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Seismic Demand in Diaphragms and Collectors

Sheikholeslami, Alireza | 2025

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 58279 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Maleki, Shervin
  7. Abstract:
  8. This study investigates the seismic demand in diaphragms, chords and collectors in typical floor systems used in steel structures. These systems include joist-block slabs, open web steel joists, and composite steel decks. The lateral load resisting systems considered were Special Concentrically Braced Frames (SCBF) and Special Moment Frames (SMF) of varying heights. For each system, the influence of using shear connectors for perimeter beams, joist orientation, geometric configuration, and number of stories on lateral force transfer was examined. Nonlinear static and time history analyses were conducted on selected models. Key findings are as follows:
    • In joist slab system without shear studs along perimeter beams, the diaphragm force transfer path is significantly influenced by joist orientation. When aligned with seismic loading, axial forces develop in the joists and are transferred to collector elements through the chords. In the orthogonal direction, joist torsional moment causes a stepped distribution of shear and axial forces to the collectors.
    • Direct slab-to-collector connection using shear studs bypasses joist participation in lateral load transfer, allowing the slab to transmit forces directly to the lateral-force-resisting system.
    • Mechanical fastening in composite steel decks improves diaphragm ductility and cyclic performance compared to welded connections.
    Overall, the study clarifies the true structural role of diaphragms, demonstrating that without proper analysis, seismic load paths can become disrupted and traditional design assumptions may fail to ensure structural safety. Applying the outcomes of this research will bring diaphragm design practice closer to actual behavior.
  9. Keywords:
  10. Seismic Design ; Diaphragm ; Collectors ; Shear Stud ; Chords ; Special Concentrically Braced Fram System (SCBFS) ; Special Moment-Resisting Frames ; Steel Joist

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