Detection of damage by tracking building natural frequencies using empirical mode decomposition of dynamic response
Abstract
The dynamic response of a building subjected to a seismic ground motion could be analyzed using the Hilbert-Huang Transform (HHT) to acquire useful information about the building. An assumption of piecewise linear behavior of the building response is necessary to apply the proposed procedure to estimate the dynamic characteristics of the building. Earthquake response records obtained from two buildings, namely, the 7story Holiday Inn building in Van Nuys, California during the 1994 Northridge earthquake and the 6-story Imperial County Services Building, Imperial Valley, California during the 1979 Imperial Valley earthquake are used to illustrate the procedure. Several examples depicting the ability of HHT, specifically the Empirical Mode Decomposition (EMD) to decompose dynamic response records to corresponding underlying dynamic process signals, also known as Intrinsic Mode Functions (IMFs), are presented. IMFs can be assumed to relate to certain natural modes of vibration of the structure (in general the first few modes). A procedure to identify the fundamental mode of vibration of a structure is developed and presented. Examples of single degree of freedom systems are presented to illustrate the use of EMD to identify the fundamental mode of vibration based on the acceleration and the displacement response of a system. Several multi-degrees of freedom systems are presented to indicate the applicability of the developed procedure to identify changes in the stiffness of the system. The procedure is based on the fact that the damage in a structure at any story other than the first (ground) story leads to an increase in the effective modal mass ratio of the second mode of vibration. In addition, the modal participation of the second mode increases at the stories below the damage story. Hence, the second natural mode of vibration is observable more prominently at the degrees of freedom below the damaged story of a building. The method was then applied to the acceleration data obtained from various floor levels of the two buildings mentioned earlier, to demonstrate its applicability to process earthquake response from actual buildings. The proposed procedure is useful and easy to apply to identify structural damage in buildings. The information obtained through the procedure contains a global as well as a local perspective of damage in the structure; however, the exact location of the damage within a story cannot be determined as the proposed procedure assumes floors to be rigid diaphragms. It was found that the data just before and just after the strong ground motion contains the most useful information to estimate the dynamic characteristics of the building. The advantages and limitations of the procedure are summarized.
Degree
M.S.C.E.
Advisors
Irfanoglu, Purdue University.
Subject Area
Civil engineering
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