SHAKING TABLE TEST OF STEEL FRAME STRUCTURES SUBJECTED TO NEAR-FAULT GROUND MOTIONS

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1 3 th World Conference on Earthquake Engineering Vancouver, B.C., Canada August -6, 24 Paper No. 354 SHAKING TABLE TEST OF STEEL FRAME STRUCTURES SUBJECTED TO NEAR-FAULT GROUND MOTIONS In-Kil Choi, Young-Sun Choun 2, Min-Kyu Kim 3, Jeong-Moon Seo 4 SUMMARY Shaking table tests on the seismic behavior of a steel frame structure model were performed. The purpose of this test is to estimate the effect of a near-fault ground motion and the scenario earthquake based on the probabilistic seismic hazard analysis on the nuclear power plant structures. Three kinds of earthquake ground motions which represent the design earthquake ground motion for the Korean nuclear power plants, the scenario earthquakes for the Korean nuclear power plant sites and the near-fault earthquake record from the Chi-Chi earthquake were used as the input motion. A 4-story steel frame structure was fabricated to perform the tests. INTRODUCTION The standard response spectrum [] proposed by US NRC has been used as a design earthquake for the design of Korean nuclear power plant structures. A survey on some of the Quaternary fault segments near the Korean nuclear power plants is ongoing [2]. It is likely that these faults will be identified as active ones. If the faults are confirmed as active ones, it will be necessary to reevaluate the seismic safety of the nuclear power plants located near the fault. Near-fault ground motions are the ground motions that occur near an earthquake fault. In general, the near-fault ground motion records exhibit a distinctive long period pulse like time history with very high peak velocities. These features are induced by the slip of the earthquake fault. Near-fault ground motions, which have caused much of the damage in recent major earthquakes, can be characterized by a pulse-like motion that exposes the structure to a high input energy at the beginning of the motion. In this study, the shaking table tests of a 4-story steel frame structure were performed to estimate the nearfault ground motion effects on the seismic response of the structure. Three types of input motions, artificial time histories that envelop the US NRC Regulatory Guide.6 spectrum [] and the probability Principal Researcher, Korea Atomic Energy Research Institute 2 Principal Researcher, Korea Atomic Energy Research Institute 3 Senior Researcher, Korea Atomic Energy Research Institute 4 Principal Researcher, Korea Atomic Energy Research Institute

2 based scenario earthquake spectra developed for the Korean nuclear power plant site [3] and a typical near-fault earthquake record, were used as input motions. STEEL FRAME STRUCTURE MODEL Model Design Most nuclear power plant structures have fundamental frequencies in the 4 to Hz range. In this study, the target frequency of the model was determined based on the fundamental frequency of the containment building. The containment building is generally a prestressed concrete shell type structure with a spherical dome. The fundamental frequency of the containment building is about 4.7Hz. Since it is difficult and expensive to make a scaled model of the prestressed concrete containment building, a steel frame structure with a similar fundamental frequency was chosen to estimate the effect of frequency contents in the earthquake ground motions. Figure shows the dimension of the test model, a 4-story steel frame structure. Figure 2 shows the photo of the fabricated test model installed on the shaking table. The specification of the test model is shown in Table. As shown in this Table, a steel pipe was used for the column. The floor slab was made with a thick steel plate. The diameter of the steel pipe and the thickness of the steel plate were determined based on the target frequency. The slab and column were connected by 6 high-tension bolts at the end plate for the rigid connection. Figure. Dimension of the Test Model Figure 2. Fabricated Test Model Table. Model Specification Member Specification Dimension (cm) Column Steel Pipe OD : 4.27, t :.36 Slab Steel Plate 2x2x4 Slab-Column Connection High-Tension Bolt at End Plate

3 Frequency Analysis To verify the fundamental frequency of the test model, preliminary frequency analysis was performed. Figure 3 shows the analysis model. The slab and column were modeled using thick plate and beam element, respectively. The fundamental frequency of the model structure from the frequency analysis was 4.7Hz, which is very similar to the target frequency. Figure 3. Analysis Model INPUT GROUND MOTIONS Three sets of earthquake ground motions were used as input motions. The first one is the artificial time histories that envelop the US NRC Regulatory Guide.6 spectrum [] which is the design earthquake for Korean nuclear power plant structures. The second one is the artificial time histories of the probability based scenario earthquake for Korean nuclear power plant sites. The scenario earthquake is specified in terms of the earthquake magnitude, M, and its distance, R, from the site under consideration. The probability based scenario earthquake is developed by the de-aggregation of the probabilistic seismic hazard analysis (PSHA) results according to the procedures of the US NRC R.G..65 [4]. The spectral shape for the scenario earthquake is developed using the attenuation equations adopted in PSHA. The magnitude and distance of the scenario earthquake used in this study is M6.4, 9km [3]. The near-fault ground motion effect is incorporated into the response spectra, since the potential active fault is located near the nuclear power plant site. The last one is the real earthquake time histories recorded at the Chi- Chi, Taiwan earthquake. The station name of the recorded earthquakes is TCU 52. Figure 4 shows the input acceleration time histories scaled to g PGA (Peak Ground Acceleration). The Chi-Chi earthquake time history shows a long period pulse-like motion. This pulse-like motion can dramatically influence the spectral content in large earthquakes. The ground response spectra of those acceleration time histories are shown in Figure 5. As shown in this figure, the frequency contents of the input ground motions are very different. The response spectra of the Chi-Chi earthquake show rich

