Control of seismic response of piping systems and components in power plants by base isolation

Control of seismic response of piping systems and components in power plants by base isolation James M. Kelly Department of Civil Engineering, University of California, Berkeley, California 94 720, USA

Daniel E. Chitty Applied Nucleonics Engineers 1nc, Santa Monica, California, USA (Received August 19 70)

Standard approaches to the aseismic design of piping systems and components for power plants - such as the floor spectrum method - are costly and neglect equipment-structure interaction. Such interaction has recently been shown to be severe when the natural frequency of a component is close or equal to one of the natural frequencies of the primary structure - a situation referred to as tuning and one almost ~ertain tO. occur in a large structural system. A base isolation system :is described that has been demonstrated to reduce dramatically the accelerations induced in structures under earthquake motion. A series of further experiments is described in the paper; this experimental work demonstrates that the response of equipment in structures so isolated is also greatly reduced. Thus, sensitive internal equipment can be protected directly from seismic attack; interaction need not be considered and inelastic analyses need not be performed during the design process. Due to rigid body action of the primary system above the base isolation system, multiple support response spectra design methods are not needed. It is anticipated that the use of base isolation will reduce the cost of the design and construction of power plant components, piping systems, and structures.

Introduction An important consideration in the design of nuclear and, recently, geothermal power plants in seismically active regions is the assurance of the structural integrity of essential equipment, such as pumps, valves, and control devices, and piping systems under earthquake-induced loading. These components are connected to the primary structure and their response is determined by the response of the primary structure to the earthquake ground motion. The design process for such equipment and for piping systems is a particularly difficult one, complicated both by uncertainties in the specification of the ground motion and by uncertainties in the specification of the primary structure. It is essential for reasons of safety that a reliable design process be used. The standard approach is the floor spectrum method which requires many time history analyses to be performed on the primary structure using a set of earthquake ground motions consistent with the design spectrum for the plant to determine the response of 0141-0296/80/030187-12/$02.00 @ 1980 IPC BusinessPress

equipment at attachment points. Each analysis is deterministic and many must be performed to reflect the probabilistic nature of the problem. A further complication arises in the case of piping systems;here, the secondary structure may be attached at many different support points which will experience different displacement time histories for the same ground motion. There then arises the problem of combining, by the floor spectrum method, the contributions to a particular response quantity from each support motion. There are several proprietary piping analysis programs which perform such analyses, but their use is controversial at best and all are extremely costly to run on the computer. A further complication arises when the equipment or piping system has a natural frequency close to one of the natural frequencies of the primary system, a situation referred to as tuning and one almost inevitable in a large system. In this case it can be shown that the interaction between the equipment and the structure can be very important even in relatively very light

Eng. Struct., 1980, Vol. 2, July

187

Control of seismic response: J. M. Kelly and D. E. Chitty

equipment. The floor spectrum method neglects this interaction and is invalid for such cases; if used, it can significantly overestimate the equipment response, and lead to excessively conservative equipment design. 1 Peak earthquake levels for which nuclear and geothermal power plants must be designed have been steadily increased by regulatory agencies over the past several years leading to the proposal 2 that inelastic action be permitted in the equipment and its supports or that energy-absorbing restrainers be used in piping systems. 3-6 Since plastic deformation produces a drop in the frequencies of the system and an energy absorption, the response of the equipment or the piping would theoretically be lowered to a level below that which would prevail if the system were to remain elastic. 7,8 However, plastic action inevitably involves some damage to the equipment supports or to the primary structure and will require nonlinear deterministic analysis of both the primary and secondary systems. In this paper we describe an alternative approach to aseismic design in which internal equipment, or piping, is protected from earthquake motion by constructing the entire power plant on a base isolation system. There are many possible systems, but in essence they all involve a double layer foundation system with a lower element fixed to the ground and an upper element separated from the lower by a decoupling system. The feasibility of a number of possible base isolation systems has been demonstrated by large-scale shaking table experiments at the Earthquake Engineering Research Center of the University of California, Berkeley. The major benefits of base isolation to a equipment and piping design are that it makes consideration of equipment-structure interaction and inelastic response unnecessary and, due to the fact that the primary structure above the isolation system moves entirely as a rigid body, it means that all support points of a piping system have the same displacement time history which means that multiple support response spectrum analysis, witti its controversial aspects, need not be used.

