Bulletin of Earthquake Science and Engineering

Bulletin of Earthquake Science and Engineering

Accidental Eccentricity Importance in the Collapse Behavior of Torsionally-Flexible Low-Rise Buildings

Document Type : Research Article

Authors
Isaac Torabpour 1
Ramin Ketabforoush Badri 2
1 M.Sc. Graduate, Department of Civil Engineering, Aza.C., Islamic Azad University, Azarshahr, Iran
2 Assistant Professor, Department of Civil Engineering, Aza.C., Islamic Azad University, Azarshahr, Iran
10.48303/bese.2024.2031961.1169
Abstract
The collapse of a building during an earthquake event signifies that some of the existing structural systems designed in compliance with the current code requirements are still unable to withstand the dynamic loads induced by earthquakes. Hence, understanding the behavior of buildings in high seismic hazard areas during severe earthquakes is crucial for preventing structural failures.
Numerous studies over the past few decades have delved into the behavior of irregular buildings, highlighting that the irregularity of a building can lead to varied demands depending on its ductile deformation capacity under lateral loads, potentially resulting in suboptimal use of materials in load-resisting members. Additionally, seismic loads may affect the lateral load-bearing members differently due to the building's irregularity.
For high seismic hazard regions, Iran's Seismic Code (Standard 2800) specifies the significant impact of accidental eccentricity in mitigating the adverse effects of torsion on the collapse of irregular buildings. However, the influence of accidental torsion on the collapse performance of torsionally flexible buildings requires further exploration. To this aim, the present study investigates the collapse performance of asymmetric models with flexible torsional behavior in accordance with FEMA-P695 guidelines, particularly in special reinforced concrete moment frame (SMF) buildings. The collapse margin ratio (CMR) is applied as a measure of the models' collapse performance. The proposed SMF buildings are designed considering various conditions of stiffness distribution. Then, the collapse behavior of asymmetric models is assessed by implementing an eccentricity that characterizes the accidental torsion effects.
The modeling and the analysis are carried out using OpenSEES software, in which the plastic behavior of the members is simulated using the concentrated plasticity method developed by Ibarra et al. The collapse mechanism of the models is scrutinized for different states of eccentricity to offer a comprehensive understanding of the accidental torsion effects on the collapse of torsionally flexible models. Utilizing the incremental dynamic analysis (IDA), the proposed collapse criterion is calculated, where the seismic intensity measure in dynamic analyses is the spectral acceleration at the fundamental lateral mode period. The commencement of instability in the building models is recognized by the concurrent formation of plastic hinges at the ends of the frame components in one or more floors. This instability criterion is overseen and managed by a processor programmed in MATLAB. The second criterion of the collapse occurrence is the inter-story drift ratio of building models, which is considered to be 10% of the story height.
The results demonstrate that by altering the position of the center of mass in the models, the safety margin against the collapse is bolstered by an average of 10%. This enhancement appears to be independent of the location of the center of mass relative to the center of rigidity in the plan. These findings raise pertinent questions about the significance of design accidental torsion criteria in this specific category of structures. However, there is a need to conduct additional studies before making any regulatory recommendations. For torsionally flexible models, the dominant mode of motion (i.e., the torsional mode) will have a great kinetic superiority with increasing eccentricity. As a result, the ro-le of that mode in absorbing earthquake energy will be greater than that of the translational modes. Accordingly, the damages that are directly caused by relative displacements in the direction of earthquake load will decrease with increasing distance of the center of mass from the center of rigidity, resulting in an increase in the safety index.
The results also demonstrate that the distribution of relative lateral displacements in torsionally flexible models is consistent with the distribution of plastic hinges. Interestingly, a distinct pattern of failure propagation has been identified for these structures, contrasting with torsionally stiff buildings.
Keywords
Collapse Margin Ratio
Torsionally-Flexible Building
Accidental Eccentricity
Seismic Performance

Subjects

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  • Receive Date 17 June 2024
  • Revise Date 23 November 2024
  • Accept Date 25 November 2024