4 frequency contents in the low frequency range. But the response spectra of the scenario earthquakes show rich frequency contents in the high frequency range greater than Hz. The response spectra of the design earthquake show rich frequency contents in the medium frequency range. The fundamental frequencies of most nuclear power plant structures are located in this range A ccel eratio n(g ) Acceleration(g) Acceleration(g) Time(sec) Time(sec) Time(sec) (a) NRC (b) Scenario (c) Chi-Chi Figure 4. Input Ground Motion Time Histories 4 NRC R.G..6 Scenario Eq. Chi-Chi Eq.(TCU52) 4 NRC R.G..6 Scenario Eq. Chi-Chi Eq.(TCU52) 4 NRC R.G..6 Scenario Eq. Chi-Chi Eq.(TCU52) Normalized 3 2 Normalized 3 2 Normalized 3 2 (a) EW Component (b) NS Component (c) UD Component Figure 5. Acceleration Response Spectra of the Input Ground Motions SHAKING TABLE TESTS Test Setup The size of the shaking table is 4 m x 4 m with a 6 degree of freedom excitation axes. The capacity of the table is limited to a maximum specimen weight of 3, kg, maximum acceleration of.5g and.g in the horizontal and vertical direction, respectively. The frequency range of the table is a maximum of 5 Hz. The table is controlled by electro-hydraulic servo system. The steel frame model was fixed to the shaking table with the high tension bolts for a rigid connection. Test Procedure The input ground motion level is gradually increased from g to.5g. In a related but separate test, random white noise was input to check the change of the natural frequency and the damage state at each step. The test procedures are summarized in Table 2. As shown in this table, the -D, 2-D and 3-D tests with three kinds of input motion were performed. The.5g level tests with the Chi-Chi earthquake records

5 could not be performed due to the displacement limit of the shaking table. The maximum displacement of the Chi-Chi earthquake record at.5g PGA exceeds the table displacement limit. Table 2. Test Procedures Step Input Motion Direction PGA (g) White Noise 3-D.5 2 Chi-Chi -D.2 3 Scenario -D.2 4 NRC -D.2 5 White Noise 3-D.5 6 Scenario -D.5 7 NRC -D.5 8 White Noise 3-D.5 9 Chi-Chi 2-D.2 Scenario 2-D.2 NRC 2-D.2 2 White Noise 3-D.5 3 Scenario 2-D.5 4 NRC 2-D.5 5 White Noise 3-D.5 6 Scenario 3-D.5 7 NRC 3-D.5 8 White Noise 3-D.5 Measurement A total of 2 accelerometers and one LVDT (Linear Variable Displacement Transformers) with a ±25 mm capacity were installed to measure the acceleration and displacement responses. The acceleration responses of three directions, two horizontal and vertical directions, at each story were measured with 2 accelerometers. The displacement responses were measured for only one horizontal direction at the slab of the 2nd floor. To evaluate the nonlinear response of the steel columns, the strain gauges were installed at the st story columns. TEST RESULTS Natural Frequencies The fundamental natural frequencies of the model measured at the elastic range for the 3-D white noise input with a peak acceleration of.25g are shown in Table 3 with the analysis results. As shown in this table, the natural frequencies from the white noise tests coincided with the analysis results well. The natural frequencies of the two horizontal directions were very similar, since the shape of the model structure is regular. Form this point of view, it is possible to foresee that the torsional responses can be ignored, and the responses of the two horizontal directions are very similar. Figure 6 shows the transfer functions and the phase angle obtained from the white noise tests. It can be seen from the transfer functions of the modal tests that the natural frequencies of the model did not change even when the model experienced a strong earthquake up to.5g. This means that the material properties of the steel structure did not change after a certain level of nonlinear behavior. In this figure, the difference of the amplitude was induced by the difference of the table motions.

