The on-line computer test control method (the on-line test) is a technique to directly simulate the earthquake response behavior of structural systems without using a shake table device. This paper presented investigations into the experimental error growth of the on-line test response. Experimental error sources included in the on-line test and their effects on the response were examined from the on-line tests applied to a two degrees of freedom and a six degrees of freedom structural systems. It was found that the displacement error, defined as the difference between the computed displacement and the measured displacement, had the greatest contribution to the error force, and the displacement error was characterized, in many cases, as the undershoot.Analytic investigations were made to quantitatively evaluate the response error growth caused by the displacement error, and guidelines were presented to specify the relationship between the amplitude of the response error and the amount of the displacement error. Capacity of the viscous damping to suppress the response error growth was also calibrated. Effects of the viscous damping assigned based upon the elastic properties of the analyzed system on the inelastic response were examined, and integration techniques having adjustable numerical

damping were introduced. Two new algorithms to suppress the response error growth caused by the displacement error, designated as I-Modification and T-Modification, were proposed. In those algorithms,the measured reactional force was modified in accordance with the detected displacement error and the initially estimated elastic stiffness (I-Modification) or the instantaneous stiffness estimated in each step of loading (T-Modification), and this modified reactional force was incorporated into the computation of the equations of motion.The validity of those algorithms to suppress the response error growth was demonstrated through the on-line tests applied to two degrees of freedom systems.

^*1 Senior Research Engineer, Production Dpartment
^*2 Assistant Research Engineer, Production Department