Journal of Concrete Structures and Materials

Journal of Concrete Structures and Materials

Evaluation of the Behavior Factor of Hybrid (Concrete-Steel) Vertically Irregular Structures

Document Type : Original Article

Authors
jalil khani 1
Aboozar Mirzakhani 2
hamed valizade 3
1 iau, shahrood, iran
2 Assistant professor, Department of Civil Engineering, Shahrood Branch, Islamic Azad University,
3 Assistant professor if iau
10.30478/jcsm.2026.558120.1407
Abstract
Hybrid reinforced concrete-steel structures, due to their optimal integration of the advantages of both materials, have found widespread application in the construction industry, particularly in regions with high seismic hazard. However, the quantitative impact of vertical irregularity on key seismic design parameters, such as the behavior factor (R), in these structures has not been sufficiently investigated. This study aims to quantitatively evaluate the behavior factor in ordinary moment-resisting frames where the lateral load-bearing system transitions from reinforced concrete to steel over the height. To this end, a series of 6-, 12-, and 18-story models with varying degrees of irregularity were modeled and assessed using nonlinear static (Pushover) analysis. Following the derivation of capacity curves, key parameters including the behavior factor (R), overstrength factor (Ω), and ductility factor (μ) were calculated employing standard bilinearization methods. The results indicate that the location of the material transition and the severity of irregularity have a direct and significant influence on the values of these factors. A comparison with code-specified values for regular structures reveals that using these values for the design of irregular hybrid structures may be non-conservative. In general, the seismic performance of hybrid frames in resisting lateral loads demonstrates a notable superiority, showing 17% to 39% higher behavior factors compared to their equivalent fully reinforced concrete counterparts. The final findings underscore the necessity of calculating the behavior factor on a case-specific basis, considering the pattern and degree of irregularity. This approach paves the way for achieving higher seismic resilience and more economical design of tall buildings in earthquake-prone zones.
Keywords
Hybrid reinforced concrete-steel structures
Vertical irregularity
Behavior factor (R)
Nonlinear static analysis (Pushover)
Moment resisting frame
Subjects
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  • Receive Date 08 November 2025
  • Revise Date 12 May 2026
  • Accept Date 20 July 2026