14-14. Investigation of the Variation of Structural Modal Properties with Vibration Amplitude Utilizing Recorded Data from Monitored Buildings
We seek a postdoctoral fellow to study the variation of modal properties within different regimes of vibrations of buildings. Comparison of modal properties that are obtained through the analysis of vibration data collected from a structure during its operational (i.e., ambient) conditions with those results extracted from the structure's response recorded during seismic events (i.e., strong shaking) reveals two major trends: decreased modal frequency and increased damping during strong shaking (see, for example, Çelebi, 1996, 2008; and Carreno and Boroschek, 2011). While the amplitude dependencies described above are well known, the tendency in the current state of earthquake engineering practice is to accept the dynamic characteristics (frequencies, damping ratios, and mode shapes) observed during low-amplitudes of motion in lieu of those that structures exhibit during strong ground shaking.
Permanent drops in natural frequencies after extreme events are conventional indicators of stiffness loss (damage) (Sohn et al., 2004). However, transient dropsas described earliermaybe associated with factors other than permanent deteriorations of structural elements. Often, the transient variations are attributed to the presence of inertial soil structure interaction effects during strong base excitation. Another speculation is to ascribe the higher frequency values to the contributions of non-structural elements to the overall stiffness of buildings during ambient or low-level ground excitations. Local nonlinearities such as opening and closing of macro‑cracks in reinforced concrete buildings may be argued to be another source of decreased stiffness during strong shaking, particularly when the structure has experienced a prior extreme event.
The above observations imply that the approach of current seismic guidelines in adopting single valued periods of vibration and damping ratios for determining the seismic forces (ASCE, 2010) may not be appropriate. Furthermore, it is possible that damage scenarios as subtle as macro-crackingeffects of which are not observable in the post-event ambient vibration signature of the structurelead to an inferior performance in future seismic events.
Although this topic of research has been of interest over the past few decades, and the variation of vibration characteristics have been frequently reported in the literature, a systematic approach to this matter has not yet been devisedat least to the degree that results useful for engineering practice are attained. This is mainly due to the uncertain and relatively complex nature of the phenomena that are believed to be responsible for the observed variations. In other words, explicit and deterministic modeling of the causes/sources of the said variability and carrying out a parametric study of their effects is not a feasible task. Therefore, the issue should be addressed in a statistical sense and through the analyses of data collected from a large number of instrumented facilities.
The launch of the ANSS (Advanced National Seismic System), which is a collection of national and regional instrumentation and seismic monitoring networks, has created a rich inventory of seismic data. Furthermore, continuous monitoring of structures to collect records of ambient vibration has become feasible, thanks to the widespread availability of adequate data acquisition and archiving technologies. A good example of such comprehensive monitoring efforts is described in Dunand et al. (2004, 2006), where strong motion and ambient vibration data are acquired from a number of buildings permanently instrumented by USGS. Parallel to these efforts, recent advances in system identification techniques provide the necessary tools for harvesting valuable information on the vibration characteristics of structures from the aforementioned wealth of data.
Research proposals are invited to identify and characterize the aforementioned variation of dynamic structural characteristics by addressing the following issues:
The effort by the postdoctoral fellow may include the development and deployment of techniques and tools to efficiently process large quantities of data.
Advanced National Seismic System: http://earthquake.usgs.gov/monitoring/anss
ASCE (2010). Minimum design loads for buildings and other structures (ASCE/SEI 7-10), American Society of >Civil Engineer, Reston, VA.
Carreño RP, Boroschek RL (2011). Modal parameter variations due to earthquakes of different intensities. Civil Engineering Topics, 4: 321-333.
Çelebi M (1996). Comparison of damping in buildings under low-amplitude and strong motions. Journal of wind engineering and industrial aerodynamics, 59(2): 309-323.
Çelebi M (2008). Comparison of recorded dynamic characteristics of structures and ground during strong and weak shaking. Special volume NATO Series E on Increasing Seismic Safety by Combining Engineering Technologies and Scientific Data [Eds. Mucarelli et al.], 99-115.
Dunand F, Rodgers JE, Acosta AV, Salsman M, Bard P-Y, Çelebi M (2004). Ambient vibration and earthquake strong-motion data sets for selected USGS extensively instrumented buildings, USGS Open-File Report 2004-1375, 31 pp.
Dunand, F., Gueguen, P., Bard, P-Y, Rodgers, J., and Çelebi, M., 2006, Comparison of the dynamic parameters extracted from weak, moderate and strong building motion, PROC. European Conference of Earthquake engineering and Seismology, paper no 1021, Geneva, Switzerland, 6-8 September 2006.
Sohn H, Farrar CR, Hemez FM, Shunk DD, Stinemates DW, Nadler BR, Czarnecki JJ (2004). A review of structural health monitoring literature: 1996-2001. Los Alamos, New Mexico: Los Alamos National Laboratory, 307 pp.
Proposed Duty Station: Menlo Park, CA; Pasadena, CA
Areas of Ph.D.: Structural earthquake engineering or related fields with extensive knowledge and skills relevant to the Research Opportunity (candidates holding a Ph.D. in other disciplines, but with extensive knowledge and skills relevant to the Research Opportunity may be considered).
Qualifications: Applicants must meet the following qualifications - Research Engineer (with particular expertise in structural dynamics, system identification, digital signal processing, computer programming and capability to organize and process large amounts of data).
(This type of research is performed by those who have backgrounds for the occupations stated above. However, other titles may be applicable depending on the applicants background, educations, and research proposal. The final classifications of theposition will be made by the Human Resources specialist).
Research advisor(s): Mehmet Çelebi, (650) 329-5623, firstname.lastname@example.org.; Ertugrul Taciroglu (U California, Los Angeles), (310) 267-4655, email@example.com.
Human Resources Office Contact: Lisa James, (916) 278-9405, firstname.lastname@example.org.
|Summary of Opportunities|