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When using a manually defined response spectrum, such as shown here, interpolated acceleration values seem to be incorrect.
From the provided values, the expected acceleration value for T = 1.293s is
assuming linear interpolation for intermediate values.
Incorrect spectral acceleration due to coarse definition of the spectrum (IGE17-176ANJH5D)
For response spectrum values, SCIA Engineer uses linear interpolation based on the frequency, not the period.
In this particular case, T = 1.293 s corresponds to f = 0.7734 Hz, which leads to
The graphical view of the spectrum is misleading: regardless of the selected display mode (period or frequency), SCIA Engineer always uses linear interpolation based on the frequency for intermediate acceleration values.
When intermediate values of the acceleration spectrum are needed, refine the definition of the spectrum to reduce the inaccuracy due to linear interpolation.
In general, prefer using the provided seismic spectrum generators, as they calculate enough intermediate points to avoid any doubt.