Effectiveness of percentage combination rules for maximum response calculation within the context of linear time history analysis

In the present paper the effectiveness of the percentage combination rules for the determination of the maximum value of any response parameter under two horizontal seismic components is investigated, within the context of linear response history analysis. Several reinforced concrete (R/C) buildings subjected to eight bi-directional seismic motions are analyzed. The maximum response values computed by the 30% and the 40% rule are compared to the maximum response over all incident angles produced by analytical formulas. In these analyses the earthquake ground motion is represented (a) by the recorded correlated accelerograms and (b) by the corresponding uncorrelated accelerograms. Furthermore, two different sets of structural axes are established and the analyses are performed with regard to both these sets. The results demonstrate that the percentage combination rule (30% or 40%) leads to unconservative response values. Moreover, the percentage combination rules produce results which strongly depend on the user's selection of the reference system.