Earthquake engineering

1. STRUCTURAL CHANGE DETECTION APPLYING LONG-TERM
SEISMIC
INTERFEROMETRY BY DECONVOLUTION METHOD TO A MODERN
CIVIL
ENGINEERING STRUCTURE (NEW ZEALAND)
Presented As A Required Task For Earthquake Engineering And Structural Design
By: MRS M
MR A

2. INTRODUCTION
Structural earthquake response mostly recorded by sensors is a combination of
 The ground input shaking into the structure,
 The coupling of the building with the ground (i.e. soil-structure interaction SSI),
 The mechanical properties of the building.
However, separating the building response from the input earthquake shaking and
from the SSI helps us estimate the current state of a structure and its safety or
operability.

3. INTRODUCTION
In 2006, Snieder and Safak [1] pioneered a method to estimate the dynamic properties
of a structure called seismic interferometry by deconvolution (SIbyD). This method
provides a wave propagation model for the building, removing the effect of seismic
ground motion by considering the sensor of the last floor as a reference for
deconvolution. However, the relationship between wave velocity and resonance
frequency proposed by Snieder and Safak [1] was based on pure-shear beam model
assumption.

4. AIM OF STUDY
To improve the understanding of the dynamic response of buildings to
earthquake shaking and to confirm the significance of Seismic
Interferometry By Deconvolution (SIbyD) method for Seismic Structural
Health Monitoring (S2HM).

5. EXPERIMENTAL SET UP
Here they analysed seismic data from the actual Te puni building and studied the building response to 208 earthquakes
recorded over 8 years. 12 seismic sensors (tri-axial strong motion accelerometers) and 5 LVDT (Linear Variable
Differential Transformer) instruments were installed in the building, a wind sensor at the roof to monitor wind
parameters (speed, direction) and a GPS for time synchronization. The building is located on a class B rock according to
NZSSDA.
Figure 1: Side view (top) and plan view (bottom) of the building seismic array with the location of the
accelerometers.

6. DATA
Table 1: Data sequences considered and corresponding number of
events
The magnitude of selected earthquakes ranges from 2.4 to 7.2 with epicentral distances from 8.6 to
630 km and depths from 5 to 241 km. The data were classified in five periods as shown below in order
to monitor changes during major earthquakes sequences.

7. METHODOLOGY
The RF of the building was first roughly estimated by computing the mean Fourier transform of the first
10 early weak motion data to estimate the resonance frequency of the building as1.56Hz (Trans.) and
1.44Hz (Long.). This is used to determine the frequency band for further estimation.
Acceleration waveform are integrated twice and filtered using the 4-th order Butterworth filter between
0.5 and 3.5Hz for transverse component and 0.5 and 2.9Hz for longitudinal.

8. METHODOLOGY
For the analysis of the velocity changes recordings were band-pass filtered using Butterworth filter with
cut-off frequencies of 0.5 and10 Hz where the seismic energy is concentrated. Deconvolution was then
performed using water-level regularization technique to determine the impulse response between the
sensors. The deconvolved traces were resampled 2 times for a better accuracy of the time delay. First,
the whole-time delay between top and bottom sensors to get information on the variation of the
global building properties, in relation to the resonance frequency was considered. Second, the SIbyD
was applied to all sensors (1–8) for locating the level along the building height with the maximum
contribution to the observed changes.

9. RESULT
Figure 2a presents variation of pulse-wave velocity β as a function of structural drift. The results in
Fig.2b present the variation of velocity in time and show a step-like decrease of the velocity values
right after each earthquake in both directions, with a significant drop in period P2.
Fig 2: Time variation of the wave propagation velocity

10. RESULT
Frequency and velocity are correlated to the nature of the structure behavior, characterized by the
beam-like structure response using the Timoshenko model.
Fig 3: Time variation of the wave propagation for frequency computed
by BTF

11. RESULT
Based on the Timoshenko beam model, the variation over time and according to structural drift of the
correction factor χ(C). χ(C) is considered as the proxy of the structural behavior used for monitoring the
co-seismic and the long-term variation of the response.
Fig 4: Time variation of the wave propagation velocity for χ(C)
coefficient

12. RESULT
We observe delay appearing in the lower part of the building, starting from sensor 6. In the transverse
direction, differences are the largest between sensors 5 and 4 and constant from sensors 4 to 1.
Fig 5: Location of damage along the Te Puni building height

13. CONCLUSION
 It is observed that the stiffness of the structure decreases over time, even though no structural damage
was observed.
 It is observed that χ(C) is decreasing in transverse direction over the time with increase of drift and in
longitudinal direction it is unstable.
 SIbyD was sensitive enough to detect the change in the design of the Te Puni building and indicate
clearly the floor with lower structural stiffness concentrating the most the seismic stress.