ABSTRACT
Bridges are structures vulnerable to the seismic action which is considered in the design stage of new structures or in retrofitting process of existing ones. The update of existing seismic provisions and the development of new design methods try to improve the level of structural protection against the seismic action. The new concept of performance based design together with the use of methods as incremental dynamic analysis (IDA) are capable to cover a larger number of seismic hazard levels and lead finally to higher safety factors.
The paper shows the results of the seismic analysis of a roundabout placed at the crossing of Bucharest Ring Road (km 52 + 220) with the county road DJ 602 (km 4 + 820). These roads register very high values of traffic when entering or leaving the city. The bridge links both roads and ensures that drivers or pedestrians can cross in any direction they choose.
The main structure is a circular overpass roundabout with the static layout of a multi-span continuous girder of 10 x 27.50 m with a composite steel-concrete cross section 13.60 m wide by 2.05 m high. It crosses two roads and a railway, thus making it a very tall, wide and heavy structure.
Considering the high level of the seismic hazard in the area around Bucharest, the need of correct estimation of the bridge vulnerability for different levels of earthquake intensity immediately stands out.
One of the recent methods used for estimating the vulnerability of a structure following the seismic action is incremental dynamic analysis (IDA). The vulnerability is expressed in terms of exceeding a certain demand of the structure, usual displacements or rotations of parts of the structure, for different levels of the seismic hazard. The method presented by Vamvatsikos & Cornell (2005), Mander et al. (2006), Buckle et al. (2006), consists in performing nonlinear dynamic analyses using sets of generated or registered ground motions, scaled to comply with different levels of the seismic hazard. The results, often in terms of displacements and rotations, plotted versus the ground motion intensity level represent the IDA curves. Further on, using these curves by means of probabilistic theory, fragility curves for particular structures can be obtained. Finally, a general image regarding the resistance or stability reserves of a structure for certain levels of seismic hazard can be obtained.
Using a finite elements model and the procedure explained above the performance level and the vulnerability of the considered bridge can be estimated.
