**Response-spectrum analysis** (RSA) is a linear-dynamic st=
atistical analysis method which measures the contribution from each natural=
mode of vibration to indicate t=
he likely maximum seismic response of an essentially elastic structure. Res=
ponse-spectrum analysis provides insight into dynamic behavior by measuring=
pseudo-spectral acceleration, velocity, or displacement as a function of s=
tructural period for a given time history and level of damping. It is practical to envelope response spectra such =
that a smooth curve represents the peak response for each realization of st=
ructural period.

Response-spectrum analysis is useful for design decision-making because = it relates structural type-selection to dynamic performance. Structures of = shorter period experience greater acceleration, whereas those of longer per= iod experience greater displacement. Structural performance objectives shou= ld be taken into account during preliminary design and response-spectrum an= alysis.

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- RSA provides insight into how damping affects structural response. A family of response curves may be develope= d with variable levels of damping. As damping increases, response spectra s= hifts downward.

- The International Building Code (IBC) is based on 5% damping. This acco= unts for incidental damping from hysteretic behavior, which is = not explicitly modeled during RSA.

- Viscous dampers do not affect structural stiffness, are not modeled dur= ing RSA, and are not accounted for in the IBC provision for 5% damping.

- All response quantities are positive, therefore RSA is not suitable for= torsional irregularity. A static lateral-load procedure is best for measur= ing accidental torsion. The same applies when considering uplift and compre= ssion during foundation design.

- Modal response may be combin= ed using SRSS, CQC, ABS, or GMC methods. CQC is best when periods are close= ly spaced, with cross-correlation between mode shapes. SRSS is suitable whe= n periods differ by more than 10%.

- Ritz vectors are rec= ommended for RSA because this formulation is computationally efficient. Onl= y pertinent mode shapes which occur in the horizontal plane are identified.= Eigen vectors use the f= ull stiffness and mass matrices, which also account for vertical modes. Eig= en formulation is useful when considering floor vibration, out-of-plane vib= ration of shear-wall systems, etc. Eigen application is also useful for loc= ating modeling errors.