Assessment of Key Factors Controlling the Modal Properties of an Instrumented Bridge after Multiple Earthquakes

This case study presents the performance of a highway bridge in New Zealand during a series of large earthquakes and the change in modal characteristics due to structural- and foundation-related damage. Substructure damage was evident following multiple large earthquakes in 2013 and 2016, including spalling and cracking at the top and bottom of multiple piers. System identification based on ambient vibration data from accelerometers installed after each earthquake showed that there was a 13% permanent increase in the fundamental transverse mode period from 2013 to 2016. A numerical model of the bridge was developed, which was able to capture the key modal response characteristics, and demonstrated that changes in foundation stiffness due to scour effects and the cross-sectional characteristics at the pier damage locations likely had the most significant influence on mode shape and period shifts between the 2013 and 2016 earthquakes. A sensitivity study demonstrated that varying the foundation stiffness and pier damage had the most overall influence on the modal characteristics; however, significant reductions in soil stiffness and increased pier damage shifted the modes shapes away from the recorded response. Importantly, the model demonstrated that irregularity of scour depth along the length of the bridge was a key aspect that controlled the transverse mode shape and fundamental period, thereby highlighting that irregular scour should be considered for the design and assessment of bridges. Response during a range of smaller aftershock events showed that the fundamental transverse period was strongly dependent on excitation level. The period was 20% higher when the peak acceleration in the middle of the superstructure was approximately 0.1 g ; however, it then returned to the ambient vibration value, suggesting that the period of elongation resulted from softening of the foundation soils, which then regained stiffness. The results presented here provide useful general insights into the difficulties inherent in the interpretation of damage from changes in the modal characteristics of structures.