ABSTRACT
Romania is an active country from seismic point of view. There are mainly two seismic regions in Romania: along a line in the direction NE-SW, where usually medium depth earthquakes occur and an area between the W and SW of the country characterized by surface earthquakes. The first type of earthquakes are in general severe, they have a predicted return period of 40–50 years and the seismic source is placed 100–150 km below the ground surface. One of the strongest earthquake registered in Romania took place on 4-th March 1977, has had a magnitude of 7.2 on Richter’s scale, killed 1500 peoples and produce severe damages on a large number of structures.
In the last decade, in Romania several medium to strong earthquakes have been registered. This is the reason why the seismic protection became a subject of major interest both for the authorities and engineers.
Fortunately, only a small number of bridges were affected by the produced earthquakes. The first seismic provision applied in Romania for the design of structures was P100–92. Taking into account the evolution on global scale regarding the protection against earthquakes, several improved versions of this standard were produced in 2004, 2006 and finally in 2013. These contain major changes regarding the ground acceleration and corner period, but most important, regarding the structural details which can lead to larger safety factors in service.
This paper presents, from the designers point of view, the influence of the seismic code changes on the dynamic behavior of a bridge previously designed according the old standard P100-92. The analyzed bridge is situated on DJ236 in Prahova County, was built with precast concrete girders and has four spans in the range 21–31.30 m.
In order to establish the dynamic response of the bridge under the seismic action, a finite element model of the bridge was built. Having as target to establish the influence of the seismic code changes on the general response of the bridge in terms of piers cross section internal forces and superstructure displacements, a simplified 3D model was chosen instead of a complex one.
Eigen vector modal analysis and linear response spectrum analysis have been performed on the finite elements model in order to obtain the seismic response.
At the piers base, the differences in terms of bending moments are in the range 8 to 80% and in terms of shear forces in the range 7 to 107%.
It is obvious that for the existing bridges, the design according the previous editions of the normative P100 is non conservative when accepting that in current practice, higher values of the seismic input parameters appear. As a consequence, speaking about existing bridges, the only measures to be applied, in order for them to regain their safety coefficient, are appropriate strengthening methods.
