What is a key limitation of using unscaled ground motion records in NRHA?

A key limitation of using unscaled ground motion records in Nonlinear Response History Analysis (NRHA) is that they may not accurately represent the seismic hazard level for the specific site being analyzed. Ground motion records are recordings from past earthquakes, and their intensity (amplitude and frequency content) depends on factors like earthquake magnitude, distance from the fault, and site-specific soil conditions. Using unscaled records means assuming that the intensity of shaking in the historical record is representative of what the structure is expected to experience at its location for a given design event (e.g., a 475-year or 2475-year return period earthquake). Since this is rarely the case, using unscaled records can lead to either an underestimation or overestimation of the seismic demand on the structure. If the unscaled record represents a smaller earthquake or a greater distance, the analysis might underestimate the actual forces and deformations the structure would experience. Conversely, using a record from a larger, closer earthquake could lead to an overly conservative and potentially uneconomical design. Therefore, scaling is crucial to align the ground motion intensity with the appropriate hazard level defined by probabilistic seismic hazard analysis for the site, ensuring a more accurate and reliable assessment of structural performance.