D-A-CH TAGUNG 2011 - SGEB

1 INTRODUCTIONThe assessment of existing structures with respect to their current load bearing capacity,their remaining life time or with respect to perspective changes in their use becomes an issue ofincreasing importance. Especially railway bridges are investigated regularly by inspections andnumerical analyses according to specific guidelines such as [1], [2], [3], and [4].The quality of any prediction based on a numerical model depends on the quality of the respectivemodel. If existing structures are considered it is possible to compare certain features ofthe model with respective parameters that were obtained from the existing structure experimentally.It has become common practice to use in this context the dynamic structural propertiessuch as natural frequencies and mode shapes because these parameters can be identified nondestructively.Even though several algorithms and sophisticated measurement equipment are generallyavailable today, the investigation of each individual structure often requires specific practicalapproaches due to limitations of the accessibility, service conditions and further problems. Inthe case of the investigations described here, two research teams were cooperating performingthe experimental modal analyses of a steel railway bridge using two different measurement systems.One of the challenges of this approach were the combination of the results obtained basedon the measurements by means of these two different systems.2 DESCRIPTION OF THE STRUCTUREWithin the European research project FADLESS, that focuses on the development of procedureswhich combine numerical and experimental methods for the assessment of fatigue resistanceof existing steel railway bridges, several structures in different countries are investigated.One of these structures is the Saalebrücke near Großheringen in central Germany.This bridge is situated at the junction of the railway lines Halle / Leipzig – Munich and Halle/ Leipzig – Erfurt. It was put into operation in 1933. The complete bridge consists of five spansconstructed as plate girder bridges and one span that was designed as a truss girder as shownin figure 1. The two tracks are supported by separate superstructures. The spans of the plategirders range from 35 to 44 m while the truss girders over the river Saale have a length of 74 m.The investigations described here concentrated on the plate girder superstructure at the northernend of the bridge (figure 2). As all other spans it was designed as a skewed simply supportedbeam with two plate girders and cross beams. Rail carriers span as simply supported beams betweenthe cross beams. Bracings between the rail carriers and between the lower flanges of themain girders give the bridge stiffness in horizontal direction. Both the cross section and thebracing systems are depicted in figure 3.3 NUMERICAL ANALYSES3.1 Preliminary numerical modelTo obtain first information of the modal behaviour of the considered superstructure a relativelysimple finite element model of the structure was created. Only the major structural elementswere taken into account. They were modelled by beam element which were connected tonodes in a single plane. Since not all centres of gravity of the structural members at the bridgeare located in a single plane, respective eccentricities of the beam elements were defined in the2982