several simplified methods based in nonlinear static analysis that are used to determine the seismic demand in structures are evaluated in this paper. reinforced concrete frames of one, two and seven stories, with a moderate and a high energy dissipation capacity, subjected to six accelerograms that represent different soil, intensity and fault-distance conditions, are considered. the seismic demand on each structure is determined with the simplified methods and compared with the ？exact demand？ obtained from a nonlinear dynamic analysis (adnl). the mpa method (modal pushover analysis) leads to the best estimation of the seismic demands, with absolute differences with the adnl that in the average are 17.2 % for the 30 cases analyzed. the inclusion of the stiffness degradation in the mpa method does not introduce an apparent improvement in the results. the fema-356 method leads to differences with the mpa that in the average are 45.5 %. the procedures a and b of the atc-40 leads to mean differences of about 24.4 %, but they do not yield results in 40 % and 27 % of the cases, respectively, because of the lack of convergence or because the limitations implicit in the procedures. when only the cases with low and moderate ductility demand are considered, the differences between all the procedures and the adnl are reduced. the lowest differences correspond again to the mpa method and in the average they are less than 9 %. although the mpa method yields the best estimates, it requires additional calculations in order to determine the nonlinear dynamic response of one-degree of freedom systems; however there are several programs that allow these calculations in a simple and direct way.