Uniform Hazard Spectra. Whilst the design response spectrum is considered an idealised approximation to constrain the spectral response in a manner that is consistent with the hazard at a site, the Uniform Hazard Spectrum is a more direct connection between hazard and spectral response. The UHS is constructed from probabilistic seismic hazard analyses undertaken for a number of ordinates that span the spectrum range of interest – usually limited by the period range of the spectral GMPEs. The spectral acceleration at each ordinate has an equal probability of being exceeded in the given time period (e.g., 50 years), irrespective of the probability of the earthquake scenarios that contribute most strongly to hazard. The UHS is therefore an important tool that allows for the identification of the probabilistic hazard at the fundamental periods of structures under consideration. The UHS cannot necessarily be considered as a direct representation of the design elastic response spectrum. There is no explicit assumption as to which parts of the UHS are controlled by acceleration, velocity or displacement, as each ordinate is independent. The shape of the hazard spectra, and the relative contribution of longer period or shorter period motion at a site, is a reflection of the nature of the seismicity giving rise to seismic hazard at a site. For example, a site at which hazard originates from larger earthquakes over greater source-site distances will likely show greater hazard contribution from longer period earthquakes resulting in a “flatter” UHS. Conversely, a site for which hazard originates from small near-field earthquakes may show a sharp peak at shorter periods, decaying to near trivial values for longer periods. In reality, hazard may originate from different sources over a range of source site distances, so the resulting UHS may reflect different source contributions over different periods. It should be recognised, therefore, that the UHS is not representative of a single given earthquake scenario. This last point is an often applied criticism of the UHS in seismic design, and is equally valid for the use of the ERS in seismic design. If the ERS is anchored via spectral accelerations for specific periods (as is the case for the NEHRP Provisions) then the earthquake scenarios controlling each of the periods will most likely differ. Where the UHS offers an important advantage is that the hazard is more closely constrained at intermediate periods, rather than assumed using approximat...