6 Table 3. Modal Frequencies obtained from the Test and Analysis Mode No. Frequency (Hz) Analysis Test Amplitude(g/g) 3 2 Modal_ Modal_2 Modal_3 Modal_4 Modal_5 Phase Angle Fqequency(Hz) (a) Transfer Functions (b) Phase Angle Figure 6. Transfer Function and Phase Angle form the Modal Tests Acceleration Response The acceleration responses of the model were measured at every floor in three directions. To compare the peak acceleration responses, it is necessary to normalize the peak acceleration response to the required peak input acceleration. In general, the shaking table can t simulate the exact required motions due to the capacity of the shaking table. So a direct comparison of the peak acceleration response is impossible. In this study, the peak acceleration responses were normalized to g input levels. Figure 7 shows the average normalized peak acceleration responses of the transverse direction from all the tests and -D tests for the three input motions at every floor, respectively. The average peak acceleration response from the NRC input is larger than that from the other inputs. The average peak acceleration from the -D tests is higher than that from all of the test data. It means that the multi-directional input does not always produce a higher response for the regular structures. From the acceleration response of the model, it was concluded that the near fault ground motion did not always induce a large acceleration response. The acceleration response shows that the relationship between the frequency content of the input ground motion and the fundamental frequency of the structures is very important for the acceleration response of the structures. From this point of view, it can be assumed that the near-fault ground motion effect is not so significant for the general nuclear power plant structures in the elastic range, since the fundamental frequency of the massive nuclear power plant structures is about 4 Hz. But if the structure is beyond the yielding point due to a large displacement, the frequency of the structure will be reduced, and the displacement response will be increased dramatically because of the characteristics of the near-fault ground motion which is the long period impulsive motion

7 occurring at the early stage. This impulsive long period motion about second period can cause a significant damage on the long period structures Roof 4th Floor 3rd Floor 2nd Floor 4. Roof 4th Floor 3rd Floor 2nd Floor Acceleration(g) Acceleration(g) TCU SCE NRC. TCU SCE NRC (a) All Test Data (b) -D Test Data Figure 7. Average Peak Acceleration Responses Normalized to g Input Levels Displacement Response The displacement responses were measured at the first floor by LVDT. The displacement responses according to the input motions, PGA levels and input directions can t be compared directly because of the difference of the input motions generated by shaking table. The displacement response of the structure is generally dominated by the spectral amplitude at the fundamental frequency of the structure rather than the peak ground acceleration. In this study, the measured displacement responses were corrected using the spectral amplitude ratio at the fundamental frequency of the structure to compare the displacement responses. It is a reasonable method to correct the displacement response for a comparison, because the modal participation factor of the fundamental mode is about 88 percents. Figure 8 shows the response spectra of the table motion and target motion in the case of the -D tests with.2g PGA. The arrow indicates the fundamental frequency of the structure. It is found that the spectral amplitude of the table motion at the fundamental frequency is lower than that of the required target motion. Table 4 shows the measured and corrected displacement with the PGA ratio and the spectral amplitude ratio. As shown in this table, in the case of the normalized displacement to the required PGA, the displacement due to the TCU 52 and the scenario earthquakes is very similar at the same intensity level. And the displacement due to the.2g and.5g PGA scenario earthquakes is similar. For a reasonable comparison, this table shows the normalized displacement by the spectral acceleration ratio at the fundamental frequency. In the case of the normalized displacement by the spectral ratio, the displacement responses due to the NRC motion are significantly larger than those due to the other input motions. It is also noted that the displacement response increases with an increase of the input intensity level. The displacement response due to the TCU 52 earthquake is much smaller than that due to the NRC motion, since the long period characteristics of the near-fault ground motion can t affect the displacement response of the short period structures.

8 ..... Table Motion Target Motion. Table Motion Target Motion. Table Motion Target Motion (a) Chi-Chi (b) Scenario (c) NRC Figure 8. Comparison of the Response Spectra of the Target and Table Motions Table 4. Displacement Responses of the Model Structure (Unit : mm) Input Motion Required PGA (g) Table PGA (g) Measured Displacement Normalized to Required PGA Normalized by Spectral Ratio TCU D SCE D SCE D NRC D NRC D It is difficult to judge the nonlinear response by the maximum displacement responses. In the tests, the strain responses were measured using strain gauges in the transverse direction at the two columns located in the diagonal direction. The strain responses of the two columns were measured to estimate the nonlinear response of the columns beyond the yielding point. The measured strain responses were also corrected by the spectral ratio at the fundamental frequency of the structure. Table 5 shows the strain responses of the columns with the spectral acceleration ratio. In general, the yielding strain of the structural steel is about.3. As shown in this table, the model structure experienced a large strain greater than the yielding strain at the.5g inputs regardless of the input motions. The strain responses due to the NRC input are larger than those due to the other input motions. And the strain responses due to the TCU 52 earthquake are relatively small. The strain response also shows the same trends as the acceleration and displacement responses. The maximum strain response shows that the strong ground motion induced the nonlinear response of the structure. But the fundamental frequency of the model after all the strong ground motion tests did not change. This means that the material properties did not change despite the experience of a large strain.

