The first of them concerns an investigation into the performance of orthotropic deck details encountered in steel bridge structures subjected to static and many times repeated loads. Several theoretical and experimental methods have been applied for their solution. The experiments involved in the investigation are focused on crack propagation, stress concentrations and the fatigue life of the bridge elements. Being fully welded the orthotropic decks often show specific problems regarding the fatigue strength under dynamic loads. The most sensitive (critical) points on this type of structure appear in the neighbourhood of the area where the bridge deck joins the cross girder and longitudinal rib. The stress concentrations, a possible source of fatigue cracks, appear in the webs of cross or longitudinal girders and, or near the cutouts.

The other research project studies the fatigue behaviour of steel plate girders whose webs repeatedly buckle ("breathe") under many times repeated loads. The main tool of the investigation is an application of numerous experiments carried out on steel thin-walled girders, the webs of which are under the action of many times repeated (i) combined shear and bending and (ii) patch loading, which are the kinds of loading frequently encountered in bridge girders. During the experimental investigation, it was observed that the main impact of the cumulative damage process in the breathing webs was the initiation and propagation of fatigue cracks which developed in the crack-prone areas, i.e. at the toes of the fillet welds connecting the flanges and transverse stiffeners with the breathing web in zones with maximum ranges of secondary stresses.

It is in the nature of the phenomenon studied that the aforesaid initiation and propagation of fatigue cracks necessarily and considerably affects the fatigue failure mechanism of the whole girder.

In the case of slender webs exposed to repeated predominantly shear, the fatigue cracks initiate near those web panel corners from which the diagonal buckled web pattern, and consequently also the diagonal tension (stress) band, emanate. Both of the aforementioned investigations are concluded by formulating recommendations for a safe and economical design of various kinds of steel bridge structures.

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