SAFETY CONCEPT FOR FASTENINGS IN NUCLEAR POWER PLANTS

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1 SAFETY CONCEPT FOR FASTENINGS IN NUCLEAR POWER PLANTS Thomas M. Sippel*, Jörg Asmus*, Rolf Eligehausen** *Engineering Office Eligehausen and Sippel, Stuttgart, Germany **Institute of Construction Materials, University of Stuttgart, Germany Abstract In nuclear power plants post-installed fastenings are often used. A decisive criteria for the use of fastenings in safety critical applications is the proper functioning of fasteners under special conditions such as impact loading and earthquake excitations and cracks in concrete with a width 0.5 mm due to earthquake loadings. Such special conditions are not covered by Technical Approvals according to [1]. Therefore, in Germany a guideline for the assessment of fasteners in nuclear power plants has been published [3]. This guideline is valid for undercut anchors with an embedment depth h ef 80 mm, which are approved according to [1]. In this paper the concept of the guideline is explained. Details of the required suitability tests and the tests to determine admissible service conditions are given. Furthermore, results of tests with different types and sizes of undercut anchors are shown. 564

2 1. Introduction In nuclear power plants post-installed fastenings are often used. A decisive criteria for fastenings in safety relevant applications of nuclear power plants is their proper functioning under special conditions, which may occur during the service life of the plant. Such conditions include impact loading, earthquake excitations and cracks in the concrete with a width 0.5 mm due to earthquake loading. These special conditions are not covered by Technical Approvals according to [1]. Therefore, a guideline for the assessment of fasteners intended for use in nuclear power plants has been published in Germany [3]. This guideline gives tests methods and assessment criteria. Furthermore modifications to the design method (CC-method) described in [1] and [2] are given. This paper describes the concept of the guideline [3]. It gives details of the test program, the assessment criteria and the modification of the CC-method. Furthermore results of tests with undercut anchors are shown. 2. Safety concept 2.1. General Post installed fasteners are often used in nuclear power plants to fasten pipe systems which are critical for the safe operation of the plant. Therefore the fasteners must safely transfer the loads acting on the base plate, even under extreme conditions which may occur during the service life of the nuclear power plant. These conditions include e.g. impact loading, earthquake excitations and cracks in the concrete with a width 0.5 mm due to earthquake loading. They are not covered by Technical Approvals according to [1]. Therefore according to [3] additional tests are necessary to check the suitability (proper functioning) of the fastener and to deduce allowable conditions of use under the above mentioned conditions. 565

3 2.2. Use Categories and related partial safety factors For the design of fasteners in nuclear power plants the safety concept of partial safety factors is used. It must be shown that the design actions S d are not larger than the design resistance R d (Eq. (1)). S d R d (1) In the simplest case (permanent load G k and one variable load Q k acting in the same direction as G k ) the design actions are calculated according to Eq. (2). S d = γ G G k + γ Q Q k (2) with γ G, γ Q = partial safety factors for permanent or variable load resp. The design resistance is given by Eq. (3). R d = R k /γ M (3) with R k = characteristic resistance (5%-quantile) γ M = material safety factor For fastenings in nuclear power plants, three different use categories (A, B and C) have to be considered [4]. These three categories assume different expected frequencies of the loading within the service life of the power plant (see Table 1). In category C the same loadings and requirements as in normal buildings are considered. Therefore the partial safety factors given in [1] and [2] must be used (see Table 1). In category A it is assumed that the loading will occur only once during the service life of the structure. Examples are the maximum expected earthquake, the hitting of the containment by a plane or explosions. Under these conditions it must be ensured that the nuclear power plant can be safely shut down. Because cooling water is needed for the shut down, fasteners of the corresponding cooling pipes must function properly. Therefore the safety factors γ G = γ Q = 1.0 and γ Mc = 1.7 are used. In category B a frequency of loading n 10 during the service life of the structure is assumed, such as "normal" earthquakes. The partial safety factors are in between the values valid for category A and C. 566

