Verification test for integrity of anchorage

Electric and mechanical equipment has often experienced seismic damage to its foundation anchorage portion. As a result of the 2007 Niigata-ken Chuetsu-Oki earthquake, it was reported that the anchor bolts of the main transformer (seismic design class C) installed outdoors, on the ground, at Kashiwazaki-Kariwa nuclear power plant were broken. In 2008, TEPCO conducted ultimate load testing by exerting dynamic loads on a mechanical foundation embedded in concrete using a large-scale shaking table at the National Research Institute for Earth Science and Disaster Prevention. The purpose was to analyse the seismic safety margin embedded in the design method [59].

FIG.54. Elastic energy spectra inside reactor building of Fukushima Daiichi Unit 6.

FIG. 55. Comparison of elastic energy spectrum of NUPEC/JNES test and observed at Fukushima Daiichi plant Unit 6.

Damping Ratio: 0.05

TABLE 31. SEISMIC MARGIN ESTIMATED FROM ELASTIC ENERGY SPECTRUM Floor in the reactor building of

Fukushima Daiichi plant Unit 6 Seismic margin estimated

Roof (OP. 65.5 m) 8

Operating floor (OP. 51.5 m) 14

Second floor (OP. 19.0 m) 24

Base mat (OP. 1.0 m) 23

In this testing program, three series of tests, namely (a) pull-out loading test, (b) shear loading test, and (c) shaking test with real scale models were conducted. No damage occurred in the tests (c). However, the tests succeeded in verifying damage modes in the pulling test of the concrete anchorage portion, tests (a), and in the shear failure testing of anchor bolts, tests (b).

Here, the correlation between DIPs and damage occurrences in the tests in which damage modes due to dynamic loads were obtained, is discussed.

4.5.2.1. Pull-out loading test of anchorage

In these tests, the elements of the anchorage system (e.g. anchor bolt diameter, embedment depth, and loading conditions) were systematically varied and 13 types of test specimens in the shapes shown in Fig. 56 were set on the concrete slab placed on the shaking table and vibrated one-dimensionally in a horizontal direction, as shown in Fig. 57. The acceleration time-history observed on the vibration table is shown in Fig. 58.

This acceleration time-history is an artificial earthquake motion that was calculated to produce a condition of resonance in the test specimens, in order to cause damage in them. Its DIPs are calculated as shown in the Table 32.

As a result of observation of the cross section of the bolt embedment after vibration, shear cone failures, which are peculiar to pull-out loading, were observed in two test specimens (Fig. 59).

The pull-out load was calculated from the maximum response acceleration value that acted on each of the test specimens. The results of comparing it with the allowable pull-out load calculated with the design formula are shown in Fig. 60. The design allowable load IIIAS shown in Fig. 60 is the allowable load calculated from the anchor bolt design method for the elastically dynamic design earthquake ground motion Sd that is employed in Japan as ‘elastically dynamic design basis earthquake ground motion’.

Test results provided important knowledge on ‘the dependency of the DIP threshold value upon the design method’, regarding the damage to the concrete side of the anchorage as discussed below.

In this testing, shear cone failures occurred in the concrete when the JMA instrumental seismic intensity was 4.3 (JMA seismic intensity grade 4). This value is considerably smaller than about grade 6 in the former JMA seismic intensity scale, experienced at general industrial facilities during the 1995 Southern Hyogo prefecture earthquake (shown in Table 10). It can be seen that both specimens in which shear cone failures were observed had anchor bolts, the embedment

FIG. 56. Test specimen for pull-out loading [59].

FIG. 57. Pull-out specimens on a concrete slab placed on the shaking table [59].

FIG. 58. Acceleration waveform observed on the shaking table in pull-out vibration test.

TABLE 32. CALCULATED DIPS OF THE INPUT MOTION (OBSERVED ON THE SHAKING TABLE)

Type of DIP DIP Value

ZPA 399 Gal

AJMA 49.7 Gal

IJMA 4.3 (JMA seismic intensity scale: 4)

Standardized CAV 1.71 g-sec

FIG. 59. Internal crack (Specimen I-1-4) [59].

FIG.60. Comparison of maximum pull-out force in vibration test and design allowable value [59].

depth of which was set to be extremely small to let shear cone failures occur (embedment depth/bolt diameter = 1.4 to 1.3). On the other hand, general anchor bolt design normally employs an embedment depth more than ten times larger than the bolt diameter, which is nearly found in test specimens I-1-1C, 5C and 6C. In the anchor bolt design method, the pull-out strength (design allowable) increases almost in proportion to the embedment depth. In reference to the margins of these test specimens, therefore, the AJMA threshold value will be larger than approximately 200 Gal (IJMA=5.6), if the maximum allowable value was employed in the design.

