Base Isolation and Energy Dissipation Questions and Answers

Answer: A

The loop area gives the energy dissipated per cycle, which is converted to an equivalent viscous damping ratio so that a linear analysis can be used. Values of ten to thirty per cent are typical, far above the five per cent assumed for ordinary concrete construction.

Answer: B

Using the secant value lets the nonlinear bearing be represented by a single spring in an equivalent linear analysis. Because the secant stiffness falls as the displacement grows, the analysis has to be iterated until stiffness and displacement are consistent.

Answer: C

An assumed displacement gives a stiffness and damping, which give a period and hence a new displacement, and the cycle is repeated until it closes. Two or three iterations usually suffice, and convergence is rapid because the dependence is gentle.

Answer: D

Damping reduces the spectral ordinate, and at long periods that ordinate is essentially a displacement. Trading damping against displacement is the central design decision in sizing an isolation system and its surrounding gap.

Answer: A

Very heavy damping transmits force across the isolation plane in proportion to velocity and excites the higher modes of the superstructure. The floor accelerations that isolation was meant to reduce can then rise again, so damping is optimised rather than maximised.

Answer: B

A building that swayed noticeably in the wind would be unserviceable, so the system needs an initial stiffness or a friction threshold that holds it still under everyday loading. The lead core and the friction interface both provide that threshold as a by-product of their damping function.

Answer: C

Rubber ages and devices can be damaged, so an accessible isolation plane with provision for jacking is part of the design. Maintenance and periodic inspection are as much a part of the system as the bearings themselves.

Answer: D

Placing the bearings at the bearing shelf isolates the massive deck from the substructure and distributes the force between piers of differing stiffness. The concept is the same as for a building, but the displacement is accommodated in expansion joints rather than in a moat.

Answer: A

Devices installed in braces or between storeys take a share of the energy that would otherwise have to be absorbed by the frame yielding. Drift and member demand both fall, which makes the approach attractive for retrofitting an existing building.

Answer: B

Forcing fluid through an orifice gives a force proportional to velocity raised to some power, so the damper peaks when the structure is passing through its neutral position. Because that is when the elastic force is smallest, the damper adds little to the peak member force.