Scale Factor Model to Estimate the Maximum Bidirectional Spectral Demand in the South American Region

The intensity of seismic motion varies considerably with changes in direction, which is commonly known as the directionality of ground motion (GM). Different seismic design codes have adopted various methods for combining the intensity of horizontal GMs. These methods typically use the median spectral ordinate over all non-redundant orientations, referred to as RotD50, and the maximum spectral ordinate over all non-redundant orientations, referred to as RotD100. More recently, an intensity measure called MaxRotD50 has also been proposed and considered, which is calculated as the 50th percentile of the maximum spectral ordinates from two orthogonal horizontal directions for all non-redundant rotation angles. This measure, which always lies between RotD50 and RotD100, offers several advantages over other intensity measures. In contrast to the approach in ASCE 7 (2010, 2022), which uses RotD100 for the design of all structures, this study proposes to use RotD100 only for the design of axisymmetric structures, that is, structures with vertical cylindrical symmetry and similar properties in terms of mass, lateral stiffness and strength. It is also proposed to use MaxRotD50 for the design of structures where the probability of exceeding the intensity of GM in at least one of the two horizontal principal components is high. Likewise, this study complements and compares the RotD100/RotD50 and MaxRotD50/RotD50 ratios, which can be used as a multiplicative factor with the RotD50 predictions to predict the RotD100 or MaxRotD50 of the GM intensity. A database of 3853 seismic acceleration records from 283 events in the South American subduction region is used for this purpose. The influence of GM parameters such as moment magnitude, significant duration, rupture distance, and mean soil period, was evaluated. The results were compared with those of previous studies for different regions of the world. It was found that the RotD100/RotD50 ratios in South America are like those in other subduction regions such as Taiwan and Japan and that the Max RotD50/RotD50 ratios are comparable to other shallow crustal earthquakes in active tectonic regions. Finally, equations are also proposed to estimate ratios depending on the different parameters of the evaluated GM to account for the found influence on the ratios.