Some presumptions on the nature of base excitation may mamayadversely affect the Anay erroneously affect the response of strongly inelastic systems systems systems

Three presumptions on how the design base ground motion is defined are entrenched in earthquake engineering codes of practice: (a) elastic Design Response Spectra (consisting of a horizontal constant-acceleration branch from very low to medium periods and a descending branch at higher periods) adequately describe the seismic threat at a site; hence they must be closely respected from the selected accelerograms–excitations, even for highly inelastic systems; (b) for relatively soft and medium soil categories (as broadly defined in the codes) the shape of the acceleration design spectra, Sa /A, is flatter than for the stiffer soil categories, with its horizontal plateau extending to higher periods — ―the softer, the flatter”; and (c) the vertical component of ground shaking can be very important in all cases and, for geotechnical systems, its effect is best accounted for by vectorially combining the vertical and horizontal effective ground accelerations. Severe limitations of the above concepts (which most often lead to unsafe results) are shown in the presentation, along with alternatives that largely avoid some of the detrimental consequences.