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SQUG cable tray and conduit evaluation procedure
Nuclear Engineering and Design 123 (1990) 241-245 North-Holland
241
SQUG cable tray and conduit evaluation procedure Paul D. Smith 1, Steve J. Eder 2 a n d Jean-Paul C o n o s c e n t e 2 J The Readiness Operation, 3871 Piedmont Avenue, Oakland, CA 94611-5351, USA 2 EQE Engineering~ Energy and Industry Division, 595 Market Street, 18th Floor, San Francisco, CA 94105, USA
Received 1 December 1988
Cable tray and conduit systems for electrical cables are a common feature of industrial facilities. They have an excellent performance history in past strong earthquake, even though they are rarely designed for earthquakes. Considerable data have been gathered on their performance in earthquakes and in shake table testing. The data have been used to develop a procedure for the verification of the seismic adequacy of cable tray and conduit systems in operating nuclear plants. The procedure is discussed in this paper. It will result in substantial savings, such as reduced engineering effort, fewer modifications of existing hardware, and simpler documentation, relative to alternate procedures like dynamic analysis or shake table testing. The procedure ensures safety-function in a unique manner since the methodology used to develop it (1) is based on a large body of historical data and (2) uses a relative approach of ensuring that nuclear plant systems will perform at least as well as systems that performed well in past earthquakes.
1. Introduction
2. Background
This paper summarizes recent efforts by the authors to develop a procedure for the verification of the seismic adequacy of cable tray and conduit systems in nuclear plants. The procedure is part of the Generic Implementation Procedure [1], which was developed by the Seismic Qualification Utility Group for application in over 60 operating nuclear plant units in the United States to resolve U.S. Nuclear Regulatory Commission Unresolved Safety Issue A-46 [2]. We have already applied similar procedures at nuclear plants in the United States and Europe. The developed procedure is based primarily on the observed and quantified performance of cable tray and conduit systems in past major earthquakes. The procedure has three major aspects: (1) qualitative and quantitative inclusion rules; (2) criteria for a limited analytical review of selected cable tray and conduit supports; and (3) guidelines for an in-plant walkdown of the cable tray and conduit systems. These are discussed in the following sections, preceded by background on the approach used in their development.
We briefly discussed the performance of cable tray and conduit systems in past major earthquakes in an earlier paper [3]. These systems have outperformed other components and installations of typical power and industrial facilities, including structure, equipment, and piping. There is only one known instance of loss of cable electrical function [4]. There are two known instances of serious structural damage to a cable tray system [3] which did not cause loss of cable electrical function. This good performance has occurred even though cable tray and conduit systems are rarely designed for earthquakes. Reasons for the good performance are discussed in ref. [3]. The observed lack of damage to so many cable tray and conduit systems that do not have a seismic design leads to the conclusion that a simple procedure will ensure cable tray and conduit systems will perform well in earthquakes. That is, if systems perform well when no attention is paid to seismic design practice, it should be easy to ensure systems will perform at least this well if some minimal attention is paid to good seismic design
0 0 2 9 - 5 4 9 3 / 9 0 / $ 0 3 . 5 0 © 1990 - Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d )
242
P.D. Smith et al. / SQUG cable tray and conduit evaluation
practice. In contrast, the seismic qualification of cable tray and conduit systems has sometimes been the focus of substantial design, testing, and construction expense in nuclear plants. The clear challenge to any simpler procedure is to reconcile these two perspectives. This discussion highlights an important point about the procedure we developed. The goal of the procedure is to ensure cable tray and conduit systems in nuclear plants are at least as rugged as cable tray and conduit systems that performed well in past earthquakes. This important point is further explained below.