acceleration and minimizing displacement. A frequency of around 0.5 Hz was deemed a suitable compromise. An experimental study on the feasibility of base isolation as an aseismic design procedure was carried out at the Earthquake Simulator Laboratory of the E.E.R.C. Specially designed multilayer elastomeric bearings were placed beneath a 20-ton model steel frame (Figure 1), and tested on a 20 × 20 ft shaking table. In Figure 2, the installation of the bearings is illustrated and in Figure 3 the details of a typical bearing are shown. The first mode frequency of the model with these bearings installed was 0.58 Hz, and it was shown by analysis that the first mode frequency of a similar full-scale structure could easily be reduced to approximately 0.35 Hz. The major differences between the response of the model when fixed to the table and when base isolated were that: (t), the isolated structure underwent large rigid body translation. The large translations necessary for a base isolation system to be effective were easily attained through the use of the rubber bearings which were capable of undergoing repeated deformation of over 3 in without deteriorating. For unscaled earthquake motion, peak acceleration was reduced by a factor of 10 when the model was isolated from the ground motion. For time-scaled motions even further reductions were found. The very stiff vertical characteristics of the rubber bearings satisfactorily reduced tilting and vertical response of the model structure. Overall, the simple earthquake isolation system isolated the model structure to a high degree from the damaging effects of earthquake ground motion. The experimental work and results are reported in references 9-11. The research reported there was directed to the protection of the main structure ; it is the thrust of this study that base isolation is also a means of protecting sensitive equipment housed

Base isolation Although the concept of isolating buildings and machinery from harmful vibration is well known, the engineering profession has rarely attempted to extend the concept to the design of structures against earthquake vibration. No wellestablished criteria exist as to what constitutes an effective earthquake isolation system nor as to proper design and construction procedures. For a structure to be isolated from earthquake vibration, two criteria must be fulfilled: (1), the lowest natural frequency of an isolated structure must be well below the frequency of most earthquake ground motion; and (2), the first mode shape of an isolated structure should approach that of a single-degree-of-freedom rigid body system so that higher mode contributions will be negligible. Although the first mode frequency of an isolated structure should thus be approximately 0.2 Hz, since most earthquake vibration is in the range 0.3-5 Hz, such a low frequency is not ideal for two reasons: (1), the lateral deflection of an isolated system of such low frequency could, for a given earthquake, approach several feet, and (2), structures need not be isolated from the low-frequency components of earthquake excitations since the peak acceleration of very longperiod structures is typically low in the first mode. Thus it is essential that a balance be struck between reducing

188 Eng. Struct., 1980, Vol. 2, July

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Recent parallel analytical work has focused on the problems of equipment-structure interaction in systems under seismic attack. Details o f this work are reported in references 1 2 - 1 4 . Equipment-structure interaction has been shown to be significant under some circumstances and the acceleration response of equipment to be highly amplified when the equipment frequency is close to or equal to one of the natural frequencies of the structure, i.e., when structure and equipment are tuned or nearly tuned. The following case illustrates why the former is true. Consider a simple two-degree-of-freedom system, subjected to ground motion, in which one mass is much greater than the other. In weakly coupled systems with the same frequency, the response of the system involves a perfect energy exchange between each component at a beat frequency that is much lower than the natural frequency o f each component. The same phenomenon, a classical beat phenomenon, occurs here. The coupling is weak because the ratio of equipment mass to structural mass is small. When such a structure is subjected to ground motion, the velocity imparted to the structure is mass independent and determined only by the ground motion. Thus, if an identical ground motion were applied directly to equipment with the same natural frequency, the same velocity would

be imparted to it. The kinetic energy, on the other hand, would be proportional to the mass of the system excited, i.e., much smaller for the equipment than for the structure. However, if the equipment is attached to the structure and the structure is subjected to ground motion, then the kinetic energy imparted to the structure will be wholly transmitted to the equipment if tuned. Consequently, the velocity imparted is anaplified by the reciprocal of the square root o f the mass ratio, a factor that can be very large.

