Piping Seismic Stress Limits: A Critical Review

Seismic stress limits for nuclear piping were published by the Section III code in 1994. Because of concerns on the technical bases for the rules, NRC has not approved their use. Modifications to the rules have been made in 2001. The 1994 seismic stress limits are reduced, and one type of joint now has a seismic stress limit that is less that the static load limit. A limit for seismic that is less than the limit for a static load contradicts the test data. Most of the technical concerns were valid. The 1994 rules are based on the premise that collapse is not a potential failure mode for a seismic event. However, collapse occurred in two of the EPRI component tests. Seismic margins in the component tests were overestimated. Revisions to the seismic margin data do not support the higher stress limits. A different approach has been taken to justify the 2001 rules. A probability approach is used where seismic capacity is related to a strength factor. The strength factor is based on the measured ultimate moment in the component tests. The capacity is the strength factor multiplied by a nonlinear dynamic factor. A small nonlinear dynamic factor is used because of concerns with off-resonance margin in stiff components. In contrast, the tests demonstrate large nonlinear dynamic factors. The intent of the new rules is to limit piping response to the SSE to the linear elastic range.

Shaking Table Tests of Full Scale Base-Isolated Structures

Conventional earthquake resistant designs depend on strengthen and ductility of the structural components to resist induced forces and to dissipate seismic energy. However, this can produce permanent damage to the joints as well as the larger interstory displacements. In recently years, many studies on structural control strategies and devices have been developed and applied in U. S. A., Europe, Japan, and New Zealand. The rubber bearing belongs to one kind of the earthquake-proof ideas of structural control technologies. The installation of rubber bearings can lengthen the natural period of a building and simultaneously reduce the earthquake-induced energy trying to impart to the building. They can reduce the magnitude of the earthquake-induced forces and consequently reduce damage to the structures and its contents, and reduce danger to its occupants. This paper is aimed at studying the mechanical behavior of the stirrup rubber bearings (SRB) and evaluating the feasibility of the buildings equipped with the stirrup rubber bearings. Furthermore, uniaxial, biaxial, and triaxial shaking table tests are conducted to study the seismic response of a full-scale three-story isolated steel structure. Experimental results indicate that the stirrup rubber bearings possess higher damping ratios at higher strains, and that the stirrup rubber bearings provide good protection for structures. It has been proved through the full-scale tests on shaking table that the stirrup rubber bearing is a very promising tool to enhance the seismic resistibility of structures.

Approximate Analytic Method of Piping Systems With Elasto-Plastic Damper

An elasto-plastic damper is one of the vibration absorbers in which energy is absorbed by elasto-plastic deformation of the hysteretic type damper. It is used for the piping system. The piping system is continuous system. Since it is difficult to find the analytical solution of the equation of motion for the system with elasto-plastic damper, the equation of motion is treated by various approximate methods in which the system is usually considered as a single- or a multiple-degree-of-freedom system, but not as a continuous system. In order to analyze the response of a nonlinear continuous system, however, it is necessary to consider the system as a continuous system. In this paper, the nonlinear steady-state response of the piping system with elasto-plastic damper is undertaken by approximate solutions, which are easily obtained by a simple procedure and are more practical than the exact solutions. As a continuous model of the piping system, a beam simply supported or clamped at one end, with elasto-plastic damper at the other end is used. The restoring force is modeled as hysteresis loop characteristics in order to consider the energy loss in the damper. In the analysis, the restoring force is expanded into the Fourier series, and only fundamental terms are considered. The resonance curves and mode shapes of the beam are obtained from the approximate solution. And effect of elasto-plastic damper on the forced response of continuous system is examined.

Impact Behavior of Rocking Bridge Piers

This research aims to investigate the energy dissipation characteristic and impact behavior of rocking piers under free vibration. Research parameters include rocking interfaces (stiff or flexible), geometry of the column cross-section (circular or rectangular), aspect ratio of the columns, anchorage of prestressing tendons and scale effect. To validate the proposed theory, five columns were constructed and will be tested. A numerical process was proposed to simulate the rocking behavior of columns. Numerical analysis revealed that aspect ratio remarkably affects the rocking behavior, however, size effect and shape of cross section had no significant influence on the rocking behavior. Contrary to the instinct, anchored columns may have less damping due to the higher restoring forces that leads to larger acceleration and slower degradation in kinetic energy.

