The response spectrum method treats a multi-storey building as
Answer: C
Each mode behaves like an independent oscillator with its own period and participating mass, which is what allows a spectrum built for single-degree systems to be applied to a real frame. The modal results are then combined.
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Seismic weight of a building for design purposes includes
Answer: D
Correct answer: dead load and a part of the imposed load
The seismic weight is the mass that will actually be accelerated with the structure. All of the dead load qualifies; only a fraction of the imposed load does, since furniture and occupants are not rigidly attached and much of the nominal live load is not present at any one time. Wind is a load rather than a mass and contributes nothing.
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Earthquake force on a structure increases with its
Answer: A
Correct answer: mass
Inertia force is mass times acceleration, so a heavier structure attracts a proportionally larger earthquake force - which is the exact opposite of the situation under gravity loading, where extra mass is merely extra load to carry. This is why lightweight construction is favoured in seismic regions, and why a heavy water tank at roof level is so unwelcome.
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Which one of the following is NOT needed to read a design spectrum?
Answer: B
Cost plays no part in determining the demand on a structure. The period fixes where on the curve the reading is taken, the soil type selects which curve applies, and the zone scales the whole of it, so those three between them settle the ordinate.
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The statement a response spectrum applies to any damping ratio is
Answer: C
Correct answer: false, since each curve is drawn for one damping value
Damping enters the computation of every ordinate, so a spectrum belongs to the damping value it was computed for. Spectra are therefore published as families of curves, and a code design spectrum specifies its damping - usually five per cent - together with factors for converting to other values when the structure warrants it.
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Assertion (A): Base isolation reduces the forces transmitted to a structure.
Reason (R): It lengthens the natural period into a region of lower spectral acceleration.
Select the correct option.
Answer: D
Shifting the period well beyond the plateau moves the structure to where the spectrum gives small accelerations, which is exactly why the forces fall. The accompanying rise in displacement is accommodated in the isolation layer.
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Consider the following statements about the design spectrum in a code.
1. It is smoothed rather than jagged.
2. It varies with soil type.
3. It is drawn for a stated damping ratio.
Which of the statements are correct?
Answer: A
Correct answer: 1, 2 and 3
All three are correct and each reflects a deliberate decision. Smoothing accommodates uncertainty in the structure’s period; the variation with soil type reflects the amplification and period-shifting that soft deposits produce; and the stated damping ratio is necessary because the ordinates depend on it. A code spectrum without all three specified would be unusable.
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The equivalent static method is acceptable in place of a dynamic analysis for buildings that are
Answer: B
Where one mode dominates and the structure is regular, distributing a base shear over the height is adequate. Irregularity or great height brings higher modes and torsion into play, and codes then require a dynamic analysis.
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A structure with a period falling on the plateau of the design spectrum experiences
Answer: C
Correct answer: the maximum acceleration the spectrum offers
The plateau is the flat top of the spectrum, so a structure whose period falls within it attracts the largest coefficient available at that site. Periods on either side attract less - shorter ones tending towards the peak ground acceleration, longer ones falling away down the descending branch. Stiffening a flexible building can therefore increase its design force by moving it onto the plateau.
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Two identical buildings stand on rock and on soft soil at the same distance from a source. The one on soft soil will generally experience
Answer: D
The soft layer amplifies the motion and shifts its energy to longer periods, which is why codes publish a separate and generally more demanding spectrum for soft sites. Distance and magnitude alone do not settle the demand.
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