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Experimental test The experimental model was the three-storey steel frame shown in Figure 1 with multilayer natural rubber bearings under each corner of the base floor. Each floor of the frame was loaded with concrete blocks weighing 8000 lb. The total weight of the mode was 39 500 lb and its height was about 20 ft. Three single-degree-of-freedom oscillators were attached to the concrete blocks at the second and third floors. Tests were conducted with the base isolation system deactivated to approximate a conventional foundation design. This was accomplished by welding straps from the frame to a beam fixed to the shaking table as shown in Figure 4. The straps were then removed and the tests repeated on the isolated system. The test structure was extensively instrumented to measure displacements and accelerations at each floor, shear in the rubber bearings, and acceleration of the mechanical oscillators. Data were taken from each transducer at the rate of approximately 50 samples per second. The three mechanical oscillators used to simulate equipment in the primary structure were constructed to correspond to the first three natural frequencies of the model structure in the fixed-base configuration. Each oscillator was comprised of a vertical cantilever beam fixed at the bottom and with a mass at the top. The upright beams were made of ¼-in thick cold-rolled steel plate and were slotted so that the position of the masses could be adjusted to change the frequency. Each mass was a 6-in by 6-in by 2-in piece of steel weighing about 20 lb. Details of the construction of the oscillator are shown in Figure 5.

190 Eng. Struct., 1980, Vol. 2, July

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and the third mode oscillator was placed on the second floor as shown in Figure 6. These positions were chosen so that each oscillator would be subjected to the greatest possible input in its mode. The earthquake signals used in the test series were the Pacoima Dam $16E (1971) and Taft (1950) and time-scaled versions of the Pacoima Dam, Taft, and E1 Centre N - S (1940) records. Some approximate square waves were also used. Since these latter waves were produced manually by moving the table displacement control, they cannot be duplicated exactly.

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Prior to the simulated equipment tests, the three-storey frame was tested on several forms of isolation system and its response compared with that of the frame when fixed to the shaking table.9, =o Several earthquake records in real and scaled time were used as input in these tests. The most significant result for the design of piping systems is that the frame above the isolation system moves as a rigid body. For example, for the E1 Centro 1940 input in real time the results for displacement and acceleration for the frame fixed to the table are shown in Figures 7a and b; results for the isolated system as shown in Figures 8a and b. The vertical response of the frame to combined horizontal and vertical El Centro input (with the peak vertical acceleration increased to 0.27 g) is shown in Figure 9. The recorded vertical accelerations are identical to the input acceleration indicating that vertical amplification either in the bearings or in the frame was negligible. Ther¢ was no response in the rocking mode. The implications for piping design are that all support points on the isolated primary structure will have the same motion. Further, the horizontal components will be almost entirely low-frequency motion and the

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vertical components almost the same as the vertical ground motion. The use of spectrum design methods is then possible with confidence. The results of the tests in which simulated equipment was included are presented in Figure lOa-h as time histories of oscillator acceleration with accelerations of the shaking table and third floor o f the model shown as well. In these figures data from the fixed-base and isolated tests for the same input are shown side by side. The table acceleration trace is plotted at an expanded scale so that it can be discemed. If for purposes of comparison the data from tests on the isolated model were plotted to the same scale as were data from tests on the fixed-based model, the table and oscillator accelerations would appear to be zero. The diagrams on the right.hand.side of Figures lOa and b show these data for the isolated tests at a scale expanded by a factor o f 10. The responses of the oscillators in the fixed-base model clearly indicate the beating phenomenon as expected from the theoretical analysis. 12-14 The very large magnification of acceleration experienced by the oscillators is immediately obvious as is the fact that the peak acceleration in the two lower-frequency oscillators was achieved considerably after the peak o f the input acceleration. The two lowermode oscillators in the fixed-base model responded predominantly at the coupled frequencies governed by the equipment-structure interaction. The response of the third mode oscillator was different; the frequency was suffi-

