The direct use of the experimental modal data for the apposite calculation of the response of an observed bridge under a live load is described in this paper.

It is important to determine properties of many bridge structures in the certain time of their durability for an objective estimation of the residual service life or the loading capacity with an adequate accuracy and eventual for a decision about a strategy and a method of their service. This is the reason to create deterioration models of bridge structures for description of their static and dynamic behaviour and their properties after the certain time of their operation. The parameters of these models can be specified by experimental methods. The experimentally - estimated frequencies, modes of natural vibration, corresponding damping values realise the real immediate state of the observed structure and its spatial behaviour. This is the reason for using the experimental modal parameters such as for identification, optimalization or a verification of a bridge design model. This model is shaped under certain physical and mathematical hypotheses. Therefore a succession of design models is possible to create for a given bridge structure in given time. The aptness of these models is important to verify by the experimental investigation of the studied bridge.

In addition, the apposite spatial model of the monitored bridge could be created directly from the experimental modal parameters of this bridge (so-called the modal model). There is possible to achieve with this model the apposite calculation of a response of an observed structure under dynamic and static load. The response of the modal model under the dynamic and static influence of the live load can be calculated by the method of the expansion with respect to the modes of natural vibration.

Today the stress is the main criterion of the load carrying capacity check of a structure and its variability in time is the main criterion of the fatigue life check. It is possible to evaluate the response of the modal model under the live load on a stress quantity scale. The calculated time histories of the stress in given details of a structure can be processed by one of classification methods. Afterwards it is possible to make the estimation of the influence of the observed dynamic load on a reduction of the fatigue life of the monitored structure on a base of the obtained results and the Palmgren - Miner hypothesis.