3. Example of procedure criteria We present examples of criteria used in the procedure in this section. The complete procedure is in ref. [4]. 3.1. Inclusion rules
We give one example of an inclusion rule; this one is for cable trays. "The span between cable tray supports should not exceed about 10 feet." This rule refers to the horizontal distance between vertical supports. (The term "inclusion rule" refers to criteria used to define those cable tray and conduit systems to which the procedure applies, that is, systems included rather than excluded from consideration.) We arrived at this rule by quantifying the spans for a large number of cases in the data base. Span is only one of the many parameters we quantified. We also quantified other parameters like tray loading, number of trays per support, and tray width. Some of these other parameters were also used to formulate other inclusion rules. Cable trays with spans of 10 ft are at the upper limit of the known spans quantified in the data base. This statement points out one of the ways these data are interpreted and evaluated. The known span upper limit is obviously less than the actual span upper limit. Since we know that spans greater than 10 ft exist (we did not quantify all the spans at the sites we visited, let alone the spans at all other sites affected by strong earthquakes), and because the known seismic damage in past earthquakes or shake table tests has not been found to have excessively large spans as a root cause, we are comfortable in using a span of 10 ft as our inclusion rule. Note that this rule contains an unquantified margin
since larger spans are known not to have experienced significant structural damage. In this sense it is similar to N R C seismic safety acceptance criteria in the Standard Review Plan or Regulatory Guides. That is, the NRC criteria are known to introduce margin even though it is not quantified. Finally, this rule points out the role of judgement. The 10 ft limitation in the rule is not absolute. In other words, spans greater than 10 ft may be acceptable. Of course, in any application the utility may choose to make its interpretation of any specific rule absolute, but this is not required. There are a number of reasons why longer spans may be acceptable, such as cases where there are only a few cables in the tray (lightly loaded trays), tight clearances with other plant features that will effectively prevent even a heavily loaded tray from collapsing, and so forth. The procedure specifically allows judgement to be used, as long as it is consistent with sound reasoning based on known engineering or other physical or mechanistic principles. This discussion has focused on one of the many rules in the complete procedure. In addition, only a small part of the discussion and consideration involved in its development is included here. This should give some idea of the many factors considered and thoroughness used in developing the inclusion rules.
4. Limited analytical review It is interesting that the first U.S. nuclear plant on which data on the performance of cable trays in past earthquakes was applied and licensed was a new rather than an operating plant [5,6]. That effort caused major improvements in our understanding of how cable trays really perform in earthquakes, which led to the development of simple but effective analytical criteria for verification of the seismic adequacy of their supports. The first important finding is that the supports of cable trays in the earthquake experience data base were at most engineered for dead load. In many cases supports were simply field installed, in other cases span limitation criteria were used as guides for field personnel, and in some of these cases the span limits were based on simple dead load considerations using the most elementary tributary area ideas in structural design. As noted above, it is rare for cable tray systems to be designed for earthquakes. We cannot stress too strongly the positive impact on our thinking caused by investigating the effects of earthquakes on industrial facilities. The typically good performance of these facilities and the occasional poor
P.D. Smith et al. / SQUG cable tray and conduit evaluation
performance have caused us to re-examine and question our analytical training and experience. In some cases, an understanding and reconciliation of performance and analysis has literally taken years. We highly recommend the process to anyone who wants to achieve a more optimum balance between practicality and safety, and between design and analysis. The second important finding is that the presence or absence of lateral bracing had no adverse effect on structural integrity in shake table tests with an without lateral braces; structural integrity was maintained whether or not the cable tray system had lateral bracing. Since cable tray systems in the earthquake experience data base rarely contain lateral bracing, these two findings were realized to be mutually supportive. These findings led to the following conclusions: (1) Cable tray