Damage avoidance

Given that the best method of damage avoidance, seismic isolation, is unsuitable in many situations, researchers are investigating other methods of avoiding damage - particularly to structural members. To a large degree, buckling-restrained braces achieve this goal but chunky diagonal braces are often unacceptable architecturally. Researchers have proposed several methods whereby damaged structural fuses are easily replaceable but none have been widely adopted.9

Precast concrete industry-sponsored research in the US has developed damage avoidance systems for precast concrete moment frames and shear walls (Figs 14.21 and I4.22).10 The two essential elements of these systems are an internal centring spring action and energy dissipation by either easily replaceable fuses or fuses that remain undamaged. Unbonded (ungrouted) post-tensioned tendons, which allow relative rotational movement between components like beams and columns, yet spring them back to their original positions, are central to both systems. The spring-back behaviour is just like that of children's toys which consist of wooden pieces joined by internal lengths of elastic thread. After distorting the toy, the elastic thread snaps the toy back to its original geometry. Energy dissipation is provided by mild steel bars or other ductile devices that yield when deformed in shear, as in vertical gaps up shear walls, or elongate in tension and squash in compression where placed within beams of moment frames. While the behaviour of these damage-free vertical systems themselves are excellent,

▲ 14.23 Post-tensioned unbonded tendons provide a damage avoidance mechanism in most of the moment frames of this 39-storey apartment and retail building, San Francisco.

Roof slab

Bearings/ dampers

Tension tie

Damper

Three building elevations

Tension tie

Damper

Three building elevations

▲ 14.24 Theoretical examples of where discontinuities and dampers are inserted into buildings with the intention of reducing the overall seismic response.

structural engineers need to pay close attention to how floor slabs affect their seismic performance. While it is desirable from the perspective of damage avoidance to separate slabs from the beams of moment frames this diminishes the essential role that floor slabs play as floor diaphragms to effectively tie a building together at each floor level.

From an architectural perspective these precast concrete structural systems are very similar in form and size to monolithic reinforced concrete construction (Fig. 14.23). Both precast and cast-in-place concrete systems experience similar seismic drifts and accelerations causing non-structural damage and structural damage to the specially designed precast concrete members during a design-level earthquake. This is expected to be minor and easily repairable.

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