On August 1, 2007, the I-35W Mississippi River Bridge in Minneapolis suddenly collapsed, sending mangled steel from the superstructure and crumbled concrete slabs from the paved deck tumbling down into the river. The rush-hour disaster resulted in 13 deaths and caused injuries to another 145 people.
Spectacular collapses such as this reverberate around the globe. So do expressions of concern about our aging infrastructure and calls to “do something about it.” To civil engineers, “failure” actually occurs much earlier than an ultimate collapse. A structural failure means that part of the structure has lost its capacity to carry the loads it was designed for or that some of its members experience bending or deformations beyond acceptable tolerances.
Tianwei “David” Ma of UH Mānoa's Civil and Environmental Engineering Department received a research grant from the National Science Foundation to perfect a method of harvesting energy from the natural motion of bridges to power smart monitoring devices. The $223,000 grant runs to April 2011.
Effective and continuous monitoring of civil engineering structures can provide timely warning that can help avert failures and ultimate collapses, just like preventive medicine. With advances in technology, wireless sensors are replacing traditional “tethered” (or hard-wired) networks.
As Ma puts it, “the use of wireless sensor networks in structural health monitoring is greatly hindered by the need to provide reliable power sources for them; batteries need to be replaced to ensure that the network is always up and running.” “Structures such as bridges,” he continues “are not just sitting still all the time as many people assume; they are almost always vibrating very slightly due to moving traffic and wind loads.”
In his research project, Ma is perfecting a device that uses these small movements to generate electricity, which in turn can power the sensors installed in the bridge. By building a tiny generator in the laboratory that can supply power to a wide variety of sensors, Ma confirmed the feasibility of the idea. With innovative design, he also managed to squeeze twice the amount of energy out of the irregular pattern of structural vibrations as compared to what regular (or “linear”) patterns can yield.
Coupled with the structure, the device amplifies the kinetic energy in the vibration that can be converted to electrical power either by electromagnetic induction or by piezoelectric means. What remains now is to increase the efficiency and robustness of the device and integrate it with a wireless sensor board for field testing.
“This is a stroke of genius,” said C.S. Papacostas, professor and chair of the Civil and Environmental Engineering department. “To me, taking a useless bridge vibration and turning it into useful energy to bootstrap a sensor-based monitoring network is an amazing accomplishment! Ultimately, it can save countless lives and property losses.”