Base Isolation and Energy Dissipation Questions and Answers

Answer: C

Adding damping at the isolation plane reduces the design displacement and hence the size of the moat, which can be decisive on a tight site. The trade is that too much damping begins to couple the superstructure back into the response.

Answer: D

Both approaches leave the members as they are and reduce what is asked of them, which distinguishes them from conventional strengthening. Isolation does it by shifting the period and damping by taking energy out of the response.

Answer: A

Correct answer: low, because the superstructure is meant to stay elastic

The whole intent of isolation is that the isolation plane absorbs the demand and the structure above it remains essentially elastic, so the ductility implied by a high response reduction factor is neither expected nor available. Designing the superstructure with a factor appropriate to a ductile fixed-base frame would under-strength it for the forces it will actually see.

Answer: B

Rubber compounds and friction interfaces vary between batches and with temperature and ageing, so prototype and production tests are specified. The design displacement and force both depend directly on the properties those tests confirm.

Answer: C

A single strong pulse in the two to three second range is exactly what an isolated building is tuned to, and the bearings may be driven to or past their capacity. Larger gaps, more damping or displacement restraints are used to deal with it.

Answer: D

It is a last line of defence against the bearings being pushed past their capacity in an event larger than the design earthquake. Impact against it transmits a sharp force into the superstructure, so it is sized to be reached only rarely.

Answer: A

Bearings are made stiff vertically because they must carry the building's weight, so vertical acceleration passes almost unchanged. Equipment sensitive to vertical shaking has to be protected separately from the isolation system.

Answer: B

Correct answer: Building a rigid ramp across the moat

A rigid element bridging the moat short-circuits the isolation plane, tying the superstructure back to the ground and defeating the system entirely. The moat must be clear through the full design displacement, and every service, stair and ramp crossing it needs a flexible or sliding detail. Jacking points, protective paint and flexible couplings are all sound practice.

Answer: C

Correct answer: 5

\[ \frac{2.5}{0.5} = 5. \] A ratio of three or more is generally needed for isolation to be worth doing, since the spectral acceleration must fall far enough to justify the cost and the movement joints. At 2.5 seconds the building sits well beyond the peak of a typical response spectrum.

Answer: D

Correct answer: 2.5 s

The mass follows from the weight, and the period from mass and stiffness: \[ m = \frac{20{,}000}{9.81} = 2039\ \mathrm{kN\,s^2/m}, \qquad T = 2\pi\sqrt{\frac{2039}{12{,}800}} = 2\pi\sqrt{0.159} = 2.5\ \mathrm{s}. \] Dividing the weight by \( g \) is the step most often skipped; using 20,000 directly as the mass gives 7.9 s, which is not among the options.