Why evaluate structural integrity?
The STRAAM structural integrity risk assessment and management system provides a valuable benefit to structure owners who are interested in any or all of four types of information:
1. A level of assurance that structural performance is being evaluated, either continuously or at regular intervals.
2. Observation of structural performance for signs of deterioration that warrant investigation or maintenance work, or to help predict useful structure life
3. Determining, quickly and with good reliability, the change in structural condition after an extreme event such as a severe typhoon or significant earthquake
4. Locating, with reasonable precision, any members or joints that warrant further inspection after an extreme event
Structural integrity evaluation can enhance the public perception that safety is of paramount interest. Questions that naturally arise after an extreme event can be quickly resolved. And if probing is determined to be necessary after such an event, locating probes more effectively can minimize cost and disruption of the structure’s main operation.
As with any diagnostic test, results must be understood and interpreted with a sense of perspective. A structural integrity assessment may show some very gradual degradation of structural behavior over the years. If observed, changes in structural behavior will likely deserve further study to determine if they are a part of the natural process of “aging gracefully” or require further action. Most structures undoubtedly undergo changes, but if undiagnosed, no one will know what changes are occurring. Characteristics being measured require detailed analysis so measurement values are not taken out of context.
How does STRAAM work?
Three key elements of the STRAAM system distinguish it from other structural assessment methods.
First, the STRAAM developers have determined that building damping varies with structure movement in a distinct three-part pattern: small or smaller movements, gradually increasing with larger movements, and reaching an upper limit or plateau. They have also determined that changes in structural properties cause detectable changes in the damping curve. This is important because the traditional approach to evaluating structure period is rather insensitive to structural changes. Period varies as the square root of stiffness, so it takes a dramatic stiffness change to make detectable period change.
Second, STRAAM methodology was developed to gather useful data on damping at different deflections by using ambient vibrations. Raw data gathered from accelerometers is a jumble of simultaneous vibrations from different mode shapes. With Fourier analysis, the component frequencies are separated out, a straightforward task. The challenge is to pick out brief intervals of each frequency that are well-behaved enough to define the damping effect at that frequency. With enough data—several weeks of continuous testing when determining initial baseline conditions, several hours or so for subsequent checks—enough appropriate intervals can be found to establish properties at a high level of statistical significance.
Third, very stable circuitry is required to assure that any variation in structural behavior observed is indeed due to structure changes and not electronic “drift.” The equipment has been built and tested to meet the required high standards.
How often should testing be done?
There are several ways to approach structural integrity assessment.
1. Only look when you think there may be a problem. Because the STRAAM system works by comparisons, at minimum a baseline measurement is required to start. Measurements taken at later dates are used to locate changes. There is a drawback to this approach: if and when changes are observed, there is no way to know if they occurred gradually, such as from changes to partitions, or suddenly from extreme events.
2. Regular check-ups. After a baseline measurement, periodic assessments are made. Eventually the time-related aspects of structure behavior can be defined and addressed when looking for event-related changes.
3. Continuous testing. This provides the clearest picture of structure behavior and the greatest sense of assurance. It also permits rapid response after an event, since installed equipment is gathering and sending data automatically instead of waiting around for a technical service call.
What is the process?
The actual measurement procedure is a two-step process, routine testing and detailed study.
For routine testing, whether periodic or continuous, several sets of accelerometers are permanently affixed to selected floors. Two accelerometers, oriented at right angles and located near a building corner, can be used to detect X, Y, and torsional motions at that level. If needed, additional accelerometer sets placed on different floors can clearly identify multiple mode shapes as the building moves. Data from these accelerometers is used to establish the damping curve and develop subsequent curves for comparison. If large or sudden changes in the damping curve are detected, immediate and further investigation would be required. Conversely, no observed changes, particularly following an extreme event, would indicate that the structure has not experienced significant damage—a great confidence booster.
Data from routine testing is also used to calibrate a computer model of the structure that corresponds to observed behaviors. That model is used for the second step in the measurement and evaluation process.
Second-step measurements are performed in the event of observed changes in damping curves or even more drastic, in modal responses. Behavior of higher modes of vibration can be more sensitive to localized damage and deterioration than lower modes, but identifying and measuring higher modes is impractical using accelerometer data from just a few levels. Portable accelerometers are then installed and moved among different locations to establish structure responses in local regions. That additional data is used to compare the new structure behavior to that of the previously calibrated computer model and the changes identified. The locations found to require modifications will indicate likely damage zones.
Conclusion
STRAAM’s structural integrity assessment system uses advanced technology to understand structure behavior and performance. It has the potential to provide a rapid check on structure behavior after an extreme event, particularly if continuous testing is performed. That check either provides reassurance that damage was avoided, or helps determine locations where damage may have occurred. In either case, improved speed and accuracy in structural evaluation are helpful in minimizing the cost and time impacts of the event.
Top of Page > |
