Parametric Studies of Inelastic Modelling of Steel Moment Frames

This report summarizes the results of a brief study examining the effects of various modelling assumptions on the predicted inelastic response of a steel moment frame. This study was conducted as part of SAC Task 3.5.

The objective of the study was to determine how sensitive the predicted inelastic reponse of a moment frame may be to the assumptions used in developing the structural model. A variety of computer models were constructed for a six story single bay steel moment frame. Two basic classes of models were considered: "refined" models, and a "baseline" model. The baseline model used simple bilinear representations of inelastic response, and modelled bare steel behavior only (no composite floor). The refined model emplyed more realistic hardening rules that better represent experimentally observed inelastic behavior, and included composite floor slab effects. In addition to comparing the refined model to the baseline model, variations in other modelling parameters were also considered: a static pushover analysis and three strong ground motion records.

Some of the major conclusions of this study can be summarized as follows:
• The results of this study indicate that the predicted inelastic dynamic response of a steel moment frame is quite sensitive to modelling assumptions. Key inelastic response parameters, such as predicted beam plastic rotation demands, can change by more than 100% as model parameters are varied.
• The predicted response of a steel moment frame can be significantly affected by the presence of a composite floor slab. Neglecting composite floor slab effects can substantially underestimate the stiffness and strength of the frame, and alter the predicted plastic rotation demands.
• The predicted inelastic response of a steel moment frame is particularly sensitive to the assumed value of yield stress for the beams and columns of the model.
• Varying the model assumptions appears to have a very important effect on the predicted location of yielding within a joint. This is particularly true for frame in which the panel zone is relatively weak compared to the beam. Changing the modelling assumptions can cause the predicted location of yielding at a joint to shift almost completely between the beam and panel zone.
• In general, the results of this study indicate that varying modelling assumptions has a substantially greater impact on local response predictions such as beam plastic rotation, than on global response predictions such as interstory drift ratio. Thus, the accuracy of the model is less important if only global response predictions, such as interstory or roof displacement, are of interest.

Inelastic dynamic analysis can be a valuable tool for developing an improved understanding of how a steel moment frame may respond to a strong earthquake. Such an analysis, guided by judgement, can be used to estimate approximate plastic rotation demands at critical locations in a steel moment frame. However, the analyst must recognize that large uncertainties exist in modelling inelastic behavior. Inelastic deformation demands cannot be predicted with great precision. Uncertainty in modelling inelastic behavior must be recognized when making design decisions.