Base isolation protects a building by
Answer: A
A flexible layer between the foundation and the structure lets the ground move while the building above stays comparatively still. The approach reduces the demand rather than increasing the capacity, which is what distinguishes it from conventional strengthening.
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Base isolation reduces the seismic force on a structure principally by
Answer: B
Shifting the period well beyond the plateau of the spectrum moves the structure to where the ordinate is small, and the accompanying rise in displacement is taken up in the isolation layer. Added damping in the bearings helps, but the period shift does most of the work.
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The typical isolated period of a base-isolated building is of the order of
Answer: C
That range lies well down the descending branch of a design spectrum, where the acceleration ordinate is a fraction of its peak value. It is also long enough to be several times the fixed-base period of the building, which is what makes the isolation effective.
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For base isolation to be effective, the isolated period should be
Answer: D
Unless the isolated period is far removed from the fixed-base one, the superstructure keeps participating in the response and the benefit is small. A ratio of three or more is the usual target, and it is easiest to achieve with a stiff building.
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Base isolation is most attractive for a building that is
Answer: A
A stiff building has a short fixed-base period, so the shift the isolators produce is large, and protecting equipment or heritage fabric is what usually justifies the cost. Tall slender buildings and very soft sites are the two cases where isolation works poorly.
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Base isolation performs poorly on very soft deep soil because
Answer: B
Soft deposits shift the energy of the ground motion towards longer periods, so moving the structure to two or three seconds can bring it closer to the site period rather than away from it. The strategy depends on the shape of the spectrum at that site, not on the isolators alone.
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Base isolation is difficult to apply to a tall slender building because
Answer: C
Elastomeric bearings take compression well but tension poorly, and a slender building rocking on its isolation plane can put its edge bearings into uplift. Tall buildings are also already flexible, so there is less period shift to be gained.
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Compared with an equivalent fixed-base building, a base-isolated building generally has
Answer: D
The superstructure moves almost as a rigid body on top of the isolation layer, so the storeys deform very little relative to one another and the contents feel much less shaking. Nearly all of the displacement is concentrated in the bearings instead.
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The large displacement that occurs across an isolation layer must be accommodated by
Answer: A
Displacements of a few hundred millimetres are normal, and every service entering the building, every stair and every surrounding pavement has to allow for them. Failing to maintain that gap is the most common way an isolation system is defeated in service.
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Flexible connections are required for the services entering a base-isolated building because
Answer: B
Pipes and cables crossing the isolation plane are stretched by the full design displacement and would shear off if rigidly connected. Flexible loops and sliding joints are provided at that plane, and their inspection is part of maintaining the system.
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