9 Table 5. Strain Responses of the Steel Columns Input Motion Spectral Ratio Column # Column #2 Average Strain TCU D TCU 2D SCE D SCE D SCE 2D SCE 2D SCE 3D NRC D NRC D NRC 2D NRC 2D NRC 3D Floor Acceleration Response The seismic responses of the structure were investigated in the above section. Based on the test results, it seems that the design earthquake for the Korean nuclear power plants is conservative, because the fundamental frequency of the major nuclear power plant structures is greater than 5Hz. From the structural point of view, the nuclear power plant structures have enough safety margins for the earthquake ground motions. In the nuclear power plant structures, many kinds of equipment important for its safety are installed on the floor or wall with a welded or bolted anchorage. The dynamic characteristics of those equipments are various. And the failure mode due to an earthquake is composed of the structural failure mode and the functional failure mode. The structural failure mode, such as anchorage failure, is mainly dominated by the global modal response of the structures. The functional failure mode is dominated by the local response of the equipment. It is noted from the fragility analyses performed for the probabilistic seismic risk assessment that the dominant failure mode of the active components is a functional failure due to the chattering of the relay attached to the electrical equipment mounts, such as a panel, rack or cabinet [5]. In general, the relay chattering is very sensitive to the high frequency ground motions [6]. The local vibration mode of the panel induces the high frequency response of the panel. In this study, the floor acceleration response was estimated by using the floor acceleration response spectra. The floor response spectrum is used for the design of the components installed in a building structure. Figure 9 show the floor response spectra at the 2 nd floor and roof. These spectra were developed from the two-dimensional tests with a.2g PGA. As shown in this figure, the difference of the spectral amplitude increases with an increase of the frequency. The floor response spectra due to the scenario earthquake show high amplitude in the high frequency range greater than Hz. Figure shows the floor response spectra corrected by the spectral acceleration ratio of the spectral acceleration of the table motion to that of the target motion at every frequency. As shown in this figure, the response spectrum shapes are very similar to the uncorrected spectra, but the spectral amplitude is somewhat different due to the simulation error of the shaking table. Based on this result, it is noted that the high frequency ground motions can affect the safety of the equipments installed in a building. The near-fault ground motion is not so damageable for the equipments that have high natural frequencies.

10 ... NRC RG.6 Scenario Chi-Chi. NRC RG.6 Scenario Chi-Chi (a) 2 nd Floor (b) Roof Figure 9. Floor Acceleration Response Spectra... NRC RG.6 Scenario Chi-Chi. NRC RG.6 Scenario Chi-Chi (a) 2 nd Floor (b) Roof Figure. Corrected Floor Acceleration Response Spectra CONCLUSIONS In this study, the shaking table tests of a steel frame structure were performed with three different input ground motions. Three kinds of input motions, design earthquake, probability based scenario earthquake developed for the nuclear power plant site and real earthquake records from the Chi-Chi earthquake, were used for the tests to estimate the safety of nuclear power plants.

11 The acceleration and displacement responses of the structure due to the design earthquake were larger than those due to the other input earthquakes. It seems that the design earthquake for the Korean nuclear power plants is conservative, and that the near-fault earthquake and scenario earthquake are not so damageable for the nuclear power plant structures, because the fundamental frequencies of the nuclear power plant structures are generally greater than 5 Hz. The high frequency ground motions that appeared in the scenario earthquake can be more damageable for the equipments installed on the high floors in a building. This means that the design earthquake is not so conservative for the safety of the safety related nuclear power plant equipments. This test is limited to steel frame structures. The steel frame structure did not show large stiffness degradation due to the large displacement in the earthquake intensity range performed in this test. However the near-fault ground motion effect should be estimated for reinforced concrete structures that the frequency shift due to stiffness degradation by cracks is inevitable. ACKNOWLEDGEMENT This research was supported by the Mid- and Long-Term Nuclear Research & Development Program of the Ministry of Science and Technology, Korea. REFERENCES. US NRC Regulatory Guide.6, Design Response Spectra for Seismic Design of Nuclear Power Plants, Korea Institute of Nuclear Safety, Development of Seismic Safety Evaluation Technology for NPP Sites, KINS/GR-26, In-Kil Choi, Young-Sun Choun, and Jeong-Moon Seo, Scenario Earthquakes for Korean Nuclear Power Plant Site Considering Active Faults, SMiRT-7, Prague, Czech Republic August 7-22, US NRC Regulatory Guide.65, Identification and Characterization of Seismic Sources and Determination of Safe Shutdown Earthquake Ground Motion, Korea Atomic Energy Research Institute, External Event Analysis Based on Probabilistic Approaches, KAERI/CM-574/, Electric Power Research Institute, Analysis of High-Frequency Seismic Effects, EPRI TR-247, 993.