The International Building Code (IBC) requires a seismic loading on all retaining walls. A common method of calculating the lateral soil pressure created by seismic activity against “yielding walls”, is the Mononobe-Okabe Method, and the Wood (1973) method, as presented in "Geotechnical Earthquake Engineering" by Steven L. Kramer, for the non-yielding walls.  Within this method, there is a coefficient “kh” that modifies the force generated by the weight of the soil failure wedge behind the wall.  "kh" is equivalent to the design spectral response acceleration, which ranges from the 0.2 sec period (SDS) to the 1.0 sec period (SD1). The value of “kh” makes a huge difference in the magnitude of the lateral force on the wall. Does anyone know if previous Seismic codes addressed whether to use the SDS or SD1 factor?