4 Table 1: Use categories and corresponding safety factors for fastenings used in nuclear power plants according to [3] use category A B C frequency during service life 1 10 >> 10 crack width [mm] 0.5 mm 0.5 mm 0.3 mm concrete class acc. to [7] C20/25 to C50/60 max. long term temperature 80 C partial safety factors action γ G = γ Q resistance (concrete) γ Mc = γ Mp Requirements on Fastenings 3.1. General In use category C cracks with a width w k 0.3 mm or w k 0.5 mm under the quasipermanent or allowable service load of the structure are assumed. Therefore only fasteners with a Technical Approval according to [1] may be used. In use category A (e.g. maximum expected earthquake) large cracks may occur in the concrete in the most stressed areas due to yielding of the reinforcement. The width of cracks have been evaluated based on the design actions in nuclear power plants. According to the evaluation in general the width of cracks running in one direction is w k 1.0 mm. Only in extreme cases the crack width may be w k 1.5 mm. Outside the most stressed regions where the reinforcement is strained below the yield strain the crack widths are much smaller. During an earthquake cyclic loading on the structure and on the fastenings is induced simultaneously. Due to this the width of the cracks will vary between a minimum and a maximum value and the fastenings will be loaded cyclically. These conditions must be taken into account in the test regime (see Section 4) Fasteners As a principle, only fasteners with a Technical Approval for use in cracked and non cracked concrete according to [1] should be used for safety relevant fastenings in nuclear power plants. The width of cracks under seismic excitations can not be assessed very accurately but some variations may occur. Therefore only undercut anchors with a sufficiently large undercut are allowed, because their behaviour will not be influenced significantly if even larger cracks than given above may occur. Undercut anchors (Fig. 1) transfer the load by mechanical interlock into the concrete. After producing of the cylindrical hole by drilling, the undercutting is produced in a second operation before installation of the anchor (Fig. 1a and b) or during installation of the anchor (Fig. 1c). The use of torque controlled expansion anchors is not allowed in [3] because their proper 567

5 functioning may be impaired significantly if they are anchored in a crack with a width larger than anticipated. For safety relevant fasteners an effective anchorage depth h ef 80 mm is required. Anchors with an effective anchorage depth h ef 40 mm are allowed only if very small loads must be transferred into the concrete (adm F 0.4 kn). a) b) c) Fig. 1: Undercut anchor systems 3.3. Concrete Concrete classes B25 to B55 according to DIN 1045 [5] or C20/25 to C50/60 according to EN 206 [7] are covered in the guideline [3] Protection against fire, corrosion and atomic radiation For the resistance against fire and corrosion the regulations in the Technical Approval for normal applications are valid. The influence of atomic radiation on the load-bearing behaviour can be neglected Installation The guideline [3] is valid only for fasteners, which are installed by skilled workers according to the installation instruction of the manufacture and additional requirements in the guideline. The correct installation must be controlled by independent personnel. Therefore, tests to investigate the sensibility of fasteners to installation inaccuracies (installation safety) can be omitted (see Section 4) Design concept Fastenings in nuclear power plants for safety related applications should be designed in such a way that they are ductile. The ductility can be ensured by the attachment, the fixture or the anchors. According to [3] in general in the design of the fastening consisting of attachment, fixture and anchors the failure mode concrete cone failure should not be decisive. 568

6 The design of fasteners in nuclear power plants closely follows the design model according to [1] or [2] (CC-method). However, the characteristic resistance for concrete cone failure is calculated according to Eq. (1). 0 N Rk, c = k 1,5 f cc h ef (1) with k = 6.0 f cc = concrete cube strength = effective embedment depth h ef The factor k = 6.0 is approximately 15% lower than the value according to [1] or [2] for cracked concrete with a crack width w = 0.3 mm. The lower value k considers the influence of larger cracks (w = 1.0 mm) on the ultimate load for concrete cone failure. Furthermore, the failure load V o Rk,c for concrete edge failure is reduced by about 15% compared to [1], [2]. The characteristic resistance for pull-out failure and steel shear failure is evaluated from the results of relevant tests. 4. Required Tests for Use Category A The required suitability tests and tests for evaluating allowable conditions of use are summarised in Table 2. Tests under monotonic loading are performed with normal loading rate, because the results of tests in [9] and [10] showed that anchor behavior is not negatively influenced by loading rates typical for earthquake excitations. The tests will be conducted with a crack width w 1 = 1.0 mm (reference tests) and w 2 = 1.5 mm. In tension tests with w = 1.5 mm the failure load should reach 0.8times the average value valid for tension tests with w = 1.0 mm (reference failure load) to take into account that the probability of occurrence of such wide cracks is relatively low. Furthermore tests with cyclic tension load on the anchor (n = 15 cycles) must be performed. The upper load is equal to N max = N Rk /γ Mc with N Rk = characteristic resistance evaluated from results of tests according to Table 2, line 4 and γ Mc according to Table 1 for use category A. The minimum load is N = 0. This loading represents the cyclic excitations of the fastening due to an earthquake or other dynamic loadings. During the test no failure of an anchor is allowed and the average failure load in the subsequent test to failure must be at least 70% of the reference failure load. To model the influence of cyclic excitations of the structure on anchor behaviour tests in opening and closing cracks are required. During the test the anchors are loaded with a 569

