Chemical anchors installed into existing concrete should be
Answer: C
Anchor capacity depends heavily on hole cleaning, resin mixing and curing, none of which can be seen once installed. A pull-out test on a proportion of the anchors is the only practical confirmation that the workmanship was adequate.
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A retrofit scheme should avoid
Answer: D
Stiffening one side, one storey or one direction can produce torsion or a soft storey where none existed. Checking the retrofitted building as a whole, rather than only the members being strengthened, is what catches it.
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An existing building that has cracked in a past earthquake should be
Answer: A
The pattern of cracking is evidence about how the building behaved and where it is deficient, and filling the cracks destroys that evidence. Assessment first, then repair and strengthening together, is the correct order.
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Retrofitting is usually cheaper than rebuilding because
Answer: B
Correct answer: the existing structure is reused
The foundations, the frame and the envelope already exist and are paid for, so the retrofit supplies only what is missing. Nothing about the code or the design effort is reduced - assessing an existing building is generally harder than designing a new one, since so much must be discovered rather than specified.
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A building whose drawings are unavailable requires
Answer: C
Correct answer: a survey of dimensions and reinforcement
Nothing can be assessed without knowing what is there, so the first task is measurement: member sizes, and the reinforcement found by cover meter and by opening up at selected points. Assuming a design rather than establishing it is the usual way a retrofit comes to be based on a building that does not exist.
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Ambient vibration measurement of an existing building gives
Answer: D
Recording the building's response to wind and traffic gives the periods and mode shapes directly, which is a valuable check on the analytical model. Repeating the measurement after a retrofit confirms the stiffness that was actually gained.
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The natural period of a building measured on site is often shorter than the analytical value because
Answer: A
Partitions, cladding and finishes all contribute stiffness that a bare frame model omits, and at the small amplitudes of ambient vibration they are uncracked and fully effective. Under strong shaking they crack and the period lengthens.
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A retrofit that relies on the infill masonry for strength is unwise because
Answer: B
A wall taken out during a refurbishment silently removes the strength the retrofit counted on, and no record of the assumption survives in the building. Where infill is relied on, it is tied into the frame and recorded as structural.
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The load path of a retrofitted building should be checked
Answer: C
Correct answer: from every floor down to the foundation
A retrofit is only as good as its weakest link, and adding a strong element does no good if the floor cannot deliver force to it or the foundation cannot receive it. The whole path must therefore be traced - which is why diaphragm and foundation checks so often determine the scope of the work.
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Diaphragm strengthening becomes necessary in a retrofit when
Answer: D
Correct answer: the floor cannot deliver force to the new elements
New shear walls or braces attract force only if the diaphragm can carry it to them, and an existing floor - timber, precast planks, or a slab now spanning further than it did - frequently cannot. Strengthening the diaphragm is then part of the retrofit rather than an addition to it.
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