The use of field monitoring data in the evaluation of seismic risk plays an important role in quantifying and reducing the epistemic as well as aleatory uncertainties, through the reconcilement between numerical and testing results or through the updating of fragility functions using data available after a damaging event. Methodologies that use model updating techniques, whether based on vibration or vulnerability parameters, can form the basis of fragility functions to use in real time seismic risk assessment activity. Different methodologies to update finite element models based on usually incomplete pre-event vibration monitoring data can be employed by typically minimizing a function given by the residuals between experimental measures and model predicted quantities. The updating process should follow specific criteria, as well as include eventual Bayesian fundamentals, so as to assure that, essentially, the engineering judgment used in the model is well kept. Furthermore, such model updating philosophy can be extended to post-event fragility updating, based on adequate engineering demand parameters taken from estimated and recorded time-history analysis of the building. Vibration data, e.g. eigenfrequencies and mode shapes, obtained from shaking table tests performed on a 3-storey RC building, are used for the implementation of a finite element model updating methodology. The influence of the different structural parameters to be estimated on the effectiveness of the updating methodology is explored and conclusions are drawn towards the optimization of the procedure for future fragility assessment application.