systems need only to be "adequately" designed for vertical gravity loads using simple tributary area structural concepts in order to perform well in earthquakes. Lateral or longitudinal bracing is not required to maintain structural integrity, as long as the primary vertical support connections are ductile and connection or member yielding cannot lead to support or system collapse. (Yielding alone will not lead to collapse of ductile supports attached to a ceiling. However, yielding could result in collapse of supports cantilevered up off a floor, for example.) (2) Cable tray supports or systems do not need to be laterally or longitudinally braced except (i) where primary vertical support connections or members are not ductile or where they lack sufficient strength to resist the lateral loads, or (ii) where lateral or longitudinal displacement must be limited to prevent adverse interferences with other plant features. (By "adverse" we mean the following: In our experience in investigating the effects of earthquakes, the vast majority of the interferences we have seen have been beneficial rather than harmful. This is because they typically introduce nonlinear response which prevents or mitigates the resonant build up of forces, loads, stresses, and displacements. In only a few cases have the interferences caused important damage to fragile components.) One important factor left undefined by the above conclusions is, of course: what exactly is meant by "adequately" in Conclusion (1)? "Adequately" is currently defined as follows. The cable tray support anchorage will be evaluated to show it can withstand a gravity load of 3 times the actual gravity load due to the cables, trays, supports, and other
243
gravity loads. The factor of 3 may be increased or decreased as the program evolves. The allowable stresses, allowable expansion anchor loads, and other Program Acceptance Criteria that are consistent with the factor of 3 are too detailed to go into here (see ref. [4]). However, it is important to discuss what is implied when the factor of 3 and the Program Acceptance Criteria (PAC) are satisfied. To do this we need to discuss how they were developed. First, the PACs were defined. Then cable tray and conduit supports in the earthquake data base that maintained structural integrity were evaluated using the PACs and the defined calculation procedure (simple tributary area, static analysis, and so forth). The outcome of these analyses was cast in the form of factors on the existing gravity loads, one factor per support. Most of the inferred factors were found to be greater than 3, but some were less than 3. This was interpreted to indicate that a factor of 3 was an appropriate criterion. Note that as in the above inclusion rule example, there is an unstated margin inherent in this process. Finally, note that the factor of 3 accounts for earthquake loading, dynamic vibrations, and so forth, in spite of the fact that it is applied as a static coefficient. The factor of 3 and the PACs are thus one way to ensure nuclear plant supports will be at least as rugged as, and thus perform at least as well as, supports in the data base whose structural integrity was maintained during the earthquake. In the ideal case this means the actual quantities selected for the PACs or the gravity load factor selected are not relevant. For example, if the allowables in the above PACs were doubled and the same data base of supports were evaluated again, the inferred factor would be found to be 1.5 rather than 3. However, both sets of criteria would accept or reject the same nuclear plant supports. Thus, the actual quantitative criteria are not that relevant. What is relevant is that the same PACs are used in the data and nuclear plant supports. The reason we adopted a analytical approach to seismic verification is that there are too many physical parameters (spans, number of trays, tray loading, tray width, support construction, and so forth) to make a similarity argument on nuclear plant and data base supports based on a direct comparison of the physical parameters. A similarity argument based solely on comparison of n physical parameters in an n-dimensional space (where n is much larger than 3) was seen to be too complex and intractable a solution based on the observed good performance of these systems. There are many other details in the calculational procedure, for which the reader is referred to ref. [4].
244
P.D. Smith et al. / SQUG cable tray and conduit evaluation
The relative comparison feature discussed above is the key point. If this point is not well understood, and it has not been an easy point to explain, then the purpose and implications of the limited analytical review can be focusing.
meant by "outliers"). All this really means is that the outlier supports do not satisfy these simple criteria. This does not mean the supports are unacceptable and must be modified. While modification may be required, a more detailed evaluation using more complex criteria may instead reveal that the supports are acceptable.
