What Is Diaphragm Design — Flexible vs. Rigid?
Diaphragms (the floors and the roofs collecting the lateral loads and delivering to the vertical elements) are classified flexible, rigid, or semi-rigid — the classification determining the load distribution (the tributary vs. the stiffness-proportional). The design: the diaphragm's in-plane (the shear, the chord, the collectors), the classification (the ASCE 7's — the deflection's ratio), and the detailing (the connections — the LFRS's). The lateral system's horizontal spine.
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Flexible vs. rigid
Flexible (the wood, the untopped steel deck — the diaphragm's deflection >2x the vertical elements'; the tributary distribution), rigid (the concrete — the <2x; the stiffness-proportional), and semi-rigid (the modeled — the actual; the FEA's). The classification — the ASCE 7 12.3's — governing the analysis.
Diaphragm components
The shear (the in-plane — the nailing, the concrete's), the chords (the boundaries — the tension/compression; the collectors — the drag struts delivering to the walls/frames), the openings (the stairs, the shafts — the trimming; the sub-diaphragms), and the connections (the wall anchorage — the out-of-plane; the subdiaphragm's).
Design and detailing
The forces (the Fpx — the diaphragm's; the 0.2SDSIw minimum), the wood (the blocked vs. the unblocked — the capacities; the SDPWS), the concrete (the topping — the shear; the chords' steel), and the irregularities (the diaphragm discontinuity — the triggers). The diaphragm is the lateral's — the loads' collector and distributor.
The bottom line
Classify diaphragms per ASCE 7 12.3 (flexible = tributary, rigid = stiffness-proportional) — design in-plane shear, chords/collectors, and wall anchorage; detail openings with sub-diaphragms.
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