What Is a Chord in Structural Diaphragm Design?
In diaphragm design, the chord is the edge member that resists the bending tension and compression when the floor or roof acts as a deep beam spanning between shear walls — analogous to the flanges of a steel beam. Chords are typically the perimeter beams, edge angles, or continuous wall top plates. Without adequate chords, the diaphragm cannot deliver lateral forces to the vertical system.
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How diaphragm action works
Picture the roof as a beam laid flat: wind pushes on the long side, the roof spans horizontally to the shear walls at each end. The 'flanges' of this flat beam are the chords at the two long edges — one in tension, one in compression. The 'web' is the sheathing or deck with its nailing or weld pattern resisting the shear. The engineer sizes chords for the computed tension/compression and details the splices to actually develop those forces.
Common chord elements
In wood construction, chords are often continuous rim joists, wall top plates with steel strap splices, or dedicated engineered lumber members. In steel buildings, the perimeter beams or angles serve as chords. In concrete, the slab edge thickening or perimeter beams do the job. The critical detail is continuity: a chord interrupted by a stair opening or a missing splice is a broken flange — the diaphragm analysis assumes it is whole.
What plan checkers look for
Chord force calculations with the load path traced to the shear walls, splice details showing how tension develops across joints, and collector (drag strut) details where diaphragm shear concentrates into the vertical elements. Openings in diaphragms (stairs, shafts, atriums) get sub-diaphragm analysis with their own chords around the opening. Missing chord continuity at openings is a top diaphragm correction.
The bottom line
Diaphragm chords are the edge members resisting bending tension/compression as the floor spans between shear walls. Detail them continuous with real splices — especially around openings.
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