Shear wall ductility

Assume an architect has chosen shear walls as the seismic resisting system for at least one orthogonal direction of a building. If short rather than long shear walls are preferred - perhaps to ease architectural planning constraints - ductile walls with their inherently lower design forces and shorter lengths are recommended.

The question then arises: How to design a ductile shear wall? For an answer we return to Chapter 3. The Capacity Design approach must be applied. In summary, a ductile overload mechanism is identified and all possible brittle failure modes are suppressed. Table 5.3 compares

▼ 5.3 Ductility of shear walls constructed of various materials

Shear wall Typical degree How ductility is achieved material of ductility

Steel Medium to high Yielding diagonal tension zones between opposite corners of the steel panels absorb seismic energy. This ductile elongation occurs before damage to the many bolts connecting the steel panels to surrounding frame members.

Once bending moments exceed the bending strength of a wall a structural fuse or plastic hinge forms at its base. Vertical reinforcement in the chords at each end of the wall yields in tension and compression. The wall shear strength and the strength of its foundations are designed stronger to preclude premature failure. In the structural fuse region, typically the lesser of a storey-height or the wall length above foundation level door or window penetrations are inadvisable due to high stresses and the need to confine the concrete with horizontal ties (Fig. 5.19).

A similar but less ductile performance as compared to a reinforced concrete wall can be achieved. It is impractical to place confining steel into narrow masonry units but short and thin steel plates inserted into the mortar joints at the ends of a wall prevent premature crushing of masonry units.

Diagonal cracking within the masonry panels may be followed by damage to the tops of columns from the diagonal compression struts. The presence of reinforcing steel ensures some ductility.

Unreinforced None No ductility is expected.

masonry

Wood Medium to high The chosen ductile mechanism is usually the bending deformations in the hundreds of nails between a plywood web and the supporting framing. Wall chords, their connections to the foundations, the foundations and the plywood itself are designed not to fail before the nails absorb earthquake energy by yielding as they bend to-and-fro (Fig. 5.20).

Gypsum plasterboard on wood framing is not usually rigorously designed for ductility. During a design-level earthquake, nails distort and damage the plaster in their vicinity and the holding-down fixings between the wall chords and foundations might be damaged. A low to medium level of ductility is achievable.

Reinforced High concrete

Reinforced Medium to low masonry

Confined Low masonry

Shear wall elevation

▲ 5.19 Structural fuse region at the base of a ductile shear wall. The vertical reinforcing yields and absorbs earthquake energy.

▲ 5.20 The plywood web of a wood shear wall is fixed to framing with closely-spaced nails that transfer forces between sheets and yield in bending to act as structural fuses. School building, Wellington.

Shear wall elevation

▲ 5.19 Structural fuse region at the base of a ductile shear wall. The vertical reinforcing yields and absorbs earthquake energy.

Coupling beam that functions as a structural fuse

Coupling beam that functions as a structural fuse

Coupled shear wall

▲ 5.21 Comparison between a shear wall and a coupled shear wall.

Shear wall

Deep foundation beam

Coupled shear wall

▲ 5.21 Comparison between a shear wall and a coupled shear wall.

▲ 5.20 The plywood web of a wood shear wall is fixed to framing with closely-spaced nails that transfer forces between sheets and yield in bending to act as structural fuses. School building, Wellington.

the ductility of shear walls with different materiality and notes how ductility is achieved. Unlike moment frames discussed in a following section the ductile design requirements of shear walls, apart from restrictions on wall penetrations in the structural fuse region, have few architectural implications.

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