In nonlinear response history analysis, what is the primary purpose of scaling ground motion records?

In nonlinear response history analysis (NRHA), the primary purpose of scaling ground motion records is to adjust their intensity to match a target seismic hazard level for the site under consideration. Ground motion records are historical recordings of earthquake shaking, and their original intensities may not accurately represent the seismic hazard at the specific location where the structure is being analyzed. Scaling ensures that the ground motions used in the NRHA are representative of the earthquake shaking expected at the site for a given return period or probability of exceedance, as determined by probabilistic seismic hazard analysis (PSHA). This involves adjusting the amplitude of the ground motion record, typically by multiplying it by a scaling factor, so that its spectral acceleration (Sa) values, averaged across a range of periods relevant to the structure, closely match the target spectral acceleration values from a uniform hazard spectrum (UHS) or a conditional mean spectrum (CMS). The scaling process aims to provide a realistic and consistent representation of the seismic demand on the structure, allowing for a more accurate assessment of its nonlinear response and performance under earthquake loading. It's important to note that scaling has limitations, and excessive scaling can distort the frequency content of the ground motion, potentially leading to inaccurate results. Therefore, careful consideration and adherence to established guidelines are crucial when scaling ground motion records for NRHA.