5. In-plant walkdown 6. Conclusions The final aspect of the procedure is to perform an in-plant walkdown of the cable tray and conduit systems. The walkdown has four major purposes: (1) to determine whether the inclusion rules are satisfied; (2) to select a sample of 10-20 worst-case supports for the limited analytical review; (3) to determine if there are outliers to the procedure; and (4) to determine if there are any unacceptable spatial interactions. If the inclusion rules are satisfied, then the supports are represented by the data base and they do not contain known poor details. The in-plant walkdown method provides a substantially improved evaluation procedure relative to conventional as-built sketching and support-by-support detailed analysis. Experienced engineers who are well trained in raceway earthquake experience data and shake table test performance, as well as in the background development of the procedure and criteria, can conduct an efficient walkdown and identify credible potential seismic hazards that might be missed by conventional analytical methods. As an example, some of the inclusion rules address electrical cable pinching or cutting that might occur due to differential motion of raceway system sub-components during an earthquake, which could result in loss of cable electrical function. The in-plant walkdown method efficiently utilizes engineering time by focusing on realistic earthquake failure modes rather than on detailed support-by-support qualification. The developed procedure allows selection of a set of worst-case supports for the limited analytical review, which addresses structural integrity issues. The selection is performed during the walkdown based on the judgement of the experienced engineers. As-installed configurations as well as seismic response aspects associated with workmanship (quality of construction) are enveloped by the set of worst-case supports. Of course, if the worst supports are acceptable, then all supports are also acceptable. This is what we call the "mosaic" approach as compared to a detailed and costly supportby-support evaluation. Finally, it is important to recognize what is implied when supports do not satisfy this procedure (what is
This paper has briefly discussed the development of a procedure for the verification of the seismic adequacy of cable tray and conduit systems in operating nuclear plants. The reason criteria based on the performance of cable tray and conduit systems in past earthquakes are practical and cost effective while they are also safety effective, is the following. Earthquake experience data provide a means for utilities and regulators to distinguish between credible and incredible consequences of earthquakes. Comparable criteria could not otherwise be developed without a large-scale test program and large-scale nonlinear structural analysis, neither of which are justified based on the outstanding performance of these systems in past earthquakes. This is in the best engineering tradition of applying resources on real rather than "What if?." issues. Cable tray and conduit systems constructed to normal industrial standards typically have a large capacity to absorb earthquake loads, even when they are not designed for earthquakes and are designed only for gravity loads.
Acknowledgements The support of the Yankee Atomic Energy Company, Tractebel (Brussels), the Tennessee Valley Authority, the Electric Power Research Institute, and the Seismic Qualification Utility Group is gratefully acknowledged. Each played an important role in helping us develop our approach to the verification of the seismic adequacy of cable tray and conduit systems.
References [1] MPR Associates, Inc. Stevenson& Associates, EQE Incorporated, URS Corporation/John A. Blume& Associates, Engineers, and Bishop, Cook, Purcell& Reynolds, The generic implementation procedure (GIP) for seismic verification of nuclear plant equipment, Prepared for the Seismic Qualification Utility Group, Revision 0 (May 1988).
P.D. Smith et al. / SQUG cable tray and conduit evaluation
[2] U.S. Nuclear Regulator Commission, Verification of seismic adequacy of mechanical and electrical equipment in operating reactors Unresolved Safety Issue (USI) A-46, Generic Letter 87-02, Washington, D.C. (Feb. 1987). [3] S.J. Eder and P.I. Yanev, Evaluation of cable tray and conduit systems using the seismic experience data base, Nucl. Engrg. Des. 107 (1988) 149-153. [4] S.J. Eder, J.P. Conoscente, P.D. Smith and S.P. Harris, Cable tray and conduit system seismic evaluation guidelines, 40010.50-R-001, Prepared for the Seismic Qualification Utility Group, San Francisco, CA: EQE Engineering (Oct. 1988).
245
[5] N. Horstman, S. Eder, P. Hashimoto and S. Swan, A comparison of cable trays at the Seabrook Nuclear Station with the seismic experience data base, Prepared for Yankee Atomic Electric Company, San Francisco, CA: EQE Incorporated (Dec. 1985). [6] U.S. Nuclear Regulatory Commission, Safety evaluation report related to the operation of Seabrook Station, Units 1 and 2, NUREG-0896 Supplement No. 5, Washington, D.C. (July 1986).
