in the response. Further extensive study is essential.