What Is Diaphragm Chord and Collector Design?

Chords

Under lateral load, the diaphragm bends like a beam: the chords carry the tension and compression couple (chord force = diaphragm moment / diaphragm depth). In wood construction, the chords are often the wall top plates with spliced connections (the splice detailing — nails, straps, bolts — must develop the full chord force); in concrete, the slab edge thickening or perimeter beams; in steel, the edge beams. Chord connections across joints and openings need the full force path — a chord that stops at a control joint is a failed diaphragm.

Collectors (drag struts)

Collectors gather shear from the diaphragm and deliver it to the shear walls or frames — critical where the lateral elements do not align with the diaphragm's edges (the interior shear wall fed by a collector, the offset braced frame). The collector force accumulates along its length (every foot of diaphragm adds shear) — the maximum at the lateral element can surprise engineers who check only the average. Connections at both ends (diaphragm-to-collector, collector-to-wall) must develop the full force — the 2015+ code's amplified seismic requirements target these connections.

Design essentials

ASCE 7 requires diaphragm design for the amplified seismic forces (the diaphragm design force, not just the story shear — diaphragms see higher accelerations); collectors in high seismic categories need the overstrength factor (the collector must survive the lateral system's full yielded capacity). The load path must be continuous and verifiable on the drawings — every chord splice, every collector connection, detailed and calculable. The special inspector verifies the connections the engineer detailed.

The bottom line

Chords resist diaphragm bending (tension/compression couple); collectors drag shear into the lateral elements — design both for amplified seismic forces with continuous, detailed load paths.

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