A design spectrum is a smoothed and simplified response spectrum prescribed by building codes for use in structural design. It represents the design-level seismic demand at a site, accounting for the site's soil class, seismic hazard, and importance of the structure, and serves as the primary input for equivalent lateral force (ELF) and modal response spectrum analysis.
A design spectrum is a standardized response spectrum defined by building codes to represent the seismic demand at a site for design purposes. Unlike a site-specific response spectrum computed from a single ground motion, a design spectrum is a smoothed envelope that accounts for the variability of ground motions, site soil conditions, and the probability of exceedance associated with the design earthquake. It is typically defined by a small set of anchor points β such as peak spectral acceleration, corner periods, and long-period decay β that make it easy to apply in routine design.
Design spectra vary by code. In ASCE 7, they are constructed from mapped spectral accelerations (S_S and S_1) modified by site coefficients (F_a and F_v) to produce the design spectrum (S_DS and S_D1). Eurocode 8 defines Type 1 and Type 2 spectra with different shapes depending on the seismic source. The Iranian Standard 2800 defines a design spectrum based on the design base acceleration (A), the importance factor (I), the response modification factor (R), and the site class. In all cases, the design spectrum incorporates safety factors and simplifications that make it conservative but not always physically accurate.
Design spectra are a code-level abstraction of real ground motion. They intentionally smooth over the peaks and valleys of site-specific spectra, sacrificing accuracy for uniformity and safety. This means that for a specific site with unusual soil conditions or near-fault effects, a site-specific response spectrum analysis may be required. Modern performance-based design often supplements or replaces the design spectrum with site-specific spectra or nonlinear time-history analysis. However, for the vast majority of ordinary structures, the design spectrum remains the standard and legally required basis for seismic design.