Structure Flexibility Identification Through Impact Testing

Structural health monitoring, as the current security system of large-scale civil engineering structures, plays an important role in the normal service conditions of structures, emergency warning, assessment of performance degeneration, decision-making of maintenance management and so on. This thesis mainly aims at researching on the method of identifying structural displacement flexibility and strain flexibility based on impact testing. Research contents and innovations are as follows:The first is about the theory of un-scaled flexibility identification from output-only data. This thesis compares three different computing methods of displacement frequency response function (FRF). The magnitudes of displacement FRF calculated from impact test data are exactly the same as those calculated from impulse response function (IRF), which equal the theoretical displacement FRF from structural inherent parameters (mass, stiffness and damping). However, the magnitudes of the displacement FRFs calculated from output-only data are different from the theoretical one, but the magnitude ratios are mode-dependent. The magnitude ratios consist of the intensities of ambient forces and modal parameters identified from ambient vibration data. The un-scaled flexibility can be calculated by FRFs from ambient vibration, whose magnitude ratios are the same with the real displacement flexibility. The identified un-scaled flexibility is practically significant in that it reveals the load-deflection characteristics of a structure, which contributes to engineers'further understanding about the structure's safety condition. Numerical and experimental examples illustrate the effectiveness of the theory of un-scaled flexibility identification from ambient data.The second is about displacement flexibility identification under the condition of unknown mass. This thesis studies the relationship between displacement flexibility and displacement FRF, and it is found that the value of displacement FRF in the frequency of zero is equal to the displacement flexibility coefficient. The calculation formulas of displacement flexibility are respectively derived from real and complex FRF. The formula from the real FRF requires normalized displacement mode shapes, while the formula from the latter requires the modal scaling factor of the structure. Basic modal parameters (natural frequencies, damping ratios and displacement mode shapes), the system poles and modal scaling factors are identified from the complex mode indicator functions (CMIF) method. Substituting these parameters into the formula of complex FRF can identify the displacement flexibility under the condition of unknown mass. This method is extremely valuable in practical application. Simply-supported steel beam structure in laboratory illustrated the effectiveness of the displacement flexibility identification method by the CMIF, and the good result of displacement flexibility was obtained.The third is about the long-gauge strain modal theory. In this thesis, a series of theoretical formulas of long-gauge strain FRF are derived and it is found that the long-gauge strain FRF is similar to that of displacement in expression. However, the strain FRF does not have symmetrical characteristic as displacement FRF. Not only can the natural frequencies, damping ratios and system poles be identified, but also strain mode shapes and displacement mode shapes can be identified respectively from any column and row of long-gauge strain FRF matrix. According to the characteristics of long-gauge strain, displacement mode shapes can be identified directly from the strain mode shapes by the conjugate beam method. In actual operation, it can lead to less testing work and improve testing efficiency.The fourth is about the combined identification of displacement flexibility and strain flexibility under the condition of unknown mass. In this thesis, traditional CMIF method of identifying displacement flexibility is improved and successfully used for strain flexibility identification under the condition of unknown mass. By comparing computing formulas respectively derived from displacement FRF and strain FRF, it is found that the strain modal parameters can also be used to identify the displacement flexibility only through the strain modal parameters. This method can speed up the testing progress and improve efficiency. The finite element model of a complex bridge structure and experimental examples are used to verify the effectiveness of the combined identification method. What's more, modal truncation error, robustness, noise immunity and sensors layout scheme are also analyzed to further verify its effectiveness.The fifth is about damage detection based on strain parameters. The thesis studies structural damage detection by employing strain mode shapes and strain flexibility matrix identified from strain modal analysis. In the research on damage detection of the simply-supported steel beam, three damage indexes, the mode shape change, mode shape curvature change and mode shape curvature square change are respectively calculated. It is found that damage indexes based on displacement is less sensitive than strain ones, which reflects the superiority of damage detection by using strain parameters. In the research on damage detection of the finite element model of a complex bridge structure, in addition to the above three mentioned damage indexes, another two indexes, the strain flexibility change and the strain uniform load surface curvature change, are studied. The damage simulated in the finite element model is successfully identified by all the five indexes. The method of damage detection based on strain parameters investigated in this thesis has practical values in engineering because the structural mass is not required.