Journal of Performance and Constructed Facilities
October 2016 | Volume 30, Issue 5
N. C. Dubbs, PE, M.ASCE; and F. L. Moon, M.ASCE
This paper presents a diagnostic case study of a long-span bridge, which uncovered the root cause of unsymmetric vibration performance that appeared to point to deterioration.
In this case study, two iterations of the structural identification (St-Id) process were applied to (1) develop a comprehensive finite-element (FE) model (inclusive of approach spans) representative of the general dynamic properties (frequencies and mode shapes), and (2) check that the response magnitudes were consistent with those expected from vehicular traffic.
More specifically, this research confirmed the perceived vibration discrepancy with direct measurements, conducted a comprehensive (including approach spans) ambient vibration monitoring of the bridge, updated an FE model to match measured modal parameters, and performed a series of simulations to both validate the updated model and understand the root cause of the unsymmetric vibrations.
Through this process, it was determined that the unsymmetric vibrations resulted from the approach span designs, which provided significantly different levels of lateral stiffness to the primary bridge.
Additionally, upon repeating the test following the replacement of all expansion bearings with neoprene bearings, similar levels of vibration asymmetry were observed, which further supported the diagnosis that this behavior is an inherent characteristic of the bridge.
In addition to introducing an iterative St-Id process that makes use of both modal parameters and magnitude information (derived from an ambient vibration test), this paper illustrates the importance of explicitly including the approach spans within the St-Id of long-span bridges.
In this case, ignoring the influence of the approach spans would likely have resulted in the erroneous conclusion that some damage or deterioration within the main spans was causing the unsymmetric vibrations observed.