Response Spectrum Analysis Checklist

Purpose

This checklist confirms that a response spectrum analysis has been set up and executed correctly. Response spectrum analysis is sensitive to the number of modes extracted, the combination method, the damping ratio and the directional combination rule. Errors in any of these produce results that look plausible but are significantly non-conservative or overly conservative.

Spectrum input

• Response spectrum is the correct one for the location and elevation (ground vs floor)
• Spectrum damping ratio matches the damping used in the modal analysis
• Spectrum units confirmed (g, m/s^2, in/s^2) and converted correctly in the solver
• Spectrum frequency range covers all modes with significant participation
• Spectrum provided for each required direction (horizontal X, horizontal Z, vertical)
• Scale factor applied to the spectrum is correct (e.g. scaling for seismic hazard level)

Modal extraction

• Sufficient modes extracted to achieve the required cumulative mass participation (typically 90% or per standard)
• Missing mass correction applied if 90% participation not achieved with practical number of modes
• Mode shapes visually inspected for plausibility — no spurious local modes at trivial masses
• Frequency range of extracted modes covers the amplified region of the spectrum
• Closely spaced modes identified (frequency ratio < 1.25) for CQC consideration

Modal combination

• SRSS used only where modes are well separated (frequency ratio > 1.25)
• CQC used where closely spaced modes exist — damping ratio in CQC matches modal damping
• Absolute value sum (ABS) not used unless specifically required by the standard
• Rigid modes combined separately from periodic modes where the standard requires it
• Combination of signed quantities (e.g. modal base shear direction) handled correctly

Directional combination

• Directional combination rule confirmed (SRSS or 100-40-40 per the applicable standard)
• Vertical direction combined with horizontal directions per the standard (typically 100-40-40 with vertical as a separate direction)
• If SRSS directional combination is used, confirm the standard permits it
• Results from each direction stored separately before combination for traceability

Post-processing

• Peak responses (displacement, acceleration, stress, force) extracted from combined results
• Modal contributions to peak response reviewed to confirm no single mode dominates unexpectedly
• Results at key locations (base, supports, anchors, welds) extracted and tabulated
• Load case combinations with other loads (dead, live, thermal) set up per the standard
• Sign of recovered forces and moments checked for consistency with expected load path