We have been provided with” Spectral Acceleration (g)-Period(s)” graphs (based on UBC1997 & IBC 2003) by our client.
To obtain g values from these charts and enter as uniform load in CII to have a static equivalent analysis, which values should be considered to be realistic for large size flare lines??
I’ve seen some companies are using max peak value (flat segment of graph) for g and enter it directly in CII, which seems too conservative.

The max value based on UBC 1997 is twice of IBC 2003; does it mean that the g value is going to be half if the structural design is based on IBC code???

Many thanks

Seismic critera have changed greatly over the last ten years. I don't think you should expect a smooth correlation between UBC and IBC. If you can establish the lowest system natural frequency (highest period), then you can pick the acceleration off that g vs. period (spectrum) graph. A conservative acceleration would be the largest acceleration between 0 seconds and your period. That might not be your max peak value.

Is your spectrum adjusted for soil? It should be.

Current (ASCE7) approach uses a single point to set the g load - the mapped short period (0.2 second) acceleration.