241
SQUG cable tray and conduit evaluation procedure Paul D. Smith 1, Steve J. Eder 2 a n d Jean-Paul C o n o s c e n t e 2 J The Readiness Operation, 3871 Piedmont Avenue, Oakland, CA 94611-5351, USA 2 EQE Engineering~ Energy and Industry Division, 595 Market Street, 18th Floor, San Francisco, CA 94105, USA
Received 1 December 1988
Cable tray and conduit systems for electrical cables are a common feature of industrial facilities. They have an excellent performance history in past strong earthquake, even though they are rarely designed for earthquakes. Considerable data have been gathered on their performance in earthquakes and in shake table testing. The data have been used to develop a procedure for the verification of the seismic adequacy of cable tray and conduit systems in operating nuclear plants. The procedure is discussed in this paper. It will result in substantial savings, such as reduced engineering effort, fewer modifications of existing hardware, and simpler documentation, relative to alternate procedures like dynamic analysis or shake table testing. The procedure ensures safety-function in a unique manner since the methodology used to develop it (1) is based on a large body of historical data and (2) uses a relative approach of ensuring that nuclear plant systems will perform at least as well as systems that performed well in past earthquakes.
1. Introduction
2. Background
This paper summarizes recent efforts by the authors to develop a procedure for the verification of the seismic adequacy of cable tray and conduit systems in nuclear plants. The procedure is part of the Generic Implementation Procedure [1], which was developed by the Seismic Qualification Utility Group for application in over 60 operating nuclear plant units in the United States to resolve U.S. Nuclear Regulatory Commission Unresolved Safety Issue A-46 [2]. We have already applied similar procedures at nuclear plants in the United States and Europe. The developed procedure is based primarily on the observed and quantified performance of cable tray and conduit systems in past major earthquakes. The procedure has three major aspects: (1) qualitative and quantitative inclusion rules; (2) criteria for a limited analytical review of selected cable tray and conduit supports; and (3) guidelines for an in-plant walkdown of the cable tray and conduit systems. These are discussed in the following sections, preceded by background on the approach used in their development.
We briefly discussed the performance of cable tray and conduit systems in past major earthquakes in an earlier paper [3]. These systems have outperformed other components and installations of typical power and industrial facilities, including structure, equipment, and piping. There is only one known instance of loss of cable electrical function [4]. There are two known instances of serious structural damage to a cable tray system [3] which did not cause loss of cable electrical function. This good performance has occurred even though cable tray and conduit systems are rarely designed for earthquakes. Reasons for the good performance are discussed in ref. [3]. The observed lack of damage to so many cable tray and conduit systems that do not have a seismic design leads to the conclusion that a simple procedure will ensure cable tray and conduit systems will perform well in earthquakes. That is, if systems perform well when no attention is paid to seismic design practice, it should be easy to ensure systems will perform at least this well if some minimal attention is paid to good seismic design
0 0 2 9 - 5 4 9 3 / 9 0 / $ 0 3 . 5 0 © 1990 - Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d )
242
P.D. Smith et al. / SQUG cable tray and conduit evaluation
practice. In contrast, the seismic qualification of cable tray and conduit systems has sometimes been the focus of substantial design, testing, and construction expense in nuclear plants. The clear challenge to any simpler procedure is to reconcile these two perspectives. This discussion highlights an important point about the procedure we developed. The goal of the procedure is to ensure cable tray and conduit systems in nuclear plants are at least as rugged as cable tray and conduit systems that performed well in past earthquakes. This important point is further explained below.
