Structures & Sensors · Β) Installation Details 8 C) Natural Period Extraction 9 C1) Natural...

Structures & Sensors

Seismic Performance Evaluation through Measurements

Structures & Sensors Seismic Performance Evaluation

through Measurements| 2

Context

Introduction 3

In general 3

Assignment 3

Information about the building 3

Measuring system - Positioning 4

Measurement Results 4

Seismic Performance Evaluation 5

Annexes 7

Α) Measuring System 7

Β) Installation Details 8

C) Natural Period Extraction 9

C1) Natural period extraction through ambient vibration measurements 9

C2) Extraction – Verification of Natural Periods through stronger seismic excitation events 11

D) Methodology of Seismic Performance Evaluation 14

References 16



Introduction In general

It is a fact that our structures lose their strength and integrity over time, this combined with increased probability of earthquakes in earthquake prone countries form the need for evaluating their structural quality and performance today. In this context Structures & Sensors combining the advances in Civil Engineering practices during the last decades as well as real measurements of structural response (natural frequencies of vibration) offers a fast and cost effective Evaluation of Seismic Performance. In this process, a prediction of the structure’s response during future earthquakes is made along with the characterization of the need (low, medium or high) for a further full scale structural seismic Assessment study along with possibly material strength tests. This Technical Report concerns the Evaluation of Seismic Performance of a building on ……… . Below are the results (measurements and seismic assessment). Also, the annexes describe the measuring system, the positioning details as well as the methodology followed for the processing of the measurements and the Evaluation of the Seismic performance.

Assignment

The assignment was made by ………..

Information about the building

The building under evaluation consists of 7 floors and ground floor without walls (Picture1), it was constructed in 1990 according to the no. ...... Building permit of Urban Planning ....... . It is a typical construction of the early 1990s in Greece of reinforced concrete. There is no reliable ground foundation information.



Picture 1. The Structure under Evaluation

Measuring system - Positioning

The measuring system was developed by S & S and the installation was done by S & S. Two measuring systems were used, one of which is located at the top of the structure and the other in the basement. The addresses given below are oriented towards the road as follows: X-X vertically to it, Y-Y parallel to it. More information can be found in Annexes A and B.

Measurement Results The proposed installation time of the system is 1 week (Session). However, in the particular building, it was placed for three sessions. The recorded time histories were subjected to a fast FFT transformation and from the set of dominant frequencies those of the structure were evaluated and are presented in Table 1. The procedure followed is based on ambient vibration measurements (Appendix C). Due to intense seismic activity southwest of Zakynthos, Greece during this structural integrity monitoring, there was confirmation of the estimated eigenfrequencies due to stronger stimulation-response of the structure (Table 1). Here it is noted that there is a variation (reduction) of 4% -6% in the eigenfrequencies from the ambient vibrations compared to those of the strongest stimulation, which is absolutely expected according to the literature [1].



Χ-Χ axis Y-Y axis

Ambient Vibr. Measurements Stronger Seismic Excitation Ambient Vibr.

Measurements Stronger Seismic Excitation

f1x (Hz) T1x (sec) f1x,e (Hz)

T1x,e (sec) Difference f1y (Hz) T1y (sec) f1x,e

(Hz) T1x,e (sec) Difference

4.9 0.204 4.64 0.216 5.9% 4 0.25 3.83 0.261 4.4%

Table 1 Dominant Eigenfrequency & Natural Period per building axis from ambient vibration measurements and stronger seismic excitation

Seismic Performance Evaluation The methodology used for seismic assessment (Appendix D) is based on the philosophy of modern seismic assessment regulations (Eurocodes, FEMA). In these, the building's response is simulated by the response of an equivalent single- degree of freedom oscillator that is excited in an earthquake. In small excitations, the oscillator response will be consistent with the measured frequency and a characteristic damping according to the type of construction. In larger excitations due to the non-linear response of the structure (crack opening, eg in concrete, hysterical damping etc.) and depending on the type of building the basic natural frequency and damping are corrected according to the code suggestions. The tool used for the above correction but also for determining the maximum peak displacement that the structure can take up until failure is the so-called Push-Over curves, which result from non-linear static analysis. Typical Push-Over curves for the typical buildings of the Greek Territory up to about 2000 are given in [2].

