No abstract

Past and recent earthquakes events demonstrate that buildings with configuration irregularity are more vulnerable to earthquake damages. So it's essential to investigate the seismic response of these structures in active seismic zones to reduce the potential seismic damages. The configuration irregularities introduce major challenges in the seismic design of building structures. One such form of irregularity is the presence of re-entrant corners that causes stress concentration due to sudden changes in stiffness and torsion amplification in the buildings; hence causes early collapse. This, the conventional design codes have not recommendations for proper evaluation of these buildings yet. Thus, a constructive research into re-entrant corner irregularity problems is essentially needed greater than ever. The objective of this study is to grasp the seismic behavior of the buildings with irregular plan of L-shape floor plan through the evaluation of the configuration irregularity of reentrant corners effects on measured seismic response demands. The measured responses include inter-story drift; story shear force; overturning moment; torsion moment at the base and along the building height; top floor displacement; and torsional Irregularity Ratio. Three dimensional finite element model of nine stories moment resisting frame buildings as reference model is developed; six L-shaped models are formulated with gradual reduction in the plan of the reference model. The models are analyzed with ETABS using Equivalent Static Load (ESL) and Response Spectrum (RS) Methods. The results prove that buildings with severe irregularity are more vulnerable than those with regular configuration resulting from torsion behavior, and the additional shear force produced in the perpendicular direction to the earthquake input. Also, in the codal empirical equation for the calculation of fundamental period of vibration could not grasp significant higher vibration modes such as torsional vibration of irregular buildings that could significantly affect seismic demands.

Recent earthquakes have demonstrated that buildings with irregular configuration are more vulnerable to earthquake damage. Moreover, the configuration irregularities introduce major challenges in the seismic design of building structures. One such form of irregularity is the presence of re-entrant corners and torsional irregularity that causes stress concentration due to sudden changes in stiffness and torsion amplification in buildings. Constructive research into re-entrant corner and torsion-irregular buildings is therefore needed to evaluate the seismic response demands for reducing the potential damage. The aim of the study reported in this paper is to grasp the seismic performance of L-shaped irregular buildings with moment-resisting frames through an evaluation of the irregularity effects on measured seismic response demands. The results for inter-storey drift, storey shear force, overturning moment, torsion–moment responses at the base and along the building height, top-floor displacement and torsional irregularity coefficient prove that buildings with irregularity are more vulnerable than those with a regular configuration resulting from stress concentration and coupled lateral–torsional behaviour.

Pushover analysis has been recommended as a reliable tool to estimate the seismic capacity of the structures. Vertical irregular structures are highly vulnerable during earthquakes due to stiffness irregularity in their elevations. Hence, seismic capacities of these types of structures need to be re-estimated during structural design stage. So, in this paper, an attempt has been made to assess the actual seismic performance of buildings with two common types of vertical irregularities such as; soft story and setback in comparison with regular (reference) building. These types of vertical irregularities are studied in individual cases, combined in one story, and combined in two different stories of the building models, while most previous studies satisfy with individual type of vertical irregularity case in the studied model. In addition, combined vertical irregularity generates extra weak points, which alter the seismic capacities, failure mode mechanism, and performance point location. Three-dimensional numerical models are created to find out significant response demand such as; the variation in periods of vibration, lateral displacement, inter-story drift, pushover curve, and plastic hinges formation. The results showed that vertical irregular buildings are subjected to early damages and have less seismic capacity than regular one. The fundamental time period becomes misleading term in seismic force calculation for vertical geometric irregular buildings and needs to be re-considered. In addition, extra lateral displacement and inter-story drift are passively generated in the vertical irregular buildings due to sudden change in stiffness. Significant negative variation in the pushover curves, ductility ratios, and plastic hinges' formation is observed when combinations of pre-mentioned vertical irregularity cases occur. Buildings with open soft ground story with asymmetric setback should have additional precautions from international codes during structural design stage according to their irregularity ratio. Therefore, response modification/reduction factor (R) may be scaled-down to adapt these negative variation in seismic capacities.

Most design codes assume the nonlinear seismic performance of structures using response reduction/modification factor (R). The R factor is sensitive to a variety of factors in terms of overall ductility and over-strength. This research assesses the actual R factor for vertical irregularity cases for RC bare buildings with moment-resisting frames (MRF) systems. Also, this research derives a significant relationship between R values and identified vertical irregularity index calculated from relative stiffness between adjacent stories. Three-dimensional numerical models are carried out for the soft story and setback irregularity scenarios using ETABS. Modal pushover analysis (MPA) is selected to obtain the inelastic seismic capacity. The obtained results demonstrate that vertical irregular buildings have weak inelastic seismic capacities compared to regular one. So, the response modification factor (R) should be scaled down before the design stage by 15% to 40% for single and combined vertical irregularity scenarios. Structures with a combined asymmetric setback with a soft ground story experience the worst R factor. Also, R factors are sensitive to the identified vertical irregularity index (Vtm) that has 80% regression percent. So, it may be used to specify the allowable vertical irregularity ratio, location, and combination for each seismic zone.