Response Spectrum Concept Questions and Answers

Q: For a site classified as soft soil rather than rock, the corner period of the plateau generally

Correct answer: moves to a longer periodA soft deposit filters the motion, amplifying the longer-period components and attenuating the short ones, so the peak of the spectrum shifts to longer periods and the plateau extends further out. The practical consequence is that flexible buildings on soft soil attract far higher coefficients than the same buildings on rock - the effect that devastated medium-rise buildings in Mexico City in 1985.

Q: The area under an acceleration time-history, integrated once, gives

Correct answer: the velocity time-historyIntegrating acceleration with respect to time gives velocity, and integrating again gives displacement. In practice both integrations require baseline correction and filtering, because a small constant error in the acceleration record integrates into a drift that grows without bound - which is why raw accelerograms are processed before displacements are derived from them.

91.

For a site classified as soft soil rather than rock, the corner period of the plateau generally

A.disappears entirely
B.is unaffected by soil
C.moves to a longer period
D.moves to a shorter period

Answer: C

Correct answer: moves to a longer period

A soft deposit filters the motion, amplifying the longer-period components and attenuating the short ones, so the peak of the spectrum shifts to longer periods and the plateau extends further out. The practical consequence is that flexible buildings on soft soil attract far higher coefficients than the same buildings on rock - the effect that devastated medium-rise buildings in Mexico City in 1985.

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92.

A structure founded on soft soil with a site period of 1.0 s should preferably be designed with a natural period

A.as close to 1.0 s as possible
B.exactly equal to 1.0 s
C.of exactly half that value in every case
D.well away from 1.0 s

Answer: D

Matching the site period invites resonance and the largest demand the site can deliver, so the structural period is deliberately shifted away. Whether it is moved up or down depends on what is practical for the structural system.

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93.

True or false: a response spectrum is a property of the ground motion, not of any one structure.

A.True, though it is computed using a family of oscillators
B.False, since it is derived for one specific building
C.True, because structural properties never enter the calculation
D.False, since a spectrum describes a mode shape

Answer: A

Correct answer: True, though it is computed using a family of oscillators

The spectrum characterises the ground motion, which is why one curve serves every structure at the site. Oscillators do enter the computation - the ordinate at each period is the peak response of an oscillator of that period - but they are a family sweeping all periods rather than any particular building, so no structure’s properties are embedded in the result.

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94.

The area under an acceleration time-history, integrated once, gives

A.the peak ground acceleration
B.the velocity time-history
C.the displacement time-history
D.the response spectrum

Answer: B

Correct answer: the velocity time-history

Integrating acceleration with respect to time gives velocity, and integrating again gives displacement. In practice both integrations require baseline correction and filtering, because a small constant error in the acceleration record integrates into a drift that grows without bound - which is why raw accelerograms are processed before displacements are derived from them.

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95.

An accelerograph is required rather than a seismograph for engineering design because it

A.measures the magnitude directly
B.records only the vertical component
C.records strong motion without going off scale
D.is more sensitive to distant small events

Answer: C

Correct answer: records strong motion without going off scale

A seismograph is built to detect very small distant events and saturates in strong shaking, precisely when the engineer needs the record. An accelerograph has a much lower sensitivity and a wide dynamic range, and it triggers on strong motion, so it captures the large amplitudes intact. Every response spectrum used in design comes from such a record.

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