7 constant tension load N max as given above. The crack widths are varied 10 times between w 1 = 1.0 mm and w 2 = 1.5 mm. After this the crack is opened to w 2 = 1.5 mm and the anchor is loaded monotonically to failure. During the tests no anchor may fail and the average failure load in the tension tests must be at least 70% of the reference failure load. During an earthquake the anchors may be cyclically loaded in tension or shear. If reversed cyclic shear loading occurs the anchor may fail by steel rupture due to low cyclic fatigue. To check this, tests in cracked concrete (w = 1.0 mm) with 15 reversals of the shear load between V max = ± V Rk,s /γ Ms (V Rk = characteristic shear resistance for steel failure for monotonic loading and γ Ms = partial safety factor = 1.5 for anchor steel with normal ductility) must be performed. The shear load is applied in direction of the crack. During the load reversals no anchor failure may occur and the average failure load in the subsequent test to failure must be 0.9times the reference value valid for monotonic loading. Table 2: Suitability tests and tests for admissible service conditions for use category A Line Purpose of test load w n Nu Vu 2) dir. mm α = or 1) N V u,0 u,0 1) 2) 3) Suitability tests 1 monotonic tension loading N α load cycles n L = 15 N α 0.7 3) 3 crack movements n R = 10 N 1.5/1.0 5 α 0.7 3) Admissible service conditions 4 monotonic tension loading N reference ultimate load for tension tests 5 monotonic shear loading V reference ultimate load for shear tests 6 load cycles n L = 15 V α 0.9 3) N = tensile load, V = shear load N u (V u ) = average failure load in tension (shear) tests, N u,0 (V u,0 ) = reference average ultimate load in monotonic tension (shear) tests with w 1 = 1.0 mm no failure during cyclic loading or crack moving 570

8 5. Test Results In the following typical test results of different types and sizes of undercut anchors performed according to the above described concept of the guideline [3] are shown. Tension tests have been performed in line cracks with a width w = 0.3 mm to 1.7 mm. In general, with the investigated fasteners concrete cone failure was observed. Fig. 2 shows typical load displacement curves of undercut anchors tested in cracks with a width w = 1.5 mm. The behaviour is not significantly different compared to tests in w = 0.3 mm; only the anchor stiffness is reduced and the scatter of the test results may be somewhat larger. In Fig. 3 the ratio of measured to calculated failure loads in cracked concrete are plotted as a function of crack width. It demonstrates that the measured ultimate loads exceed the characteristic resistance for concrete cone failure according to Eq. (1). Furthermore, with increasing crack width no significant reduction of failure loads can be observed up to a crack width w 1.7 mm. The tension behaviour of anchors in cracks with constant width may be influenced by cyclic loads on the anchor. Therefore tests under repeated loading have to be carried out in cracked concrete (w = 1.5 mm) applying 15 load cycles. Fig. 4 shows a typical loaddisplacement curve of an anchor subjected to repeated loading with subsequent tension loading to failure. During cyclic loading the anchor displacement increase, however the failure load is generally not much influenced by the previous load cycles. Typical results of tests with opening and closing cracks using different types and sizes of undercut anchors are shown in Fig. 5. In Fig. 5 the average displacement of 5 test per series with anchors in line cracks loaded with a constant tension load N p = 0.6 to 0.7 N 0 Rk,c (N 0 Rk,c according to Eq. (1)) is plotted as a function of the number of crack openings. In the tests the maximum crack width amounts to w 2 = 1.5 mm and the minimum crack width was about 1 mm. With increasing number of crack openings the displacements increase. This increase depends mainly on the load bearing area of the undercut system, the magnitude of the constant tension load on the anchor and the number of crack openings. During the crack openings none of the tested anchors failed. However, anchors with an insufficient load bearing area may be pulled out after a small number of crack openings. In general the increase of displacements during tests with opening and closing cracks is larger than in the tests with cyclic loading on anchors located in a crack with a constant width (compare Fig. 5 with Fig. 4). 571

9 140 Load [kn] Test 1 Test 2 Test 3 Test 4 Test undercut anchor M16, type 1 embedment depth h ef = 190 mm f cc = 28.5N/mm²; w = 1.5 mm Displacement s [mm] Fig. 2: Load displacement curves for an undercut anchor M16 in cracked concrete w = 1.5 mm 2,50 Ratio N u,test/n o Rk,c (cracked concrete) 2,25 2,00 1,75 1,50 1,25 1,00 0,75 N o Rk,c (cracked concrete) = 6,0 * h ef 1,5 * f cc 1,5 crack 0,50 0,25 0,00 undercut anchor, type 1 undercut anchor, type 2 undercut anchor, type 3 anchor 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 Crack width w [mm] Fig. 3: Ratio measured to calculated failure load as a function of crack width (line cracks) 572

