In this paper, a base isolator called a multiple direction optimized-friction pendulum system (Multiple DO-FPS) with numerous sliding interfaces is proposed. To understand the mechanical behavior of the Multiple DO-FPS isolator under multidirectional excitations, an analytical model called the multiple yield and bounding surfaces model is proposed. On the basis of the derived mathematical formulations for simulation of the characteristics of the Multiple DO-FPS isolation bearing, it is revealed that the natural period and damping effect of the Multiple DO-FPS isolator are a function of the sliding displacement and sliding direction. By virtue of the proposed model, the phenomena of the sliding motions of the Multiple DO-FPS isolator with numerous sliding interfaces subjected to multidirectional excitations can be understood in a simple manner. The analytical results indicate that the natural frequency and damping effect of the Multiple DO-FPS isolator with numerous concave sliding interfaces change continually during earthquakes and are controllable through appropriate designs.

This paper investigates the use of dynamic vibration absorbers as a means to reduce the vibration of a floating roof due to sloshing caused by long-period earthquakes. It is shown that the damping ratio of the primary system caused by the dynamic vibration absorbers increases with rising filling level, hence increasing the probability of liquid overspilling, although the ratio of the vibration absorbers’ mass to the liquid mass decreases as the filling level rises. We also study dynamic vibration absorbers that can be more easily tuned to the filling-level dependent sloshing frequency. A feature of these vibration absorbers is that they use time-integral feedback of the primary structure’s displacement near the tuning frequency unlike ordinary vibration absorbers. Computer simulation is carried out using a sinusoidal wave function and an actual earthquake ground motion record as the excitation applied to the tank.

This paper describes the results of vibration tests using a 1/10 reduced scale model of large-scale cylindrical water storage tanks to clarify their dynamic behavior under seismic excitation. The thin sidewall of the tanks is not so rigid that the vibration modes (sloshing and bulging) induced by earthquake can affect the distribution of their liquid pressure and seismic load. It is, therefore, important for the seismic design of water storage tanks to consider such elastic deformation theoretically and experimentally. In this study, vibration tests by shaking table are conducted using a reduced scale tank model partially filled with water to investigate the dynamic fluid pressure behavior and seismic-proof safety of the tanks. A small sinusoidal excitation test, large amplitude sinusoidal excitation test and seismic excitation test are conducted. The measured values are compared with the calculated ones by some conventional seismic design methods. The results reveal that the distribution shape and magnitude of the dynamic fluid pressure are different between under positive and negative pressures and depend on the magnitude of input acceleration. Further examination concludes that the oval-type vibration, which is a high-order vibration mode, occurring on the sidewall of the tanks affects the distribution shape and magnitude of dynamic fluid pressure. However, it is demonstrated that the vibration does not act as a seismic load in the conventional evaluation of seismic-proof safety.

Pressurized piping systems used for an extended period may develop degradations such as wall thinning or cracks due to aging. It is important to estimate the effects of degradation on the dynamic behavior and to ascertain the failure modes and remaining strength of the piping systems with degradation through experiments and analyses to ensure the seismic safety of degraded piping systems under destructive seismic events. In order to investigate the influence of degradation on the dynamic behavior and failure modes of piping systems with local wall thinning, shake table tests using 3D piping system models were conducted. About 50% full circumferential wall thinning at elbows was considered in the test. Three types of models were used in the shake table tests. The difference of the models was the applied bending direction to the thinned-wall elbow. The bending direction considered in the tests was either of the in-plane bending, out-of-plane bending, or mixed bending of the in-plane and out-of-plane. These models were excited under the same input acceleration until failure occurred. Through these tests, the vibration characteristic and failure modes of the piping models with wall thinning under seismic load were obtained. The test results showed that the out-of-plane bending is not significant for a sound elbow, but should be considered for a thinned-wall elbow, because the life of the piping models with wall thinning subjected to out-of-plane bending may reduce significantly.

Motivated by the earthquake response of industrial pressure vessels, the present paper investigates externally-induced sloshing in horizontal-cylindrical and axisymmetric liquid containers. Assuming ideal and irrotational flow, small-amplitude free-surface elevation, and considering appropriate trigonometric functions for the sloshing potential, a two-dimensional eigenvalue problem is obtained for zero external excitation, which is solved through a variational (Galerkin) formulation that uses triangular finite elements. Subsequently, based on an appropriate decomposition of the container-fluid motion, and considering the eigenmodes of the corresponding eigenvalue problem, an efficient methodology is proposed for externally-induced sloshing through the calculation of the corresponding sloshing (or convective) masses. Numerical results are obtained for sloshing frequencies and masses in horizontal circular cylindrical, spherical, and conical vessels. It is shown that, in those cases, consideration of only the first sloshing mass is adequate to represent the dynamic behavior of the liquid container quite accurately. For the case of a horizontal cylinder subjected to longitudinal external excitation, its equivalence with an appropriate rectangular container is demonstrated. The numerical results are in very good comparison with available semi-analytical or numerical solutions and available experimental data.

