Dampers

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Dampers perform the same function in a building as shock absorbers do in a motor vehicle. They absorb vibration energy in the system, reducing acceleration and movement to provide a smoother ride. Dampers are also known generically as energy dissipaters. They transform dynamic energy into heat, either reducing horizontal drifts in buildings (and therefore damage), or allowing designers to specify more slender structural members. The role of dampers in seismic isolation where they have most often been used to date has already been mentioned. But dampers can also be placed within superstructure frameworks; perhaps as diagonal members within a new or even an existing moment frame to absorb energy and reduce inter-storey drifts (Fig. 14.15).

Cylinder - containing orifices \

Piston

Chamber Section through a viscous damper

Piston Cylinder Lead

TWWW-

Section through a lead-extrusion damper

Visco-elastic material

Visco-elastic material

Centre plate

Visco-elastic damper

T - section

Centre plate

Visco-elastic damper

▲ 14.17 Dampers placed at the ends of tubular steel braces. Office building, San Francisco.

(Reproduced with permission from David Friedman, R. Cranfield, photographer).

Direction of movemen

Rod pushes and pulls cantilever

Mild steel cantilever

Direction of movemen

Rod pushes and pulls cantilever

Mild steel cantilever

Steel cantilever damper

Steel cantilever damper

▲ 14.16 Four types of damping devices.

▲ 14.16 Four types of damping devices.

▲ 14.18 A lead-extrusion damper bolted to the foundations (left) and to the isolated superstructure (right) Police Station, Wellington.

▲ 14.17 Dampers placed at the ends of tubular steel braces. Office building, San Francisco.

(Reproduced with permission from David Friedman, R. Cranfield, photographer).

The shapes, materials and methods for absorbing energy vary greatly among dampers (Fig. 14.16). In a classical viscous damper, like those sometimes found in college physics labs, a piston with an orifice is forced through a fluid-filled cylinder (Fig. 14.17). The resistance of the damper is proportional to the velocity of the piston. In a lead-extrusion damper, the cylinder is filled with lead instead of fluid. The movement of its bulb is minimal until the resistance of the lead peaks. Then the pressure of the steel bulb causes the lead to deform plastically, allowing the bulb to move within the enclosing steel cylinder (Fig. 14.18). A thin film of visco-elastic material, which absorbs energy when subject to shearing actions, is the basis of another type of damper. If the visco-elastic material is removed and the layers of metal are clamped by bolts in slotted holes, one has the beginnings of a friction damper. The ability of mild steel, whether deformed in bending or torsion leads to yet another set of damping devices, possibly the most simple. Dampers utilizing shape memory alloys that absorb energy, yet return to their original shape after being deformed, are currently under development but are yet to be installed in a seismic resisting system.

Piston head

Fluid rod

Encasing mortar

SI Core

Steel tube

Unbonding layer over steel core

Yielding steel core

Brace

▲ 14.20 Buckling-restrained braces improve the seismic resistance of an existing building. Berkeley, California.

▲ 14.19 Schematic details of a buckling-restrained brace.

A buckling-restrained brace is another structural device able to dissipate energy (Figs 14.19 and 14.20). It consists of an inner steel member capable of resisting tension or compression forces that also functions as a structural fuse - yielding and absorbing energy. An outer tube and a grout-filled gap between outer and inner members prevent the inner member from buckling, and ensure a highly ductile and dependable performance. Depending on the strength of a brace it can be considered either a damper, or if very strong, as a diagonal member of a braced frame. The design of buckling-restrained braces prevents any chance of their buckling and minimizes their damage when deformed into the plastic range. They are preferable to concentrically braced steel frames whose diagonals are prone to buckling. They are also an alternative to eccentrically braced frames whose damaged fuse regions would be difficult to repair after a major quake.

The dampers discussed above are classed as passive, in contrast to active systems which rely upon computer-controlled hydraulic rams or other devices. Conceptually, active systems represent an exciting advance over passive devices. The idea of structural members or devices responding to signals from sensors so as to reduce or even eliminate seismic energy input into a building is most attractive, particular to those people drawn to high-tech solutions. Unfortunately, although conceptually simple, the practical application of active control is still some years away and even then is likely to be very expensive.

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