3. Example of procedure criteria We present examples of criteria used in the procedure in this section. The complete procedure is in ref. [4]. 3.1. Inclusion rules
We give one example of an inclusion rule; this one is for cable trays. "The span between cable tray supports should not exceed about 10 feet." This rule refers to the horizontal distance between vertical supports. (The term "inclusion rule" refers to criteria used to define those cable tray and conduit systems to which the procedure applies, that is, systems included rather than excluded from consideration.) We arrived at this rule by quantifying the spans for a large number of cases in the data base. Span is only one of the many parameters we quantified. We also quantified other parameters like tray loading, number of trays per support, and tray width. Some of these other parameters were also used to formulate other inclusion rules. Cable trays with spans of 10 ft are at the upper limit of the known spans quantified in the data base. This statement points out one of the ways these data are interpreted and evaluated. The known span upper limit is obviously less than the actual span upper limit. Since we know that spans greater than 10 ft exist (we did not quantify all the spans at the sites we visited, let alone the spans at all other sites affected by strong earthquakes), and because the known seismic damage in past earthquakes or shake table tests has not been found to have excessively large spans as a root cause, we are comfortable in using a span of 10 ft as our inclusion rule. Note that this rule contains an unquantified margin
since larger spans are known not to have experienced significant structural damage. In this sense it is similar to N R C seismic safety acceptance criteria in the Standard Review Plan or Regulatory Guides. That is, the NRC criteria are known to introduce margin even though it is not quantified. Finally, this rule points out the role of judgement. The 10 ft limitation in the rule is not absolute. In other words, spans greater than 10 ft may be acceptable. Of course, in any application the utility may choose to make its interpretation of any specific rule absolute, but this is not required. There are a number of reasons why longer spans may be acceptable, such as cases where there are only a few cables in the tray (lightly loaded trays), tight clearances with other plant features that will effectively prevent even a heavily loaded tray from collapsing, and so forth. The procedure specifically allows judgement to be used, as long as it is consistent with sound reasoning based on known engineering or other physical or mechanistic principles. This discussion has focused on one of the many rules in the complete procedure. In addition, only a small part of the discussion and consideration involved in its development is included here. This should give some idea of the many factors considered and thoroughness used in developing the inclusion rules.
4. Limited analytical review It is interesting that the first U.S. nuclear plant on which data on the performance of cable trays in past earthquakes was applied and licensed was a new rather than an operating plant [5,6]. That effort caused major improvements in our understanding of how cable trays really perform in earthquakes, which led to the development of simple but effective analytical criteria for verification of the seismic adequacy of their supports. The first important finding is that the supports of cable trays in the earthquake experience data base were at most engineered for dead load. In many cases supports were simply field installed, in other cases span limitation criteria were used as guides for field personnel, and in some of these cases the span limits were based on simple dead load considerations using the most elementary tributary area ideas in structural design. As noted above, it is rare for cable tray systems to be designed for earthquakes. We cannot stress too strongly the positive impact on our thinking caused by investigating the effects of earthquakes on industrial facilities. The typically good performance of these facilities and the occasional poor
P.D. Smith et al. / SQUG cable tray and conduit evaluation
performance have caused us to re-examine and question our analytical training and experience. In some cases, an understanding and reconciliation of performance and analysis has literally taken years. We highly recommend the process to anyone who wants to achieve a more optimum balance between practicality and safety, and between design and analysis. The second important finding is that the presence or absence of lateral bracing had no adverse effect on structural integrity in shake table tests with an without lateral braces; structural integrity was maintained whether or not the cable tray system had lateral bracing. Since cable tray systems in the earthquake experience data base rarely contain lateral bracing, these two findings were realized to be mutually supportive. These findings led to the following conclusions: (1) Cable tray