Seismic response of steel frames modeled as MDOF systems using concentrated and distributed plasticity: A point of view of several response parameters

In dynamic analyses of steel buildings, the most common approach to model the material nonlinearity is the concentrated plasticity (CP) model. In this paper, the accuracy of this practice is evaluated by comparing the seismic responses obtained with this approach with those of the distributed plasticity (DP) model. Regular plane steel moment frames with 4, 10, 15 and 20 levels, representing low-, mid- and high-rise steel buildings, were considered in the study. Results indicate that bending moments in beams and columns, axial forces in exterior columns, and interstory shears, are underestimated on average by up to 30%, 21%, 28%, respectively, if the CP formulation is adopted, a reason for this is that, the CP approach inadequately capture the nonlinear behavior inside of the members, since they are assumed to remain fully elastic except at plastic hinges which develop at the member ends. Axial loads in interior columns, unlike those of the exteriors, are accurately predicted with the CP approach, this is due, in part, to the fact that axial loads on columns come from three sources whose combined effect on the exterior columns is different than that of the interiors. Unlike the case of forces, the interstory displacements can be underestimated or overestimated, if the CP approach is used in the seismic analyses, the maximum values of underestimations and overestimation are 16% and 14%, respectively. One reason for having overestimation for the case of interstory displacements is due to the fact that the contribution of the higher modes to the response in terms of displacements is different than that of forces. In addition, the contribution of such higher modes may have been enhanced for the case of the CP formulation. The earlier results clearly demonstrate that, in general, the seismic response of steel buildings with moment resisting frames are underestimated if the CP approach is adopted, leading to nonconservative designs. Hence, it is strongly suggested to use the DP formulation in the seismic analysis. A rough correction can be made by increasing the seismic response by a 25% when using the CP formulation. More research is needed considering other situations to reach more general conclusions.