Response Spectrum Concept Questions and Answers

Answer: A

A well-detailed ductile frame is assigned a high value and a brittle system a low one, so the same spectrum yields different design forces for different systems. Zone, importance and soil are handled by separate terms.

Answer: B

The reduced force is justified only if the structure can actually sustain large inelastic deformations, which depends entirely on the detailing of reinforcement and connections. Taking the reduction without the detailing is unsafe.

Answer: C

Correct answer: The mass of the structure being designed

A response spectrum is a property of the ground motion together with the damping ratio, computed by exciting a family of oscillators of every period. Mass enters when the spectral ordinate is turned into a force, \( F = m\,S_a \), but not into the spectrum itself - and mass affects which ordinate you read only through its effect on the period.

Answer: D

Each mode is treated as a separate single-degree-of-freedom system, read against the spectrum at its own period, and the modal responses are then combined. No time history is needed, which is what makes the method economical.

Answer: A

It measures how well the mode shape matches the pattern of inertia forces from a uniform base acceleration, so a mode with a small factor contributes little however large its spectral ordinate. The first mode usually dominates in a regular building.

Answer: B

Adding the peaks directly would assume they coincide, which they do not, so the result would be unrealistically large. The square root of the sum of squares is the statistical estimate for modes with well separated periods.

Answer: C

Correct answer: have closely spaced periods

The square root of the sum of squares rule assumes the modal peaks are statistically independent, which fails when two periods are close together, because those modes respond in a correlated way. The complete quadratic combination rule includes cross terms weighted by a correlation coefficient that grows as the periods approach each other, and it reduces to the simpler rule when they are well separated.

Answer: D

Codes typically require modes summing to 90 per cent of the mass, because a mode carrying little mass contributes little to base shear. Truncating too early understates the response, particularly for irregular buildings.

Answer: A

The fundamental mode carries most of the participating mass in a regular structure, which is what justifies the equivalent static method for such buildings. Higher modes matter increasingly with height and irregularity.

Answer: B

The first two describe the method exactly. The third is its principal limitation: the analysis yields peak magnitudes with no information about when they occur or about signs, which is why combining results needs care.