Diaphragm Unit Shear Force
Diaphragm Unit Shear Force
A roof or floor diaphragm acts like a deep, horizontal beam, collecting lateral load (from wind or seismic forces) across its plan area and delivering it to the vertical lateral-force-resisting elements — shear walls or braced frames — at its edges. Just like a vertical shear wall, the diaphragm has a shear capacity per unit length that its sheathing and fastening must be designed to develop.Diaphragm shear capacity is a fastener-and-sheathing problem, not a raw material strength problem — it is governed by nail spacing, panel thickness, and blocking at panel edges, so the diaphragm design output (unit shear) needs to be matched against a nailing schedule or a manufacturer-rated shear capacity rather than a simple stress limit; using the wrong schedule for the calculated unit shear is a common construction-document error.
The unit shear along a diaphragm is v = V/b_d, the total shear force divided by the diaphragm depth (the dimension perpendicular to the direction of load). where V is the total shear force delivered into the diaphragm and b_d is the diaphragm depth (the plan dimension parallel to the direction of the shear walls receiving the load), giving the resulting unit shear v.
Inputs
V := 150 kN=150000 N
b_d := 20 m=20 m
Dividing the total shear by the diaphragm depth spreads the force evenly along the diaphragm's length to get a shear-per-unit-length value that can be matched against a sheathing and fastening schedule.
v := V/b_d=7500 kg/s^2
Results
A unit shear of 7.5 kN/m here is a moderate value that a well-nailed plywood or OSB diaphragm can typically develop without needing blocked, heavily-nailed panel edges throughout. If the unit shear exceeds what the diaphragm's sheathing thickness and standard nailing can provide, the usual fixes are tighter nail spacing, adding blocking at panel edges, or increasing sheathing thickness — in roughly that order of cost. This simplified average-shear approach assumes a uniformly loaded diaphragm; real diaphragms often have a parabolic shear distribution and openings that concentrate shear locally, both of which need separate checks.
Assumptions & limitations
- Uniform shear distribution assumed along the diaphragm depth (average value, not peak local shear).
- Diaphragm treated as flexible or rigid per the analysis method assumed elsewhere — not distinguished in this simplified check.
- Openings, re-entrant corners, and collector/drag elements are not accounted for in this basic unit-shear value.
- Actual capacity comparison requires matching this demand against a sheathing/fastening schedule, not shown here.