10 Load [kn] N max undercut anchor M10, type 3 N max = 0,59 N o Rk,c Displacement s [mm] Fig. 4: Load-displacement behaviour of an undercut anchor M10 in cracked concrete (w 1.5 mm) under repeated loading with subsequent tension loading to failure average displacement d [mm] undercut anchor, type 1, M10 undercut anchor M12, type 2 undercut anchor, type 1, M12 undercut anchor M20, type 3 undercut anchor, type 1, M16 w max = 1,5 mm w min = 1,0 mm N p = 0,6... 0,7 N o Rk,c with N o Rk,c = 6 x h 1,5 0,5 ef x f cc number of crack openings Fig. 5: Average displacements of different undercut anchors in tests with opening and closing cracks as a function of the number of crack openings 573

11 If fasteners are located sufficiently far from edges and loaded in shear, steel failure may occur. Under seismic excitation the fastener may be subjected to large shear loads with changing load directions. Therefore, in [3] tests under reversed cyclic shear loading in cracked concrete w = 1.0 mm are required. In Fig. 6 typical shear load shear-displacement curves for undercut anchors tested under reversed shear loads in cracked concrete (w = 1.0 mm, loaded in direction of the crack) are plotted. The behaviour in these tests is significantly influenced by the maximum load V max and the number of load cycles. Furthermore, the type of fastener (throughpositioning anchor or pre-positioning anchor) is decisive. Fig. 6a and 6b show the displacement behaviour for different shear load levels. A regular behaviour with relatively small displacements after 15 shear load cycles can be seen in Fig. 6a. The peak shear load at the load cycles was about 45% of the average shear load measured in monotonic tests. After cyclic loading, the shear load was increased monotonically up to failure. The anchor failed by steel rupture. When increasing the amplitude of the shear load by about 40% for the same anchor type, steel failure occurred after 4 load cycles (Fig. 6b). Load [kn] undercut anchor M12, type Displacement s [mm] Load [kn] undercut anchor M12, type Displacement s [mm] a) b) Fig. 6: Shear force shear displacement curves for an undercut anchor M12 (throughpositioning anchor) tested in cracked concrete (w = 1.0 mm) a) repeated maximum shear load V max = ±47.5 kn and subsequent shear test to failure b) repeated maximum shear load V max = ±66.4 kn; steel failure after 4 load cycles 574

12 6. Summary In nuclear power plants post-installed fasteners are often used. A decisive criteria for the use of fastenings in safety critical applications is the proper functioning of fastener under special conditions such as large dynamic loadings and large cracks (w 0.5 mm) due to earthquakes. Such special conditions are not covered by Technical Approvals according to [1]. Therefore, in Germany a guideline for the assessment of fasteners in nuclear power plants has been worked out [3]. In this paper the concept of the guideline is explained. Details of the required suitability tests and the tests to determine admissible service conditions are given. Furthermore, results of tests with different types and sizes of undercut anchors are shown. 7. References [1] Guideline for European Technical Approval of Anchors (Metal Anchors) for Use in Concrete ; Mitteilungen des Deutschen Instituts für Bautechnik; Sonderheft Nr. 16; 31.Dezember [2] Deutsches Institut für Bautechnik: Bemessungsverfahren für Dübel zur Verankerung im Beton (Design Concept for Fasteners fastened in Concrete); Berlin, Juni 1993 [3] Deutsches Institut für Bautechnik, Berlin: Verwendung von Dübeln in Kernkraftwerken und kerntechnischen Anlagen. Ausgabe 9/98. [4] DIN 25449: : Auslegung der Stahlbetonbauteile von Kernkraftwerken unter Belastung aus inneren Störfällen [5] DIN 1045: Tragwerke aus Beton, Stahlbeton und Spannbeton, Bemessung und Konstruktion [6] DIN 1055: Lastannahmen für Bauten [7] ENV 206: Beton; Eigenschaften, Herstellung, Verarbeitung und Gütenachweis. [8] Eligehausen, R.; Mallée, R.: Befestigungstechnik im Beton- und Mauerwerkbau (Fastening Technique to Concrete and Masonry Structures). Ernst & Sohn, [9] Eibl, J.; Keintzel, F.: Behavior of expansion anchors and undercut anchors under dynamic loads. Institut für Massivbau und Baustofftechnologie, Universität Karlsruhe, 1989 [10] Eibl, J.; Keintzel, F.: Behavior of expansion anchors and undercut anchors under high impact and alternate loads. Institut für Massivbau und Baustofftechnologie, Universität Karlsruhe,

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