Computation of seismic hazard at rock or firm sites has been a well established technique for many years. Hazard is generally expressed in terms of exceedance rates (ER), defined as the mean annual number of times in which a given value of intensity is exceeded. The inverse of the ER of some intensity is called its mean return period. Usually, the practice was to compute seismic hazard in terms of ER of peak motion values, such as peak ground acceleration, velocity and displacement,. More recently, uniform-hazard response spectra (UHS) have been constructed in a more direct and precise way, by means of attenuation equations that relate, typically, magnitude and distance with response spectral values for a range of periods. By performing a conventional hazard analysis on a period by period basis, it is possible to construct response spectra whose ordinates are, all, associated to the same return period, thus constituting a UHS. In order to compute approximate uniform-hazard response spectra at places affected by local soil amplifications, several approaches have been adopted in the past to overcome analytical difficulties. We present a coherent approach to construct UHS of different intensities at soft soil sites by means of interactive Internet-software named Zp, which will be illustrated with deterministic validations on Armenia city during the 25 January Armenia Earthquake and probabilistic examples for Bogotá city.