systems need only to be "adequately" designed for vertical gravity loads using simple tributary area structural concepts in order to perform well in earthquakes. Lateral or longitudinal bracing is not required to maintain structural integrity, as long as the primary vertical support connections are ductile and connection or member yielding cannot lead to support or system collapse. (Yielding alone will not lead to collapse of ductile supports attached to a ceiling. However, yielding could result in collapse of supports cantilevered up off a floor, for example.) (2) Cable tray supports or systems do not need to be laterally or longitudinally braced except (i) where primary vertical support connections or members are not ductile or where they lack sufficient strength to resist the lateral loads, or (ii) where lateral or longitudinal displacement must be limited to prevent adverse interferences with other plant features. (By "adverse" we mean the following: In our experience in investigating the effects of earthquakes, the vast majority of the interferences we have seen have been beneficial rather than harmful. This is because they typically introduce nonlinear response which prevents or mitigates the resonant build up of forces, loads, stresses, and displacements. In only a few cases have the interferences caused important damage to fragile components.) One important factor left undefined by the above conclusions is, of course: what exactly is meant by "adequately" in Conclusion (1)? "Adequately" is currently defined as follows. The cable tray support anchorage will be evaluated to show it can withstand a gravity load of 3 times the actual gravity load due to the cables, trays, supports, and other
243
gravity loads. The factor of 3 may be increased or decreased as the program evolves. The allowable stresses, allowable expansion anchor loads, and other Program Acceptance Criteria that are consistent with the factor of 3 are too detailed to go into here (see ref. [4]). However, it is important to discuss what is implied when the factor of 3 and the Program Acceptance Criteria (PAC) are satisfied. To do this we need to discuss how they were developed. First, the PACs were defined. Then cable tray and conduit supports in the earthquake data base that maintained structural integrity were evaluated using the PACs and the defined calculation procedure (simple tributary area, static analysis, and so forth). The outcome of these analyses was cast in the form of factors on the existing gravity loads, one factor per support. Most of the inferred factors were found to be greater than 3, but some were less than 3. This was interpreted to indicate that a factor of 3 was an appropriate criterion. Note that as in the above inclusion rule example, there is an unstated margin inherent in this process. Finally, note that the factor of 3 accounts for earthquake loading, dynamic vibrations, and so forth, in spite of the fact that it is applied as a static coefficient. The factor of 3 and the PACs are thus one way to ensure nuclear plant supports will be at least as rugged as, and thus perform at least as well as, supports in the data base whose structural integrity was maintained during the earthquake. In the ideal case this means the actual quantities selected for the PACs or the gravity load factor selected are not relevant. For example, if the allowables in the above PACs were doubled and the same data base of supports were evaluated again, the inferred factor would be found to be 1.5 rather than 3. However, both sets of criteria would accept or reject the same nuclear plant supports. Thus, the actual quantitative criteria are not that relevant. What is relevant is that the same PACs are used in the data and nuclear plant supports. The reason we adopted a analytical approach to seismic verification is that there are too many physical parameters (spans, number of trays, tray loading, tray width, support construction, and so forth) to make a similarity argument on nuclear plant and data base supports based on a direct comparison of the physical parameters. A similarity argument based solely on comparison of n physical parameters in an n-dimensional space (where n is much larger than 3) was seen to be too complex and intractable a solution based on the observed good performance of these systems. There are many other details in the calculational procedure, for which the reader is referred to ref. [4].
244
P.D. Smith et al. / SQUG cable tray and conduit evaluation
The relative comparison feature discussed above is the key point. If this point is not well understood, and it has not been an easy point to explain, then the purpose and implications of the limited analytical review can be focusing.
meant by "outliers"). All this really means is that the outlier supports do not satisfy these simple criteria. This does not mean the supports are unacceptable and must be modified. While modification may be required, a more detailed evaluation using more complex criteria may instead reveal that the supports are acceptable.
