Discrepancies between program results and USGS tool results |
  
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Discrepancies between program results and USGS tool results |
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The gridded USGS text files contain SS and S1 parameter values from the International Building Code. Please note that these values will not exactly match those provided by the U.S. Seismic Design Maps. The maps interpolate the underlying probabilistic and deterministic ground motions, as well as the risk coefficients, before calculating SS and S1. Should you choose to interpolate the gridded values provided by USGS, you will be doing so after the SS and S1 calculations have already been performed at the grid points. Proceeding with this order of operations will produce results that slightly differ from those calculated in the Design Maps.
In addition, interpolation for a specific lat/lon coordinate requires a two-step interpolation. There must be an interpolation in the north-south direction, and then another interpolation in the east-west direction. If ENERCALC SEL interpolates in a different order than the USGS tool, the results could be different, even though both methods use a perfectly logical approach to the interpolation.
Remember that if you feel more comfortable specifying an SS and S1 value from another source, that is always an option in ENERCALC SEL.