4.5.2.2. Shear loading test of anchor bolts

In this test, weights were added to a steel plate (1600 mm x 1600 mm x 25 mm) simulating a baseplate for mechanical equipment, which was anchored to the concrete slab with four bolts (nominal diameter: 8 mm). Twelve test specimens (see Fig. 61) were placed and vibrated on the shaking table (see Fig. 62). The design was varied so that the load in the test is 0.5, 1.0, and 2.0 times the design allowable load. Other major variations were the presence or absence of a sleeve and the presence or absence of a concrete foundation base. The initial tightening torque for the bolts was reported to be 12 N m. A greased stainless-steel plate was inserted between the steel plate simulating the baseplate and the concrete, to create a very conservative testing condition without friction force between the baseplate and the concrete foundation.

As the waveform of the vibration input to the shaking table, the acceleration time-history observed on the reactor building basemat of the Kashiwazaki-Kariwa nuclear power plant Unit 1 in the east-west direction during 2007 Niigata-ken Chuetsu-Oki earthquake was used. The maximum acceleration observed actually on the concrete slab on the shaking table was reported to be 1270 Gal. Table 33 shows the results of DIP calculations with the maximum acceleration being 1270 Gal.

Due to this excitation, the anchor bolts of two of the test specimens listed in Table 34 broke, as shown in Fig. 63. The test specimens were observed to be moving on the concrete slab.

This result has given valuable information that the margin against the shear failure of anchor bolts under Japan’s seismic design method is from one to two, when expressed as the quotient between the maximum load produced by earthquake acceleration and the design allowable load

(IIIAS), when the friction force between the mechanical foundation and the concrete is ignored.

This result is worth analysing from the perspective of DIPs.

According to Japan’s anchor bolt seismic design method, test specimens 1-2, 1-3 and II-1-4 would reach their design allowable values when the static seismic coefficient is 0.70, 0.41, and 0.25, respectively. Based on this vibration test result, therefore, test pieces with the design static seismic coefficient of 0.41 (400 Gal) could withstand the seismic motion input of AJMA=492 Gal, while those with the design static seismic coefficient of 0.25 (245 Gal) could not. Considering that the strength of actual materials is greater than the design standard value, it can be said that the AJMA , as a DIP for first excursion damage, is a parameter close to the concept of static seismic coefficient.

On the other hand, friction force is working between the mechanical foundation and the concrete due to dead weight. The initial tightening torque is also contributing to the friction force. Nevertheless, it needs to be considered that the frictional force due to dead weight will decrease because of vertical motions.

FIG. 61. Test model for shear loading (Type B) [59].

TABLE 33. DIP ESTIMATION OF THE ACCELERATION INPUT TO THE SHAKING TABLE

DIP

Input to the shaking table

(Estimated)

Earthquake motion observed in the east-west direction on the reactor building basemat of

Kashiwazaki-Kariwa plant Unit 1 (2007 Niigata-ken Chuetsu-Oki earthquake)

Maximum acceleration (Gal) 1270 680

AJMA (Gal) 492 263

IJMA(JMA seismic intensity scale) 6.3 (6-upper)

5.7 (6-lower）

Standardized CAV (g-sec) 2.44 1.16

FIG. 63. Damage in bolt and surrounding concrete [59].

When JMA seismic intensity grade 6-upper (approximately 340 Gal in AJMA) is observed, it can be concluded that anchor bolts of conventional facilities, including Seismic Class C components at Japanese nuclear power plants, are to be checked, for example, with respect to the design margin (ratio of the design load to the design allowable load) and so on, in order to find potential hidden damage.

4.6. APPLICATION OF DAMAGE INDICATING PARAMETERS TO POST-EARTHQUAKE ACTIONS AND SEISMIC INSTRUMENTATION SYSTEM This section proposes the application of the DIPs discussed in chapters 3 and 4 to enhance the reliability of during and post-earthquake actions addressed in IAEA Safety Report Series 66, as mentioned in Section 1.

Table 35 summarizes the ideas on how to utilize DIPs and it gives the corresponding sections where the suggested utilization is discussed in detail.

TABLE 34. SPECIFICATION AND RESULTS OF SPECIMENS IN VIBRATION TEST FOR SHEAR LOADING [59]

Specimen Mode Type¹ Bolt

diameter Embedment

Notes: (1) Type A: Without concrete equipment base / Type B: With concrete equipment base

(2) Observed Load = (Maximum load by maximum acceleration) / (Design allowable load IIIAS)