The feeder pipes of a typical CANDU ® reactor supply each of the 380 fuel channels with coolant and are individually connected to the large bore primary piping. In some designs, the longer pipes are interlinked with spacer rods, which mitigate vibrations and prevent contact between adjacent feeder pipes. Under severe loading conditions, the spacer rods will yield, which will prevent overstressing the pipes as the stiffness of the coupling gets reduced. Spacer yielding also provides increased damping for dynamic loads. It has recently become practicable to model the entire interlinked system in a single model. This paper presents the modeling approach for seismic analysis and for the fuel channel creep (a radiation induced slow unidirectional anchor motion). Results with and without spacer bars are compared to illustrate the effect of spacers. The importance of representing spacer yielding in the model is discussed.

The estimation of reliability of system subjected to earthquake excitations is an important problem for aseismic design. The reliability of such system should be evaluated in probabilistic manner. The first excursion failure is one of the most important failure modes of structures and also one factor of reliability. Many structures have nonlinear characteristics. Hysteresis loop characteristic caused by plastic deformation is one of the most common nonlinear characteristics observed in pressure vessels and piping systems. In this paper, an estimation method for the first excursion probability of structure with hysteresis loop characteristic is proposed. The first excursion probability is the function of many parameters, which is obtained by using artificial time histories. It is shown that when the tolerance level is normalized by the expected values of the maximum response of the structures, the first excursion probability can be shown to be independent of many parameters.

In Japan, mechanical structures installed in nuclear power plants, such as piping and equipment, are usually designed statically in an elastic region. Although these mechanical structures have sufficient seismic safety margin, understanding the ultimate fatigue endurance is very important in order to improve the seismic safety reliability for unexpected severe earthquakes. Moreover, clarifying a margin of seismic resistance of mechanical structures that suffered a severe earthquake is being required. In this study, the energy balance equation that is one of valid methods for structural calculation is applied to the above-mentioned issues. The main feature of the energy balance equation is that it explains accumulated information of motion. Therefore the energy balance is adequate for the investigation of the influence of cumulative load such as seismic response. The investigation is implemented by forced vibration experiments. The experiment models are simple single–degree-of-freedom models that are made of stainless steel and carbon steel. In the experiment, random waves having predominant frequency similar to natural frequency of the experimental model are input in order to obtain adequate response not only in the elastic region but also in the plastic region. As a result, experimental models vibrate under resonance condition, so response acceleration is approximately seven times as big as the input. The excitation is continued until the experimental models fracture, and is carried out with various input levels. In the experiment, models suffered cracks at the bottom end, and fractured finally. As a result, input energy for failure increases as time for failure. In other words, more input energy for failure is needed in case of small input. Moreover the correlation between increment in input energy and input energy for failure is investigated. It was confirmed that input energy for failure is inversely proportional to increment in input energy per unit time. Additionally energy for failure of stainless steel is about twice as big as carbon steel. The correlation between fatigue failure and energy is confirmed from the vibration experiment. Therefore it is expected that time for fatigue failure can be evaluated by the energy balance equation.

The characteristics of dynamic strength and fracture in structural steels and their welded joints particularly for pipelines should be evaluated based on the effects of the strain rate and service temperature. The temperature, however, rises so rapidly in structures due to the plastic work under the high strain rate such as ground sliding by earthquake when the effect of the temperature cannot be negligible for the dynamic fracture. It is difficult to predict or measure the temperature rise history with the corresponding stress-strain behavior, including the region beyond the uniform elongation, though the behavior at the large strain region after the maximum loading point is very important for the evaluation of fracture. In this paper, the coupling phenomena of the temperature and stress-strain fields under dynamic loading were simulated by using the finite element method. A modified rate-temperature parameter was defined by accounting for the effect of the temperature rise under rapid plastic deformation, and it was applied to the fully coupled analysis between the heat conduction and thermal elastic-plastic behavior. The temperature rise and stress-strain behavior, including the coupling phenomena, were studied including the region beyond the maximum loading point in structural steels and their undermatched joints, and then compared with the measured values.

An explicit analytical solution is derived for sloshing in a cylindrical liquid storage tank with a single-deck type floating roof under seismic excitation. The floating roof is composed of an inner deck, which may be idealized as an isotropic elastic plate with uniform thickness and mass, and connected to an outer pontoon, which can be modeled as an elastic curved beam. The contained liquid is assumed to be inviscid, incompressible, and irrotational. By expanding the response of the floating roof into free-vibration modes in air and applying the Fourier–Bessel expansion technique in cylindrical coordinates, the solution is obtained in an explicit form, which is exact within the framework of linear potential theory. Numerical results are presented to investigate the effect of the type (single-deck or double-deck) and stiffness of the floating roof on the sloshing response.

