Modal Analysis of a Highway Bridge: Experiment, Finite Element Analysis and Link

The Westend Bridge is part of the Berlin City Hi Belt where it connects downtown Berlin and Tegel 2. hway ~rport. This 242 m long and roughly 30 years old prestressed concrete bridge is exposed to relatively dense heavy commercial traffic and does no longer match toda ‘s requirements. It has therefore been strengthened 16 y means of longitudinal prestressed steel plates externally added to the box girder bottom flange in 1994. Within the framework of these measures two experimental modal analyses (EMA) have been planned: one before and the other one after strengthening of the structure. The reject is a joint venture encompassmg EMPA and BAM. T E e elm of the tests is to assess the effectiveness of the structural modifications by determining their influence on the bridge’s dynamic parameters such es natural frequencies, corresponding mode shapes and damping ratios. In addition, these experimental investigations provide the basis for adjusting of a finite element (FE) model of the bridge structure to the real bridge behavior. Making use of such a verified FE model any possible significant load case can then be analysed with confidence. The paper firstly deals with the EMA having been performed in autumn 1993. A randomly driven servohydraulic actuator producing a 4 kN maximum vertical force amplitude and acceleration measurements in 247 selected points in three directions each were used to obtain the frequency response functions and subsequently the dynamic characteristics of the bridge. Based on these results an MSC/NASTRAN finite element model was then updated making use of a specific ‘“Link” software package. The hence validated FE model will be enhanced to include the strengthening measure’s effects. The second EMA is scheduled for April 1995. Subsequent updating of the FE-model of the strengthened structure will give reliable indications concerning the effectiveness of the strengthening measures chosen.