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Eng. Struct., 1980, Vol. 2, July

193

Control o f seismic response: J M. Kelly and D. E Chi~ty

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Eng. Struct., 1980, V o l . 2, July

195

Control of seismic response: J. M. Kelly and D. E. Chitty

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Control o f seismic response: d. M. Kelly and D. E. Chitty

ciently high (around 15 Hz) that the peak response occurred during the input acceleration. It is clear that the third mode oscillator responded at its coupled frequencies of equipment-structure interaction, but that the peak response predicted by the theory was obscured by contributions from the lower modes. Maximum values of table, floor, and oscillator acceleration are presented in Table I for both the isolated and fixed.base conditions. From these data it can readily be seen that with the conventional foundation, oscillator accelerations were greater than table accelerations by factors of 10-34 and that acceleration of the oscillators exceeded that of the third floor by factors of 6-17. By contrast, the accelerations of the simulated equipment in the isolated structure were uniformly lower than the acceleration of the input to the shaking table. For the same input signal, oscillator accelerations in the fixed-base tests were, in some cases, 50 times greater than those measured in the isolated structure. The levels of earthquake input in these tests were controlled by the fact that it was important to keep the fixed-base response of the oscillators below 5 g. Thus, only very low levels of earthquake input could be tolerated. At these levels of input, the measured accelerations in the isolated condition were extremely low. The isolated tests were repeated at much higher levels of input acceleration and the resulting accelerations in the structure are given in Table 1. It is interesting to consider the physical basis of these large reductions in equipment response. It should be noted that when the structure is isolated, the lowest mode is predominantly a rigid body motion of the entire structure. The earthquake input ag appears in the equations of motion of the system:

where n ~ 1. It is unlikely that a piece of equipment would have a natural frequency in the same range as that of the isolated rigid body motion and thus the response of the equipment will be included in the higher modes of the combined equipment-structure system. Since these will be orthogonal to the input, the equipment response will be negligible when the main structure is isolated. Conclusions This experiment has clearly shown that equipment with a natural frequency close to one of the frequencies of the primary structure in which it is housed can experience accelerations several times greater than those in the primary structure. It has further demonsrated that base isolation of the primary structure can greatly reduce the accelerations that will be induced in sensitive equipment by a damaging earthquake. When base isolation is used to protect a primary structure, the cost of designing and installing components and equipment - which cost has escalated greatly due to increases in levels of acceleration that such components must now be designed to resist - is anticipated to decrease. Specifically, the cost of the following would be affected: (a) site preparation and foundation costs; (b) building structures; (c) mechanical components; (d) electrical components; (e) piping systems, ventilation ducts, and electrical distribution elements. While the cost under (a) may increase, all other costs would decrease, in some cases substantially - for example, under (c), (d), and (e) - since mass-produced items could be substituted for components that must under current regulations and design methods be specially strengthened. Costs under (b) will decrease as will the structural mass and, because wall thicknesses can be reduced and shear walls eliminated, greater internal square footage will be available.

M 2 + C~ + k x = P

in the form of an equivalent force Pequ = - Mriig where r is a vector that represents the rigid body displacements that correspond to unit base displacement. Thus the higher modes of the structure are orthogonal to the input motion, i.e.:

References

¢~Mr = 0

Kelly, J. M. and Sackman, J. L. 'Conservatism in summation

since ~ = cr;

rules for closely spaced modes', Int. J. Earthquake Eng. Struct.

