Effects of Multi-Directional Seismic Input on Non-Linear Static Analysis of Existing Reinforced Concrete Structures

Recent studies have shown the importance of including the seismic input directionality in nonlinear analyses for an accurate prediction of the structural demand on frame structures. This paper proposes a new method that includes the multi-directionality of the input seismic forces in Nonlinear Static Analyses (NSAs). Conventionally, the pushover (PO) analyses apply monotonically increasing lateral loads in two directions that typically correspond with the building X and Y directions, that in the case of a rectangular plan are parallel to the building sides. Since in general the direction of the seismic input is a priori unknown, the effects of applying the PO load patterns along varying angles are studied in this paper. Two non-code-conforming reinforced concrete buildings are used as a case study. They have identical structural design but the first one is doubly symmetric while the second one has a significant plan asymmetry due to the translation of the center of mass. PO loads are applied to both structures at angles between 0° and 360° with 15° increments. The results of the NSAs are compared with those of multi-directional NHAs applied at the same angles. The structural demands show that the multi-directional NSAs are more conservative than the conventional NSAs, especially at the corners of the asymmetric- plan building where they can yield significantly higher demands. The base shear capacities in the X and Y directions decrease for intermediate angles due to the interaction between the responses in the X and Y directions that can be captured thanks to the columns’ fiber section discretization. On average the results of the multi-directional NSAs are closer to those of the NHAs, even though they are generally lower.