Developing cyclic Dual Loading Protocols for the Columns of Moment Frames Based on Their Seismic Demands, and Evaluation of Their Cyclic Behavior

The column members of moment frames are subjected to high axial forces as well as inelastic rotations during a seismic event. Estimating the boundaries of these simultaneous structural demands on the columns of special moment frames are one the aims of this research. These demands were evaluated in this research by performing a vast number of non-linear time history analyses on some archetype frames. Totally, eight archetype buildings were designed under two levels of spectral accelerations represent the Maximum Credible Earthquake (MCE) and the Design Earthquake (DE). Both geometric and material nonlinearities were taken into account. Far-field earthquake record set proposed by FEMA P696 was utilized in this study to conduct the required time history analyses. This set includes 44 individual seismic records with different magnitudes and other specifications. Each sample SMF was excited by each of the individual records with two different intensity levels referring to DE and MCE respectively. After performing numerous time history analyses, the boundaries and the characteristics of the axial force and the story drift ratio demands on columns were fairly stablished for the seismic events with MCE and DE intensity levels. It was proved that they might experience the axial force around 0.3 of their yielding capacity as well as the drift ratios up to 0.06. Moreover, based on the results of these analyses, a representative loading protocol was also developed for the column members of special moment frames because the effect of simultaneous change of the axial force demand on columns along with the change of plastic rotation in these members was neglected in all of introduced loading framework for these members. On the other hand, the utilized loading frameworks in different research studies were introduced quite arbitrarily and with no root in the real seismic demands of these structural members. Hence, a statistical approach implemented to extract the loading cycles based on the results of the mentioned non-linear time histories. This loading protocol includes cycles of varied axial force ratios as well as simultaneous cycles of lateral loading.
Providing such a loading regime can pave the way to investigate the behavior of the columns of moment frames using more sophisticated numerical models that cannot be ordinarily employed to perform time history analyses under seismic records due to excessive computational costs. The overall response of steel columns may be dominantly affected by the local effects like as local bucking at large deflections. Hence, the response of some archetype individual columns was fairly investigated under the proposed loading protocol by utilizing continuum finite element simulations which are expected to be able to capture deliberately these kinds of local effects. The investigation not only revealed more on the details of the behavior of these members, but also proved the versatility of the proposed loading protocol in comparison to the previous loading procedures implemented on columns. In order to do so, the numerical models of the archetype columns were also loaded under the proposed lateral loading protocol besides constant levels of axial loading and the results of both cases were compared. It was shown that implementing a constant axial load besides the cyclic lateral cyclic loading may not be an appropriate loading framework and the results would not represent the seismic demands in these members accurately.