## 7 Conclusions  
The responses of steel moment-resisting frames (MRFs) considering the mass matrix as concentrated (\(M_{\text{L}}\)) are calculated and contrasted with those calculated by considering that such matrix is consistent (\(M_{\text{C}}\)). The responses are also calculated by considering just one element per beam and are compared with those of assuming two and three elements. Steel frame models, representative of low, medium, and tall steel buildings, modeled as multi-degree-of freedom systems under the action of 15 seismic records, representative of the area of the model locations, are used in the numerical study. The comparison is made in terms of overall parameters (inter-story shear and displacements) as well as in terms of local parameters (axial loads and flexural moments at exterior and interior beams and columns). The main findings are:  
Figure 15: Mean values of \(R_{B2}\) beams, \(NS\): (**a**,**b**) exterior and interior, respectively, Model 1; (**c**,**d**) exterior and interior, respectively, Model 2. 1. Depending on the parameter, the structural model, the seismic intensity and the member location, the responses may be precisely calculated, significantly overestimated or underestimated, when \(M_{\text{L}}\) is used. The inter-story shears, on an average basis, are underestimated by up to about 15%, but underestimations of up to 30% can be seen for some individual seismic records. The lateral drifts are precisely calculated, but axial loads at columns can be significantly overestimated (up to 60% and 100% on average and individually, respectively). Flexural moments at columns, on the other hand, are underestimated; average and individual underestimations of up to 35% and 60%, respectively, can be seen. The flexural moments at beams are precisely calculated. 2. Inter-story shears can be overestimated if just one element per beam is used in the structural modeling; average and individual overestimation of up to 21% and 33%, respectively, can be seen. The lateral drifts, however, are precisely calculated. For significant deformations, the corresponding amounts of average and individual overestimation for axial loads at columns are about 95% and 140%, while those of flexural moments are 48% and 68%. The flexural moments at columns for the lowest structural deformations, on the other hand, can be underestimated; the maximum amounts of average and individual underestimations are about 14% and 29%, respectively. Underestimations by up to 35% and 60% (on average and individually) are observed for flexural moments at beams for small deformations, but for intermediate and large deformations, they are precisely calculated. The underestimation or overestimation obtained by comparing the responses of the models without intermediate node with those of one intermediate node is quite similar to that of comparing the responses of the models without intermediate nodes with those of two, or three, intermediate nodes; it indicates that convergence is reached in the responses when three elements per beam are considered. 3. Structural members in MRFs are normally designed as beams or beam-columns based on the strength concept to support the member forces; then lateral drifts are revised. The results of this study indicate that the introduced errors for lateral drifts are neglectable if \(M_{\text{L}}\), or one element per beam is used, but significant errors may be introduced in the design due to the overestimation and underestimation of the design forces. For those reasons, it is strongly suggested to use the \(M_{\text{C}}\) model and at two elements per beam in the structural modeling. 4. One of the main justifications in the past for not using the consistent mass matrix was the great computation (implying greater cost) time demand.