A finite element method (FEM)-based formulation is developed for an effective computation of the eigenmode frequencies, the decomposition of total liquid mass into impulsive and convective parts, and the distribution of wall pressures due to sloshing in liquid storage tanks of arbitrary shape and fill height. The fluid motion is considered to be inviscid (slip wall condition) and linear (small free-surface steepness). The natural modal frequencies and shapes of the sloshing modes are computed, as a function of the tank fill height, on the basis of a conventional FEM modeling. These results form the basis for a convective-impulsive decomposition of the total liquid mass, at any fill height, for the first few (two or three at most) sloshing modes, which are by far the most important ones in comparison to all other higher modes. This results into a simple yet accurate and robust model of discrete masses and springs for the sloshing behavior. The methodology is validated through comparison studies involving vertical cylindrical tanks. Additionally, the application of the proposed methodology to conical tanks and to the seismic analysis of spherical tanks on a rigid or flexible supporting system is demonstrated and the results are compared to those obtained by rigorous FEM analyses.

This paper investigates the characteristics of the suppression of sloshing in a vessel by a bulkhead that divides the vessel vertically into two sections but allows communication between two sections at the lower part of the vessel. The sloshing behavior is thus separated into a U-tube sloshing mode and a sloshing mode confined to each of the separated compartments. The effect of the aperture height beneath the bulkhead and the amplitude of the excitation on the characteristics of the suppression of the U-tube sloshing mode is investigated experimentally. As a result, the following phenomena are clarified. Three types of flow patterns were observed: flow pattern A, with unsteady vortices, flow pattern B, with a single swirl and a vortex, and flow pattern C, with twin swirls. The flow pattern changes from A to B or C as the amplitude of the excitation increases. The damping ratio with the bulkhead is much larger than that without the bulkhead. The damping generated by the bulkhead depends on the amplitude of the excitation. The flow pattern plays an important role in the dissipation of the energy of the sloshing. In particular, the vortices and swirls increase the rate of energy dissipation of the sloshing.

It is important to assess the failure strengths for pipes with wall thinning to maintain the integrity of the piping systems and to make codification of allowable wall thinning. Full-scale fracture experiments on cyclic loading under constant internal pressure were performed for 4 in. diameter straight pipes and 8 in. diameter elbow pipes at ambient temperature. The experiments were low cycle fatigue under displacement controlled conditions. It is shown that a dominant failure mode under cyclic loading for straight pipes and elbows is crack initiation∕growth accompanying swelling by ratchet or buckling with crack initiation. When the thinning depth is deep, the failure mode is burst and crack growth with ratchet swelling. In addition, failure strengths were compared with the design fatigue curve of the ASME Code Sec. III. It is shown that pipes with wall thinning less than 50% of wall thickness have sufficient margins against a seismic event of the safety shutdown earthquake.

An analytical solution is presented to predict the sloshing response of a cylindrical liquid storage tank with a floating roof under seismic excitation. The contained liquid is assumed to be inviscid, incompressible, and irrotational, while the floating roof is idealized as an isotropic elastic plate with uniform stiffness and mass. The dynamic interaction between the floating roof and the liquid is taken into account exactly within the framework of linear potential theory. By expanding the response of the floating roof into free-vibration modes in air and employing the Fourier–Bessel expansion method in cylindrical coordinates, the solution is obtained in an explicit form, which will be useful for parametric understanding of the sloshing behavior and preliminary study in the early design stage. Numerical results are also provided to investigate the effect of the stiffness and mass of the floating roof on the sloshing response.

Most of the base isolated buildings or structures are built on laminated rubber bearings in order to give them certain natural periods. This situation, however, also encourages structural engineers to research and develop nonrubber-type isolation systems such as linear motion bearing isolators and friction pendulum systems. It is considered that the nonrubber-type isolation systems can be applied to important industrial facilities, such as LNG tanks, boiler facilities, and so on, to refine their seismic reliabilities. This device of a nonrubber-type isolation system uses the energy loss associated with sliding to reduce the deleterious effects of earthquakes. However, when using nonrubber-type isolation systems with sliding in the atmosphere, long term durability of the systems must be taken into account. It may be difficult to maintain the friction coefficient of the system. In this paper, a stochastic study of the effect on rotational motion and isolation performance of two structures subjected to an earthquake with a friction pendulum bearing is analyzed with a Monte Carlo method.