Experiments on Large Structures - Dynamic Behavior of Concrete and Seismic Engineering - page 212

Figure 6.7. Instrumentation of the building
Figure 6.8 presents the frequency response curves measured on the first floor and
the roof before damage, and after applying the standardized El Centro earthquake
signal with a 0.129 g maximum acceleration. The effects of the damage caused by
the earthquake are obvious in the figure and mostly involve an amplitude reduction,
a notable increase of the modal damping (by a factor of 2) and a downward shift of
the vibration frequencies. It is possible to quantify the damage and to locate it from
this data, using appropriate system identification techniques [DOE 96]. Ultimately,
we obtain damage “quantities” according to the stiffness reduction of the structural
components (frames either parallel or perpendicular to loading directions, for
example).
Figure 6.8. Frequency responses of the undamaged and damaged building