5. In-plant walkdown 6. Conclusions The final aspect of the procedure is to perform an in-plant walkdown of the cable tray and conduit systems. The walkdown has four major purposes: (1) to determine whether the inclusion rules are satisfied; (2) to select a sample of 10-20 worst-case supports for the limited analytical review; (3) to determine if there are outliers to the procedure; and (4) to determine if there are any unacceptable spatial interactions. If the inclusion rules are satisfied, then the supports are represented by the data base and they do not contain known poor details. The in-plant walkdown method provides a substantially improved evaluation procedure relative to conventional as-built sketching and support-by-support detailed analysis. Experienced engineers who are well trained in raceway earthquake experience data and shake table test performance, as well as in the background development of the procedure and criteria, can conduct an efficient walkdown and identify credible potential seismic hazards that might be missed by conventional analytical methods. As an example, some of the inclusion rules address electrical cable pinching or cutting that might occur due to differential motion of raceway system sub-components during an earthquake, which could result in loss of cable electrical function. The in-plant walkdown method efficiently utilizes engineering time by focusing on realistic earthquake failure modes rather than on detailed support-by-support qualification. The developed procedure allows selection of a set of worst-case supports for the limited analytical review, which addresses structural integrity issues. The selection is performed during the walkdown based on the judgement of the experienced engineers. As-installed configurations as well as seismic response aspects associated with workmanship (quality of construction) are enveloped by the set of worst-case supports. Of course, if the worst supports are acceptable, then all supports are also acceptable. This is what we call the "mosaic" approach as compared to a detailed and costly supportby-support evaluation. Finally, it is important to recognize what is implied when supports do not satisfy this procedure (what is
This paper has briefly discussed the development of a procedure for the verification of the seismic adequacy of cable tray and conduit systems in operating nuclear plants. The reason criteria based on the performance of cable tray and conduit systems in past earthquakes are practical and cost effective while they are also safety effective, is the following. Earthquake experience data provide a means for utilities and regulators to distinguish between credible and incredible consequences of earthquakes. Comparable criteria could not otherwise be developed without a large-scale test program and large-scale nonlinear structural analysis, neither of which are justified based on the outstanding performance of these systems in past earthquakes. This is in the best engineering tradition of applying resources on real rather than "What if?." issues. Cable tray and conduit systems constructed to normal industrial standards typically have a large capacity to absorb earthquake loads, even when they are not designed for earthquakes and are designed only for gravity loads.
Acknowledgements The support of the Yankee Atomic Energy Company, Tractebel (Brussels), the Tennessee Valley Authority, the Electric Power Research Institute, and the Seismic Qualification Utility Group is gratefully acknowledged. Each played an important role in helping us develop our approach to the verification of the seismic adequacy of cable tray and conduit systems.
References [1] MPR Associates, Inc. Stevenson& Associates, EQE Incorporated, URS Corporation/John A. Blume& Associates, Engineers, and Bishop, Cook, Purcell& Reynolds, The generic implementation procedure (GIP) for seismic verification of nuclear plant equipment, Prepared for the Seismic Qualification Utility Group, Revision 0 (May 1988).
P.D. Smith et al. / SQUG cable tray and conduit evaluation
[2] U.S. Nuclear Regulator Commission, Verification of seismic adequacy of mechanical and electrical equipment in operating reactors Unresolved Safety Issue (USI) A-46, Generic Letter 87-02, Washington, D.C. (Feb. 1987). [3] S.J. Eder and P.I. Yanev, Evaluation of cable tray and conduit systems using the seismic experience data base, Nucl. Engrg. Des. 107 (1988) 149-153. [4] S.J. Eder, J.P. Conoscente, P.D. Smith and S.P. Harris, Cable tray and conduit system seismic evaluation guidelines, 40010.50-R-001, Prepared for the Seismic Qualification Utility Group, San Francisco, CA: EQE Engineering (Oct. 1988).
245
[5] N. Horstman, S. Eder, P. Hashimoto and S. Swan, A comparison of cable trays at the Seabrook Nuclear Station with the seismic experience data base, Prepared for Yankee Atomic Electric Company, San Francisco, CA: EQE Incorporated (Dec. 1985). [6] U.S. Nuclear Regulatory Commission, Safety evaluation report related to the operation of Seabrook Station, Units 1 and 2, NUREG-0896 Supplement No. 5, Washington, D.C. (July 1986).