The building under evaluation consists of 7 floors and a ground floor, it has a regular form of wall-filling on the floors other than the ground one as well as a reinforced concrete elevator core. It was constructed at the beginning of 1990 according to the 1959 Greek Antiseismic Regulation and the additional articles of 1984. Therefore, the type of building is obtained according to the suggestion in [2] as T2K80A84. For the estimation of seismic assessment, the methodology proposed by KANEPE (Greek Adjustment of Eurocode 8 part III for reinforced concrete buildings, Art. 5.7.4.2) and FEMA 356 is used. Initially, a targeted displacement (demand) is calculated according to the measured natural period, the EC8 design seismic spectrum and the appropriate correction coefficients, then this value is compared with the failure roof displacement given in [2]. The regulation suggests computation of at least two different Push-Over curves. The first is computed after charging with a triangular load distribution, and is representative for normal buildings that are satisfactorily simulated by a single degree of freedom oscillator model. The failure roof displacement as well as the targeted displacement per axis for this case are given in Table 2.

Δufailure(m) Δudemand(m)

x-x 0.035 0.019

y-y 0.035 0.028

Table 2 Failure Roof Displacement and Targeted Displacement per axis for triangular distribution



The second curve arises after loading with a different type of distribution that expresses the possible influence of higher mode vibrations, a phenomenon that becomes more pronounced in non-normal buildings. A rectangular load distribution is applied to [2]. The failure displacement as well as the targeted displacement per direction for this case are given in Table 3.

Δufailure(m) Δudemand(m)

x-x 0.015 0.019

y-y 0.015 0.028

Table 3 Failure Roof Displacement and Targeted Displacement per axis for rectangular distribution

The building under evaluation is considered as normal due to regularity in plan and elevation but also due to the fact that no other significant higher eigenfrequencies are identified in a 0-20 Hz band (see Annex C) both in ambient vibration and stronger seismic excitation measurements.

Considering the measurement results the following analysis and interpretation, there is a sufficient probability (and not a certainty) that the building will perform adequately in a major earthquake event. Therefore, the need for performing a further full scale structural seismic Assessment study along with possibly material strength tests is low.



Annexes Α) Measuring System



Β) Installation Details

System installation (Picture 2) is a basic parameter for the quality of the measured results. It is preferred to place the sensor directly on the reinforce concrete element after the plaster coat has been removed. Alternatively, if the surface is judged intact (without cracking through), it can also be placed on top layers, including paint. Placement is done by providing a strong adhesive that stabilizes a metal plate within a few minutes. The sensor is then mounted on the particular plate with a powerful magnet and is suitably oriented according to the installer's judgment, who assumes the responsibility of proper positioning. The installation position for buildings made of reinforced concrete for the first unit is in the last satisfactorily dimensioned reinforced concrete slab. Where this is not feasible, placement can be carried out vertically on the side of a load-bearing element (eg column, beam). In the building in question, the recording system was placed on the 7th-floor ceiling plate. The position of the second unit is at the foundation level and when this is not possible on the basement floor.

(a)

(b)

Picture 2 Measuring Units and Installation points (a) Building Top (b) Building Base



C) Natural Period Extraction

C1) Natural period extraction through ambient vibration measurements



(a) (b)

(c) (d)

Figure 1 Average Fourier transorm magnitude for (a) Top Χ-Χ axis, (b) Top Y-Y axis, (c) Bottom Χ-Χ axis, (d) Bottom Y-Y axis

Figure 2 Spectrogram from 0Hz to 25Hz for Top in X-X direction per week



Figure 3 Spectrogram from 0Hz to 25Hz for Top in Y-Y direction per week

C2) Extraction – Verification of Natural Periods through stronger seismic excitation events

During the second week of measurements there was a seismic M6.8 intensity event centered 32km southwest of the village of Mouzaki in Zakynthos, Greece (distance from the structure under evaluation > 200 Km). The event was recorded by the installed measuring systems (Figure 4). Figures 5 and 6 illustrate the power of the earthquake signals vs frequency for each direction from the recording systems located at the top and bottom of the building. Finally, in Figure 7, the structure’s transfer function is depicted where the dominant oscillation frequencies are clearly displayed per direction. It is calculated for each frequency by the division of the Fourier coefficient of the top to that of the base of the building

Figure 4 Time histories per axis Χ-Χ & Υ-Υ of the response of the structure under evaluation due to Zakynthos Earthquake

(~200Km Athens-epicenter)



