After Hurricane Irma hit, concerns arose over the structural integrity of South Florida’s bridges, mainly those in the Florida Keys. All 42 bridges that connect the mainland to the Keys were inspected and declared safe by county officials.
Now a researcher at Florida International University and one of the world’s leading bridge engineers shares what he says is an easy and cost-effective way to test a bridge for safety.
The technique establishes the signature of a bridge before a hurricane or other natural disaster occurs and allows engineers to familiarize themselves with its characteristics, says Atorod Azizinamini, director of FIU’s Accelerated Bridge Construction-University Transportation Center (ABC-UTC), and chair of the College of Engineering & Computing’s Department of Civil & Environmental Engineering. A bridge’s signature refers to its stiffness. That stiffness, says Azizinamini, can change due to damage.
One tool that can be used to detect a bridge’s signature is the impulse-response test (IR), originally used to test foundations. The test has been adapted to assess bridge decks and columns and other infrastructure types.
With the IR technique, stress waves are generated using a hammer with a load cell attached at its end to establish the mobility and stiffness of the bridge elements and identify any areas with deficiency.
If no deficiencies were present in the testing before the hurricane, any new deficits that turn up would likely be a result of the storm’s impact and could quickly be assessed. The data collected displays as a graph.
A transducer measures the velocity at a given point on the bridge and the hammer measures force. When an engineer strikes the hammer, it causes the structure to vibrate. That generates waves that cause the structure in the vicinity of the transducer to displace. The information from the transducer and load cell attached to the hammer establish the structure's stiffness at the grid points. The bridge’s structure is divided into a grid, and measurements of the vibrations are taken at each grid point allowing engineers to establish the stiffness of the structure in a given area.
The key is to test bridges prior to the storm, not just afterwards, in order to identify changes, which translates to damage.
IR, according to Azizinamini, is the best technique for bridge damage assessment at a local level. There are other tools available such as ultrasound.