Ground Motion Characteristics Questions and Answers

Q: Peak ground acceleration is usually expressed as a fraction of

Correct answer: the acceleration due to gravityQuoting acceleration as a fraction of \( g \) is more than a convenience: because the inertia force is mass times acceleration and the weight is mass times \( g \), a peak of 0.3 g means a horizontal force of roughly three tenths of the structure’s weight. The figure can therefore be compared directly with the lateral strength available.

61.

Peak ground acceleration is usually expressed as a fraction of

A.the acceleration due to gravity
B.the velocity of the shear wave
C.the height of the structure concerned
D.the duration of the strong shaking

Answer: A

Correct answer: the acceleration due to gravity

Quoting acceleration as a fraction of \( g \) is more than a convenience: because the inertia force is mass times acceleration and the weight is mass times \( g \), a peak of 0.3 g means a horizontal force of roughly three tenths of the structure’s weight. The figure can therefore be compared directly with the lateral strength available.

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62.

The ground motion used for design is normally taken at

A.the top of the deepest rock layer only
B.the free-field surface or the foundation level
C.the roof of the completed structure
D.the focus of the expected earthquake

Answer: B

Correct answer: the free-field surface or the foundation level

Design motion is specified at the free-field ground surface, or at foundation level, because that is where the structure is excited. It is not the rock motion, which must be propagated up through the soil column first, and it is certainly not the roof motion, which is the structure’s response rather than the input to it.

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63.

Which one of the following is NOT normally used as a ground motion parameter?

A.Bracketed duration of strong shaking
B.Arias intensity of the record
C.The number of stations that recorded the event
D.Peak ground velocity

Answer: C

Correct answer: The number of stations that recorded the event

How many instruments happened to be operating is a property of the network, not of the motion, and it tells you nothing about what the ground did at a site. The other three are all genuine parameters: bracketed duration measures how long, Arias intensity how much energy, and peak ground velocity how large the intermediate-frequency motion was.

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64.

An accelerogram recorded on rock is used with a site response analysis in order to

A.estimate the magnitude of the earthquake
B.locate the epicentre more accurately
C.determine the duration of the aftershocks
D.obtain the motion expected at the soil surface

Answer: D

Correct answer: obtain the motion expected at the soil surface

A site response analysis takes the rock motion as input at the base of the soil column and propagates it upward through layers whose stiffness and damping vary with strain, producing the motion at the surface. That surface motion may be several times the rock motion and at a longer predominant period, and it is what the structure will actually experience.

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Ground Motion Characteristics Questions and Answers

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