Seismic Proving Test of Ultimate Piping Strength: Test Results on Piping Component and Simplified Piping System

In 1998FY, the 6 year program of piping tests was initiated with the following objectives: i) to clarify the elasto-plastic response and ultimate strength of nuclear piping, ii) to ascertain the seismic safety margin of the current seismic design code for piping, and iii) to assess new allowable stress rules. In order to resolve extensive technical issues before proceeding on to the seismic proving test of a large-scale piping system, a series of preliminary tests of materials, piping components and simplified piping systems is intended. In this paper, the current status of the piping component tests and the simplified piping system tests is reported with focus on fatigue damage evaluation under large seismic loading.

Development and Application of Innovative Anti-Seismic Systems for the Protection of Cultural Heritage: New Achievements of ENEA

As described in detail at previous ASME-PVP Conferences and also reminded by separate papers presented this year, large efforts have been devoted by the Italian Agency for New Technology, Energy and the Environment (ENEA), with the cooperation of several further members of the Italian Working Group on Seismic Isolation (GLIS), to the development, validation and application of innovative anti-seismic (IAS) techniques since 1988. To date, considered have been base and floor seismic isolation (SI), energy dissipation through various types of passive devices, hydraulic coupling by means of innovative shock transmitters, systems formed by shape memory alloy devices and more recently, semi-active control of vibrations. New activities at ENEA, which are in progress in the framework of both international and national collaborations, concern the development of new IAS techniques of the aforesaid kinds to be applied to: • civil structures and industrial plants; • cultural heritage structures (CUHESs) to be restored or reconstructed, or masterpieces to be seismically protected. Progress of the work performed for civil and industrial structures has been separately presented at this Conference, while this paper deals with the new development, validation and application activities concerning the IAS techniques applicable to the seismic protection of CUHESs, to which particular attention has been devoted by ENEA for several years. The ongoing activities for CUHESs are being performed in the framework of: • PROSEESM, a national project which foresees pilot applications of the IAS techniques to the restoration of CUHESs damaged by the 1997–98 Marche and Umbria earthquakes; • a feasibilily study for the reconstruction in the original site, with SI and the original masonry materials, of Mevale di Visso, a village in the Marche Region destroyed by the aforesaid event; • a study for the design and application of an innovative three-dimensional SI system for seismic and ambient vibration protection of a roman ship excavated at Ercolano, near Naples.

Analyses of Seismic Isolation Systems in Kyrgyzstan

The purpose of the present work is the analysis of existing methods of seismoisolation in the Kyrgyz Republic at which 95% of territory has seismically active zone with intensity 8, 9 and more. Also an opportunity of application of system seismic protection as rubber-metal bearings.

Application of Semi-Active Oil Damper System to Base Isolation Systems

This paper deals with one of the applications of the Semi-Active Oil Damper system, which applies base isolation systems reducing the maximum acceleration. The theory of the Semi-Active Oil Damper system is based on Karnopp Theory. The theory has been actually now in use for a Semi-active suspension system of the latest Shinkansen (New trunk lines) trains to improve passenger’s comfortable riding. Various experiments have been conducted using a single mass model whose weight is 15 ton on the shaking table. This model is supported by the rubber bearing. The natural frequency is 0.33Hz of this system. Two Semi-Active Oil Damper were installed in the model and excited the table for one horizontal direction. The maximum damping force of each Semi-Active Oil Damper used for the model is 4.21 kN. The damper can change the damping coefficient by utilizing two solenoid valves. Therefore, the dynamic characteristic of the damping force has two modes. One is a hard damping coefficient and the other is a soft one. It was confirmed that the maximum acceleration of the Semi-Active Oil Damper system can be reduced more than 20% in comparison with the passive Oil Damper system in our tests.

Three Dimensional Seismic Isolation System Using Hydraulic Cylinder

A three-dimensional (3D) seismic isolation system for FBR building is under development. The proposed vertical isolation system consists form hydraulic cylinders with water-based liquid and accumulators to support large vertical static load and to realize low natural frequency in the vertical direction. For horizontal isolation, laminated rubber isolator or sliding type isolator will be combined. Because the major part of the feasibility of this isolation system depends on the sealing function and durability of the hydraulic cylinder, a series of feasibility tests of the hydraulic cylinder have been conducted to verify the reliability against seismic load and seismic motion. This paper describes the specification of the seismic isolations system, seismic response characteristics and the results of the feasibility tests of the seal. This study was performed as part of a government sponsored R&D project on 3D seismic isolation.

Failure Analysis of Thinned Wall Elbows Under Excessive Seismic Loading

A series of finite element analyses has been carried out in order to investigate the failure behaviors of degraded bent pipes with local thinning against seismic loading. The sensitivity of such parameters as the residual thickness, locations and width of the local thinning to the failure modes such as ovaling and local buckling and to the low cycle fatigue damage has been studied. It has been found that this approach is useful to make a reasonable experimental plan, which has to be carried out under the condition of limited cost and limited period.