Ground Motion Characteristics Questions and Answers

Answer: C

Correct answer: 0.10 s

Convert both to consistent units before dividing: \[ \frac{0.50\ \mathrm{m/s}}{0.5 \times 9.81\ \mathrm{m/s^2}} = \frac{0.50}{4.905} = 0.10\ \mathrm{s}. \] The ratio has the dimensions of time and is a rough indicator of the predominant period of the motion, so a high value points to a long-period record likely to be severe for flexible structures.

Answer: D

Correct answer: absolute value of acceleration over time

Cumulative absolute velocity integrates the modulus of the acceleration, so every excursion adds to the total regardless of sign and the result grows with both amplitude and duration. It was developed as a damage measure for nuclear plant, where it discriminates far better than peak acceleration between motions that damage equipment and those that merely shake it.

Answer: A

Correct answer: integrating the velocity spectrum over a period range

Housner spectral intensity is the area under the pseudo-velocity response spectrum between about 0.1 and 2.5 seconds, the range in which most structures have their fundamental periods. Because it integrates rather than taking a single ordinate, it measures the severity of the motion across the band that matters rather than at one period.

Answer: B

Both liquefaction of sand and progressive damage in members depend on how many large cycles are applied, not merely on the largest one. Duration and cycle count are therefore converted into an equivalent number of uniform cycles.

Answer: C

Correct answer: more damaging, because of the additional cycles

Damage accumulates cycle by cycle: each large inelastic excursion degrades stiffness and strength and consumes part of the available ductility. Four times the duration means roughly four times the number of damaging cycles at the same amplitude, so the longer record is substantially the more severe despite the identical peak.

Answer: D

Correct answer: 1, 2 and 3

All three are established site effects. Soft soil amplifies motion and shifts it to longer periods; ridge crests are shaken more strongly than the ground below them through topographic focusing; and a sediment-filled basin traps waves that reverberate within it and generate surface waves at its edges, so the shaking lasts considerably longer than at a rock site nearby.

Answer: A

The design spectrum states the level of demand the code requires, so records used in place of it must deliver comparable demand across the periods of interest. Matching magnitude, distance and site class as well makes the selection stronger still.

Answer: B

Magnitude fixes what is released at the source, but what arrives depends on how far the waves travelled, what they passed through and what lies beneath the site. Two equal earthquakes can produce quite different motions at the same place.

Answer: C

Correct answer: three-dimensional and irregular in time

A real ground motion has two horizontal components and a vertical one, all irregular, all changing in amplitude and frequency from instant to instant. The idealisations used in analysis - a single horizontal component, a smooth design spectrum - are convenient simplifications of this, and the difference is why recorded motions are used for verification.

Answer: D

Correct answer: inertia forces on the mass of the structure

An earthquake shakes the base, and the structure resists that motion through its own inertia; the force on each mass is that mass times its acceleration. This is why earthquake force is proportional to weight, and hence why lightweight construction is favoured in seismic regions - the exact reverse of the situation under gravity loading.