Investigating the Dynamic Stability Index of Steel Structures

Articles in Press

Document Type : Research Note

Authors

1 Department of Civil Engineering, Faculty of Engineering, University of Qom, Qom, Iran

2 International Institute of Earthquake Engineering and Seismology, Tehran, Iran

3 Associate Professor, Department of Civil Engineering, Faculty of Engineering, University of Qom, Qom, Iran

Abstract

The stability index, defined as the ratio of moments caused by secondary P-Delta effects to those resulting from design story shear forces, serves as a critical parameter in assessing structural behavior. These P-Delta effects, arising from the interaction between axial loads and lateral displacements, can significantly influence the overall structural response. While current design codes primarily provide static relationships for calculating the stability index - typically developed for lateral loads such as wind - this study adopts an innovative approach by investigating the stability index from both dynamic and nonlinear static perspectives. To obtain reliable results, three reference structures with varying heights (3-, 9-, and 20-story buildings) were analyzed using two advanced analytical methods: Incremental Dynamic Analysis (IDA) and Nonlinear Static Analysis. The research findings reveal that for low-rise buildings, when inelastic deformations are considered, static stability index values closely approximate their dynamic counterparts. However, for medium- and high-rise structures, significant discrepancies emerge between results obtained from the two methods. A key finding demonstrates that the dynamically computed stability index exceeds code-specified threshold values (0.1 and 0.25) for all cases except those with low acceleration levels (below 0.2g). The study further establishes that dynamic stability index values differ substantially from those recommended in current design codes. These results suggest that traditional stability assessment methods may prove non-conservative for tall structures subjected to severe dynamic effects. Significant influence of building height on the relationship between static and dynamic stability indices. Critical importance of considering nonlinear effects in stability calculations for tall structures. Necessity for developing new relationships that properly incorporate dynamic effects in stability index computation. These findings provide a solid foundation for updating structural design codes and developing more precise analytical methods. Future studies should investigate a broader range of structures with various lateral load-resisting systems to establish more comprehensive relationships for stability index calculation. Implementation of height-dependent correction factors in stability evaluation. Development of dynamic amplification coefficients for P-Delta effects. Inclusion of higher-mode effects in stability assessments for tall buildings. Consideration of material nonlinearity and geometric imperfections in analytical models. The study's outcomes emphasize the need for code revisions to address stability concerns in modern tall building designs, particularly in seismic-prone regions. The proposed methodology offers a more realistic assessment of structural stability under extreme loading conditions.

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Sharif Journal of Civil Engineering

Articles in Press, Accepted Manuscript
Available Online from 15 July 2025

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