Redundancy in RC Moment Resisting Frames II: Evaluating the Effects of Redundancy on Seismic Behavior

Structural redundancy is a non-independent concept in structural engineering and has inherent dependence on structural parameters such as overstrength and ductility, so that both of overstrength and ductility capacities should change corresponding to any variation in structural redundancy. Nevertheless, most of researchers notified that taking any increase in structural redundancy should be a desirable property to deal with more effectively against earthquake loading. Furthermore, this issue can reduce structural sensitivity to abnormal loads. In this research to clarify the pure role of redundancy in earthquake resistant design and to distinguish the role of redundancy from total overstrength capacity, a number of 3D reinforced concrete special moment resistant frames (RC-SMRF) with equal ultimate base shear coefficient were designed. The dynamic behavior parameters of the designed structures under natural strong ground motion were evaluated, especially with regards to configuration of nonlinear deformations. The analytical outputs obtained from analyzed structures are illustrated ensembles of maximum acceleration, maximum velocity and maximum drift of each story. Furthermore, adequacy and accuracy of response modification factor which should be assigned as general indicator of quality of total seismic behavior has been studied conceptually. The results of this research indicate that: (i) Assigning an increase in structural redundancy would not always lead to efficient improvement in structural seismic behavior. Furthermore, notification to process of increased redundancy should not be consider as a criterion for any basic improvement in structural performance. This issue means that it is better to consider the effects of redundancy on important seismic parameters such as both the structural member ductility and the overstrength capacity. (ii) The calculated response modification factors as mentioned in this research, can consider as an index of quality of structural dynamic performance which is corresponding to a certain level of redundancy. Accordingly, the above statement should be notified in general cases of those earthquake loadings which would cause a certain level of story drift. This certain level of story drift would denote the structural behavior typically follows the calculated response modification factor. Oppositely, if an earthquake loading causes more story drift from that assigned certain level, structural behavior typically does not follow the calculated response modification factor. (iii) The codified procedure of calculation of response modification factor which were discussed and assessed in this study, cannot be realized subjected to those input strong ground motions that able to display high amplitude and long period pulse or pulses in their velocity time history. It is important to know that strong near-fault ground motions often have an impulsive feature and impose large amounts of sudden intense kinematic energy which must be dissipated by structural system during a short period of time. This issue causes amplified deformation demands in structures which are associated with very few cycles of cumulative plastic deformations. Hence, the earthquake damages due to these seismic load cases are effectively related to maximum deformation as well as maximum ductility. Yet, structures cannot accomplish based on the calculated response modification factor in the mentioned cases.