The most frequently overlooked consequence of adding shear walls in a retrofit is
Answer: A
A wall collects a large share of the storey shear and delivers a large overturning moment to its base, and the existing footings were never designed for it. New pile caps or raft extensions are commonly the largest single item of the work.
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Steel bracing added to an existing concrete frame has the advantage that it
Answer: B
A braced bay is light, can be installed from outside in some cases, and interferes less with the use of the building than a solid wall. The connections to the existing frame and the demand on the foundations remain the difficult parts.
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An external buttress or exoskeleton is chosen as a retrofit when
Answer: C
Correct answer: the building must stay in use during the works
An exoskeleton is built outside the envelope, so the interior is scarcely touched and the occupants remain in place. That is its principal justification, and it is bought at the cost of appearance and of new foundations outside the existing footprint.
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Reducing the mass of an existing building is an effective retrofit because
Answer: D
Removing heavy floor finishes, replacing masonry partitions with light ones, or taking off an upper storey all reduce the demand in direct proportion. It is one of the few measures that improves matters without adding anything new to the structure.
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Removing the topmost storey of an existing building as a retrofit measure
Answer: A
Mass removed from the top has the greatest effect on base shear and on overturning, because of the height at which the code applies the force. It is drastic and reduces the lettable area, so it is used where nothing gentler will serve.
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Base isolation as a retrofit measure is most often chosen for
Answer: B
Correct answer: heritage buildings and essential facilities
Base isolation reduces the demand on the superstructure rather than strengthening it, so the fabric above can be left alone - which is what a heritage building needs. And it keeps the structure close to elastic, so an essential facility remains usable after the earthquake rather than merely standing.
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Supplemental dampers are attractive in retrofit because they
Answer: C
Cutting drift by taking energy out of the response can bring the existing members within their capacity, so the intervention is confined to a few bays. That makes the work far less disruptive than jacketing columns throughout an occupied building.
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A retrofit that stiffens only one side of a building in plan risks
Answer: D
Moving the centre of stiffness away from the centre of mass produces twist under shaking and overloads the far corners. Retrofit schemes are therefore checked for the irregularity they introduce as well as the one they remove.
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A retrofit that stiffens only the upper storeys of a building risks
Answer: A
Correct answer: creating a soft storey lower down
Stiffening part of a building concentrates the deformation in whatever is left unstiffened, so the storey below the strengthened region takes the drift the storeys above no longer do. A retrofit must therefore be checked over the whole height - strengthening a part can make the whole worse.
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Concrete jacketing of an existing column consists of
Answer: B
The new cage provides confinement, additional longitudinal steel and additional section, so both strength and ductility improve. Continuity of the new bars through the floors is the detail that decides whether the jacket works.
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