New Criteria for the Determination of Seismic Performance Levels of Dual-Steel Concentrically Braced Frames Based on the Park-Ang Damage Index

The Performance-Based Earthquake Engineering (PBEE) has emerged three decades ago in order to improve the safety assessment of structures. The computational advances in past decades paved the way for researchers and engineers to be able to use sophisticated algorithms for analysing complicated structures. PBEE aims at assessing and control the performance in different structural elements during and after a target ground motion. The immediate occupancy, life safety and collapse prevention are three common structural limit states that have been used in most of the regulations around the world. However, quantification of these limit states for different structural systems is somehow not very crystal clearly defined. FEMA 273, FEMA 356, and ASCE 41-13 are some pioneers in this subject in which they compare demand and capacity at each element. Excessing demand to the capacity ratio in one element means that the whole structure fails to comply with the regulations, which might somehow be a conservative approach. However, FEMA 350 is one of the few regulations that uses an approach for the structural assessment as a whole.
Dual systems usually provide enough strength, stiffness, and ductility, especially for tall buildings in seismic prone regions. The combination of steel moment frame and the concentric bracing system is one of the most convenient dual systems around the world. The bracing system bears the majority of the base shear; however, after significant buckling of braces, the moment frame behaves as a second line of defence. It is worth mentioning that few attempts have been made in order to quantify the limit states in dual systems. The limit state criteria for individual moment resisting or bracing systems might be different in essence if being used for dual systems. Therefore, limit states for these dual systems should specifically being studied. Therefore, this vital issue is the main focus of the current study.
In designing and evaluating structures based on performance, identifying the performance levels of the structure at specific seismic hazard levels is of particular importance. In assessing the seismic performance of dual-steel concentrically braced frames consisting of moment-resisting frames and concentrically braced frames with incremental dynamic analysis, there are no accepted and reliable criteria for determining performance levels. The present study proposes suitable and simple criteria for identifying the performance levels of this structural system. For this purpose, the mean value of the Park-Ang damage index is calculated for a given moment-resisting frame system at the performance levels based on the FEMA 350 guideline approach. On the basis of these values, the performance levels of 22 steel buildings with the dual system with different configurations of non-geometrical irregularity along the height are determined by means of incremental dynamic analysis. The mean damage index for the immediate occupancy and collapse prevention are, respectively, equal to 0.21 and 0.9. These damage indices are accounted for a reasonable representation of the structural performance. Finally, a set of appropriate and straightforward criteria for determining the performance levels of the dual-steel concentrically structural system are proposed based on the average of the capacity of the inter-story drift ratio in each performance level. The inter-story drift ratio equal to 0.9 per cent is proposed for the immediate occupancy limit state. In the case of collapse prevention limit state, the inter-story drift ratio equal to 7.5 per cent or the slope reduction of IDA curve to 20 per cent of the initial slope, each reaches earliest, is proposed as the performance limit state.