Modal Property Changes of a Seismically Damaged Concrete Bridge

The Lavic Road Bridge in San Bernardino County, California, was severely damaged during the October 16, 1999, Hector Mine Earthquake (7.1 Mw). The earthquake epicenter was approximately 16 km (10 mi) south-southwest of the structure’s location on Interstate 40. Prior to the earthquake, periodic modal field tests were being conducted on the structure to evaluate the impact of on-going reactive-aggregate induced structural deterioration. A scheduled field test was performed on the structure two weeks prior to the event. A second, unscheduled field test was performed three days after the event in order to capture any changes in the structure’s modal properties that might have resulted from damage incurred as a result of the earthquake. This paper provides a visible description of the damage sustained by the structure, documents the field test procedures used to determine modal properties (preevent and postevent modal frequencies, damping, and mode shapes), and discusses the changes observed in the preevent and postevent modal properties. The extracted modal results indicated that lower-mode natural frequencies decreased 18% relative to preevent values, that the frequency order of modes did not change, and that the modal damping levels increased by 25% relative to preevent levels.