Figure 5 Power spectrum from 0Hz to 25Hz for building top for axis Χ-Χ & Υ-Υ of the response of the structure under

evaluation due to Zakynthos Earthquake (~200Km Athens-epicenter)

Figure 6 Power spectrum from 0Hz to 25Hz for building base for axis Χ-Χ & Υ-Υ of the response of the structure under




Figure 7 Transfer function from 0Hz to 50Hz for building top for axis Χ-Χ & Υ-Υ of the response of the structure under




D) Methodology of Seismic Performance Evaluation

The building under evaluation consists of 7 floors and a ground floor, it has a regular form of wall-filling on the floors other than the ground one as well as a reinforced concrete elevator core. It was constructed at the beginning of 1990 according to the 1959 Greek Antiseismic Regulation and the additional articles of 1984. Therefore, the type of building is obtained according to the suggestion in [2] as T2K80A84. Typical Push-Over curves for triangular and rectangular load distributions for this building type as well as the points of flexural and shear type failure are depicted in Figure 8.

(a)

(b)

Figure 8 Typical Push Οver curves of Τ2Κ80Α84 building type (a) For triangular load distribution, (β) For rectangular load distribution

The targeted displacement is computed from the relation proposed in FEMA 356 as well as in Greek KANEPE (ar. 5.7.4.2)

( ) ( )2 20 1 2 3 / 4 et C C C C T Se Tδ π= (D1)

where C0 to C3 are coefficients that take into account the phenomenon of difference in response at the top of the actual elastoplastic structure and the elastic equivalent system, the influence of the shape of the hysteresis loop and the influence of the second order phenomena (P-δ), where Se (T) is the acceleration of the elastic design spectrum by EC8 - 1 for the equivalent period Te of the structure calculated according to the following equation.

0e

e

T KTK

=

(D2)

Where T is the elastic dominant natural period in the direction considered, K0 is the elastic lateral stiffness for the building type under consideration and Ke is the equivalent lateral stiffness calculated after the Push Over curve has been two way



linearized according to the code provisions. The elastic dominant natural period is the value resulting from the results of the acceleration measurements for the building under consideration [3].

The values for coefficients, natural and equivalent periods, spectral acceleration, failure and targeted displacements per direction and per Push Over curve type for the building under consideration are given in Tables 5 and 6.

C0 C1 C2 C3 Sa T(sec) Te(sec) Δufailure(m) Δudemand(m)

x-x 1.50 1.50 1.405 1.00 0.48g 0.204 0.226 0.035 0.019

y-y 1.50 1.50 1.367 1.00 0.48g 0.25 0.277 0.035 0.028

Table 5 Data for Evaluation of Seismic Performance for Push-Over curve resulting from triangular load distribution for the 2 main building axis

C0 C1 C2 C3 Sa T(sec) Te(sec) Δufailure(m) Δudemand(m)

x-x 1.50 1.50 1.405 1.00 0.48g 0.204 0.227 0.015 0.019

y-y 1.50 1.50 1.367 1.00 0.48g 0.25 0.277 0.015 0.028

Table 6 Data for Evaluation of Seismic Performance for Push-Over curve resulting from rectangular load distribution for the 2 main building axis

The graphical representation of the results is given in Figures 8 and 9.

(α)

(β)

Figure 8 Graphical representation of Evaluation of Seismic Performance for Push-Over curve resulting from triangular load distribution. (a) x-x axis (b) y-y axis



(α)

(β)

Figure 8 Graphical representation of Evaluation of Seismic Performance for Push-Over curve resulting from rectangular load distribution. (a) x-x axis (b) y-y axis

References

[1]: Dunand, François, Pierre-yves Bard, and Janise Rodgers., [2006]. "COIMPARISON OF THE DYNAMIC PARAMETERS EXTRACTED FROM WEAK, MODERATE AND STRONG MOTION RECORDED IN BUILDINGS."

[2]: Vintzilaiou E., Zeris X., Repapis K., [2003]. "Assessment of the behavioral factor of existing reinforced concrete buildings", Final Report, Organization for Earthquake Protection (OASP), Athens, Greece.

[3]: The use of the equation (D2) for the calculation of the equivalent period basically combines the value of the actual measured elastic natural period with the assumption that the Push-Over curve for the building under evaluation agrees both regarding to the failure displacements as well as the shape (slope ratio) to that of the appropriate category given in [2]

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