Strength or force reduction factors for steel buildings: MDOF vs SDOF systems

Alfredo Reyes-Salazar, Mario D. Llanes-Tizoc, Edén Bojórquez, Juana Luz Rivera-Salas, Arturo Lopez-Barraza, Achintya Haldar

Research output: Contribution to journalArticlepeer-review

1 Scopus citations


The nonlinear seismic responses of steel buildings with perimeter moment resisting frames (MRF), modeled as complex 3D MDOF systems, are calculated and the ductility demands, ductility reduction factors and the force reduction factors (R), are studied. Equivalent 3D models with spatial MRF, two-dimensional models, and equivalent SDOF systems (SD), are also considered. Results indicate that the global and local force reduction factors significantly vary from one structural representation to another and that they are much larger for the SD models. One of the reasons for this is that, although there is equivalence between the SD and MDOF models, the dissipated energy and the number of incursions in the inelastic range are significantly larger for the SD models. In addition, while for the SD models total plasticization occurs, for the SAC and EQ models, even for significant yielding, plastic hinges are developed only in a relatively small number of structural members; therefore, using the R factors of the SD models may be conservative. According to the results obtained in this research for the more realistic representation of the steel building (3D), the value of 8 suggested in many codes for the R factor for ductile MRF cannot be justified. It is only justified for the SD systems. The implication of this is that non-conservative designs may be obtained if so large value is used. More transparence is needed in the codes regarding the magnitude and the components involved in the R factor.

Original languageEnglish (US)
Pages (from-to)2680-2702
Number of pages23
JournalJournal of Vibroengineering
Issue number4
StatePublished - 2017


  • 3D models
  • Ductility and force reduction factors
  • MDOF and SDOF systems
  • Moment resisting frames
  • Nonlinear seismic analysis
  • Steel buildings

ASJC Scopus subject areas

  • General Materials Science
  • Mechanical Engineering


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