Spectral acceleration (Sa) is the peak acceleration response of a single-degree-of-freedom oscillator with a given natural period and damping ratio when subjected to a specific ground motion. It is the primary quantity plotted in a response spectrum and the fundamental demand parameter in modern seismic design codes.
Spectral acceleration (Sa) is the maximum absolute acceleration experienced by a single-degree-of-freedom (SDOF) oscillator of a given natural period (T) and damping ratio (ΞΆ) when subjected to a specific ground motion. When plotted against period for a range of SDOF systems, it forms the acceleration response spectrum β the most widely used representation of seismic demand in structural engineering.
Sa is the central demand parameter in modern seismic design. Equivalent lateral force procedures use Sa at the structure's fundamental period to compute base shear. Modal response spectrum analysis uses Sa at each modal period to compute modal forces. Performance-based design frameworks β including FEMA P-58 and the PEER framework β use Sa at multiple periods to define intensity measures for fragility and loss estimation. Sa is also a common input to ground motion selection and scaling procedures, where records are chosen to match a target spectrum across a period range of interest.
Sa is period-dependent and site-specific, which makes it both powerful and nuanced. At short periods, Sa is dominated by high-frequency content and correlates with PGA. At long periods, Sa depends on the low-frequency energy of the motion and is sensitive to source distance and site conditions. Near a structure's fundamental period, Sa amplifies significantly, which is why resonance is a critical design concern. Sa is not a fixed property of a ground motion β it depends on the damping ratio assumed in the SDOF system, typically taken as 5% for design purposes but modified for other damping levels via damping modification factors.