Using the Random Decrement (RDD) technique to obtain free response estimates and combining this with time domain modal identification methods to obtain the poles and the mode shapes is acknowledged as a fast and accurate way of analyzing measured responses of structures subjected to ambient loads. Using commonly accepted triggering conditions however, one is limited to use a combination of auto-RDD and cross-RDD functions with high noise contents on the cross-RDD functions. Using only the auto-RDD functions, estimated independently for each channel, causes the loss of phase information and thus the possibility of estimating mode shapes. In this paper, a new algorithm is suggested that is based on pure auto-triggering. Auto-RDD functions are estimated for all channel to obtain functions with a minimum of noise, but using a vector triggering condition that preserves phase information, and thus, allows for estimation of both poles and mode shapes. The proposed technique (VRDD) is compared with more commonly used triggering conditions by evaluating modal parameters estimated by time domain technique on simulated data.
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