Ground Motion Characteristics Questions and Answers

Answer: C

A neighbouring building does not change the free-field motion in any way a code accounts for, which is why free-field stations are sited clear of structures. The other three are the standard source, path and site terms.

Answer: D

Correct answer: 1-Q, 2-R, 3-P

Three parameters measuring three different things. Peak ground acceleration is the largest single ordinate of the record. Bracketed duration is the time between the first and last crossings of a threshold. And Arias intensity, being the integral of the squared acceleration, is a measure of the total energy the record carries.

Answer: A

Records are chosen and scaled so that their spectra match the design spectrum over the range of periods that matter to the structure. Matching magnitude, distance and site class as well makes the selection more defensible.

Answer: B

Correct answer: multiplying its ordinates by a constant factor

Scaling multiplies every acceleration ordinate by the same factor so that the record matches a target peak or a target spectral ordinate, leaving the frequency content and the duration untouched. That is its virtue and its limitation: the shape of the spectrum is unchanged, so a record whose shape is wrong for the site cannot be made right by scaling.

Answer: C

Correct answer: match the target spectrum closely at every period

An artificial record is generated so that its response spectrum matches the design spectrum across the whole period range, which removes the scatter that real records show and makes the analysis reproducible. The cost is that such a record is not a real earthquake: its energy content is generally too high and its phasing artificial, so real records are preferred where enough are available.

Answer: D

Two motions with the same peak can have very different frequency content and duration and produce quite different responses, so codes require a suite of records and use the average or the envelope. Records are certainly used in design; one is simply not enough.

Answer: A

Correct answer: long structures such as bridges and pipelines

A structure whose supports are hundreds of metres apart does not see the same motion at every support: the wave arrives at different times, is incoherent over that distance, and may cross different ground conditions. The resulting differential displacements impose forces that a single-support analysis misses entirely, which is why multi-support excitation is used for long bridges and buried pipelines.

Answer: B

Correct answer: 2, 3, 1

The motion is created at the fault by the rupture, modified as it travels through the crust by geometric spreading and attenuation, and modified again by the soil column immediately beneath the site, which may amplify it strongly and shift its predominant period. Each stage is modelled separately in hazard analysis, and the site term is usually the largest single source of variability.

Answer: C

Correct answer: better represents the motion that affects structures

The recorded peak may be a single high-frequency spike lasting a few hundredths of a second, too brief to move any real structure. Effective peak acceleration is derived instead from the spectral ordinates over the period range that matters, so it correlates far better with damage - which is what a code parameter needs to do.

Answer: D

Correct answer: the sustained level of shaking over the record

Root mean square acceleration averages over the whole strong motion, so it reflects the sustained intensity rather than the largest instant. It is therefore a better companion to duration than the peak, and it appears in energy-based measures such as Arias intensity, which is essentially the mean square acceleration multiplied by the duration.