Structural Engineering & Statics
where the load goes
Concept

Coupling beam — where it appears

A short, deep beam spanning the opening between two shear walls, which makes the pair act partly as one wall. It works in shear as much as in bending, and its yielding is where a coupled wall is meant to dissipate an earthquake's energy.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

Also named here as degree of coupling — the same set of essays touches all of them, so they are one junction rather than several.

Two cantilevers, or one wall, and the beams decide which. The deflected shape of a coupled pair of 6 m walls, drawn against the two limits it lies between. Release the coupling beams entirely and the pair is two independent cantilevers, deflecting 111 mm. Make them rigid and it is one composite wall of the full width, deflecting 16 mm — 6.8 times stiffer, because the lever arm between the wall centroids is 8.40 m and everything inside either wall is smaller than that. Real beams of 600 × 350 mm over a 2.4 m opening land at 23 mm and carry 63% of the base overturning as an axial couple rather than as wall bending. The degree of coupling never reaches one, because a beam of finite depth cannot suppress the walls' curvature entirely.

Two walls that agreed to be one

A pair of shear walls with a row of doors between them is the commonest lateral system there is, and it has two readings that differ by a factor of seven. What decides which one applies is a beam 600 mm deep over a 2.4 m opening — and most of the overturning ends up as an axial couple that no bending diagram contains.

internal-forces · Wall coupling The shear term cuts the beam's stiffness, not its share. For 20 storeys of two 6.0 m walls 0.35 m thick either side of a 2.4 m opening, coupling beams 0.35 m wide, 60 kN at every floor, against the coupling beams' depth: each beam's stiffness with its shear deformation counted over its stiffness in bending alone, the degree of coupling likewise, and the largest beam's shear likewise. At a depth of 0.6 m (span over depth 4.0) the stiffness is 0.85 of the bending figure, the coupling 0.977 and the shear 0.961; at 2 m (1.2), 0.33, 0.973 and 0.937. Where the shear term is large the beam is stiff enough that its stiffness has stopped deciding anything.

The coupling beam asked for shear

The coupling beams of a pair of shear walls are the stubbiest members in a building, and the shear term in their deflection is largest exactly there: a beam two metres deep over a 2.4 m opening is a third as stiff as bending alone says. The walls hardly notice. The pair's degree of coupling moves by under 3%, the beams' shears by under 7%, because a beam that stiff has stopped deciding how much of the overturning the walls carry as a couple. What decides it is the shear the beams can give. A 600 mm beam limited to 2 N/mm² starts to yield at 1.9 times the storey force; past the last yield the pair is two cantilevers again, and the beams supply rotation instead.

deflection · Shear deflection

Named alongside it

The objects these essays reach for when they reach for this one.

Degree of couplingDuctilityLateral systemOverturningShear wallAxial forceCapacity designComposite actionDriftInternal forcesLever armPlastic hinge

All concepts