CTtMOn = 0 Table I

Dyn. 1980, 8, 63

Peak accelerations in t u n e d oscillators under fixed base and base isolated conditions

Signal

EC (2.45 ts) E C (3.46 ts)

Maximum frame accal. (g)

Max. accel. oscill. 1 (g)

Maximum table accel. (g)

Fixed base

Isolated

Fixed base

0.158 0.099

0.326 0.196

0.038 0.031

3.419 2.074

E C ( 3 . 4 6 ts)

0.381

--

0.084

--

Pac. D. Pac. D. (1.73 ts)

0.131 0.172

0.620 0.302

0.057 0.044

4.097 2.715

Isolated

0.071 0.041 0.117 0.184 0.062

Pac. D . ( 1.73 ts )

0.704

--

0.162

--

0.218

Taft T a f t (2.45 T a f t (2.45 T a f t (3.46 T a f t (3 A 6 SQWV* SQWV* SQWV** SQWV**

0.118 0.073 0.309 0.095 0.410 0.144 0.275 0.313 0.270

0.436 0.189 -0.262 -0.344 -0.559 --

0.077 0.029 0.093 0.018 0.065 0.049 -0.059

3.107 1.911 -2.611 -2.128 -2A43 --

0.183 0.036 0.133 0.066 0.230 0.049 -0.097

ts) ts) ts) ts)

Max. accel. oscill. 2 (g)

Max. accel. oscill. 3 (g)

Fixed base

Isolated

Fixed base

Isolated

3.135 3.280 -1.562 3;276 -1.943 2.471 -2.982 -3.386 -3.257 --

0.065 0.066 0.202 0.066 0.096 0.384 0.103 0,054 0.188 0.055 0,233 -0.105 -0.140

0.938 0.579 -0.885 1.259 -0.708 0.370 -0.896 -0.441 -0.812 --

0.079 0.061 0.149 0.076 0.129 0.302 0.129 0.045 0.229 0.062 0.337 -0.054 -0.084

* one pulse; * * t w o pulses

Eng. S t r u c t . , 1 9 8 0 , V o l . 2, J u l y

197

Control o f seismic response: J. M. Kelly and D. E. Chitty 2 3

4

5

6

7

198

Newmark, N. M. 'Inelastic design of nuclear reactor structures and its implications on design of critical equipment', Trans SMiRT4, Vol. K(a), paper K 4/I, Berlin, 1977 Spencer, P. N. et al. "The design of steel energy absorbing restrainers and their incorporation into nuclear power plants for enhances safety: Vol. 1 - Summary report',Rep. UCB/ EERC. 79/07, Earthquake Engineering Research Center, University of California, Berkeley, 1979 Lee, M. C. et al. 'The design of steel energy absorbing restrainers and their incorporation into nuclear power plants for enhanced safety: Vol. 2 - The development of analyses for reactor system piping', Rep. UCB/EERC-79/08, Earthquake Engineering Research Center, University of California, Berkeley, 1979 Owen, W. S. et al. 'The design of steel energy absorbing restrainers and their incorporation into nuclear power plants for enhanced safety: Vol. 3 - Evaluation of commercial steels', Rep. UCB/EERC. 79/09, Earthquake Engineering Research Center, University of California, Berkeley, 1979 Kelly, J. M. and Skinner, M. S. 'The design of steel energy absorbing restrainers and their incorporation into nuclear power plants for enhanced safety: Vol. 4 - A review of energyabsorbing devices', Rep. UCB/EERC- 79/10, Earthquake Engineering Research Center, University of California, Berkeley, 1979 Lee, M. C. et al. Seismic performance of piping systems sup-

Eng. Struct., 1980, Vol. 2, July

ported by nonlinear hysteretic energy absorbing restrainers',

Prec. Second US Nat. Con[ Earthquake Eng., Earthquake 8 9 10 11

12 13 14

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