As explained in communities.bentley.com/.../how-to-prescribe-a-dynamic-displacement, the amplitude of the motion is double when body wave reaches a free surface due to impossibility of transmitting stresses and therefore ux,start,ref= 0.5m to account for that. However, it's still a question when the result shows peak acceleration is about half and one time of input signal at the bottom and the surface, respectively. In Plaxis version 2011 the displacement is only set as 0.01 to convert cm to m for input SMC file. Something is still not clear for me to understand the above explanation and to decide the input in our project case. Would anybody explain me more clearly?
Dear Vu Ngo,
Indeed, the example in the article you shared explains the case that the signal is recorded in the outcropping rock at surface. Then, assuming the theory of wave propagation, the signal is double of that corresponding to the outgoing wave and therefore a factor of 0.5 should be used when that motion is to be applied at the base of a FE model.
The factor applied, based on what you mention, in PLAXIS 2D 2011 due to units is not related to this but more of unit conversion (cm to m).
It is important to consider that signals may be recorded at various locations and not always at the outcropping rock surface, so this depends on your data. Based on that you may need to apply more sophisticated procedures (e.g. a deconvolution analysis). More insight into this topic is provided in Kramer (1996).