By Kate Kurtin
Richard Christenson (Civil & Environmental Engineering) and Jiong Tang (Mechanical Engineering) both specialists in structural dynamics, control and monitoring are joining forces on a project that takes advantage of their differing disciplines. Dr. Christenson’s primary area of expertise is the overall behavior of civil structures subjected to dynamic loading. Meanwhile, Dr. Tang focuses on the behavior of mechanical systems and the local behavior of such systems. Sharing a common interest in reducing damaging vibrations and identifying the structural health through vibration measurements brought the two researchers together. Specifically, Dr. Tang introduced Dr. Christenson to the piezo electric sensors used in mechanical engineering to transport information about global, as well as local, structural damage. Meanwhile, Dr. Christenson is sharing his knowledge of applying such technologies to large-scale civil structures.
“This project came together through the idea that bridge health monitoring and structural control in a broader sense can be leveraged to better improve the performance of each component,” Dr. Christenson explained. “Whereas traditionally they have been looked at separately, our intent is to study them as one unit in a synergistic sense,” he finished. This interdisciplinary approach rests on the old saying, ‘the whole is greater than the sum of its parts.’ By looking at structural control and health monitoring individually, it is possible to get good performance; however, if you look at the whole, the parts not only help themselves, but also one another.
On a technical level, structural control asks ‘How do we apply external forces to the structure to reduce the vibrations and response of the structure to some excitation?’ Continuing on, structural health monitoring poses the question: ‘How can you use vibration response of the structure to identify the structural health?’ Piezo electric sensors can be used in both structural control and health monitoring. In practice, if the structural health monitoring component identifies damage on a certain element, structural control may be able to react accordingly and help protect, or favor, that element.
An additional benefit to this fusion of structural control and health monitoring, Dr. Christenson explained, is the unknown excitation factor. Alone, health monitoring can look only at the output effect of heavy blast loadings on a bridge; it does not convey the size or weight of the blast. Including structural control provides the ability to excite the structure offering known input. “Bridge structural health monitoring is often put into terms of human health,” Dr. Christenson clarified. “If you are running along and you sprain your ankle, one thing you can do to diagnose the injury would be to put some weight on your ankle or try and run a little bit to see how bad it is. In a similar way, the structural control monitor could sense that something is wrong and redirect the energy and vibrations to query it to decide if you really have damage, and how bad the damage is.” In application, it is the piezo electric sensor that can excite the structure at a high frequency, allowing it to precisely diagnose the damage.
This project began really as a discussion between the researchers long before pen was put to paper. The conversation was transformed into action by the School of Engineering’s selection as a Department of Homeland Security (DHS) National Transportation Security Center of Excellence (NTSCOE). Bridge structural integrity is an area of keen interest for DHS so it provided a perfect opportunity for Drs. Christenson and Tang. “We had talked about doing a project on this and when NTSCOE came along it was our chance to actually look at this, so we started down that road,” said Dr. Christenson.
Specifically, DHS is focused on so-called multi-hazards. “If you have this control and monitoring system, you can design the system to be multi-hazard – to look for damage due to traffic or seismic events like earthquakes or blasts,” Dr. Christenson explained. Continuing with the example of an earthquake, Dr. Christenson illustrates, “First off, during the earthquake you can control the responses – try to reduce the damage right then. Following the earthquake, you could determine whether there was damage to the structure or not.” This is of vital importance to DHS. It would indicate if the structure was stable enough to withstand the weight of the vehicles currently on the structure. In addition, it would accurately show if the structure was strong enough for first responders to cross over, or if they needed to find an alternate route.
A second point of interest for DHS is how to build structures to withstand heavy blast loads from terrorist attacks or natural disasters. The control and monitoring system being developed by Drs. Christenson and Tang would allow the greatest ability to assess damage to the structure. “During a blast it very difficult to get an estimate for structural control because it is immediate, but immediately after the blast you can use this synergistic control and monitoring to identify damage, and if there is damage, try to redistribute the load to make the structure as safe as possible,” said Dr. Christenson.
With these structural monitoring and control goals in mind, the general framework the researchers are looking at is multi-hazard protection. At the current stage in their research, a 12-foot highway bridge model has been assembled, complete with piezo electric sensors. The researchers intend to start proof of concept demonstrations on the model in the upcoming months.