Application of seismic input motions for deeply embedded structures

The application of seismic input motion for deeply embedded structures has been a controversial subject. The current position of the U.S. Nuclear Regulatory Commission (NRC) is that the seismic input motion whose response spectra envelop the NRC Regulatory Guide 1.60 should be applied at the foundation base level for soil-structure interaction analysis, regardless of the depth of embedment. This position is considered questionable since the motion at this lower level is strongly influenced by the soil column above. This paper shows by comparative analyses that strict implementation of the NRC position yields unreasonable results for a deeply embedded structure. Available published recorded data and analytical results are also briefly discussed to substantiate the advocated position of reducing the input motion with depth embedment. (orig.)