Seismic Analysis: Static, Response Spectrum and Time History

Three seismic analysis methods

– Equivalent static analysis: generates lateral seismic loads by direction using floor mass, elevation, centres of mass and stiffness, accidental eccentricity and torsional effects.

– Response spectrum analysis: estimates maximum joint displacements, member forces and support reactions from a user-defined acceleration spectrum.

– Time-history analysis: evaluates the structural response through a specified earthquake record.

Equivalent static seismic analysis

For each floor, SAFI can organize mass, mean elevation, the centre of mass and the centre of stiffness. Results include floor seismic force, torsional moment, structural dimension perpendicular to the seismic direction, eccentricity, torsional-sensitivity ratio and overturning moment.

Response spectrum combinations

Modal results can be combined using the SRSS or CQC method. The resulting maximum response can then be combined with static results for structural design. Positive and negative earthquake-load combinations should be considered to represent load reversibility.

Base shear and P-Delta considerations

Response-spectrum base shear calibration is performed against the corresponding static seismic load. The technical paper also explains how stability coefficients and P-Delta effects should be reviewed under the applicable NBCC or ASCE provisions. Engineers should verify the requirements of the current adopted code and project jurisdiction.

Reviewing results

The influence of each mode can be reviewed graphically and in tables. Engineers can examine displacements, forces, reactions, centres of mass and rigidity, floor forces and accidental-torsion effects.