Simplified model for the seismic performance of unanchored liquid storage tanks

Ground-supported unanchored liquid-storage cylindrical tanks, when subjected to strong seismic loading may exhibit uplifting of their bottom plate, which has significant effects on their dynamic behavior and strength. Those effects mainly concern: (a) the increase of axial (meridional) compression at the tank base, resulting in premature buckling in the form of elephant's foot and (b) the significant plastic deformation at the vicinity of the welded connection between the tank shell and the bottom plate that may cause failure of the welded connection due to fracture and fatigue. The present study focuses on base uplifting mechanics and tank performance with respect to the shell/plate welded connection through a numerical two-step methodology: (1) a detailed finite element shell model of the tank for incremental static analysis, capable of describing the state of stress and deformation at different levels of loading and (2) a simplified modeling of the tank as a spring-mass system for dynamic analysis, enhanced by a nonlinear spring at its base to account for the effects of uplifting. Two cylindrical liquid storage tanks of different aspect ratios are modeled and analyzed in terms of local performance of the welded connection. The results are aimed at better understanding of tank uplifting mechanics and motivating possible amendments in existing seismic design provisions. Copyright © 2015 by ASME

A simple and efficient model for seismic response and low-cycle fatigue assessment of uplifting liquid storage tanks

Ground-supported unanchored liquid-storage cylindrical tanks, when subjected to strong seismic loading may exhibit uplifting of their bottom plate, which may have significant effects on their dynamic behavior and structural integrity. In particular, due to uplifting, a substantial amount of plastic deformation develops at the vicinity of the welded connection between the tank shell and the bottom plate that may cause failure of the welded connection due to fracture or fatigue, associated with loss of tank containment. The present study focuses on the base uplifting mechanism and tank performance with respect to the shell/plate welded connection through a numerical simple and efficient methodology that employs primarily a simplified modeling of the tank as a spring-mass system for dynamic analysis, enhanced by a nonlinear spring at its base to account for the effects of uplifting, supported by a detailed finite element model of the tank for incremental static analysis. The latter model is capable of describing with accuracy the state of stress and deformation at different levels of lateral loading, in order to obtain the overturning moment-rocking angle relationship to be used in the simplified model. The methodology is applied in two cylindrical liquid storage tanks of different aspect ratios focusing on local performance of the welded connection, towards assessing the strength of the welded connection. The numerical results provide better understanding of tank uplifting mechanics and strength against failure of the welded connection at the tank bottom. Furthermore, the proposed methodology can be used for efficient assessment of uplifting effects on tank structural safety, towards minimizing seismic risk in industrial facilities. © 2017 Elsevier Lt

Elephant's foot buckling and imperfection sensitivity of internally pressurized thin-walled metal cylinders subjected to bending

Upright cylindrical steel storage tanks, subjected to lateral loading, may fail in the form of local buckling near their bottom, due to the simultaneous action of excessive axial compression in the presence of internal pressure, a failure mode known as "elephant's foot buckling". The present paper is aimed at examining this failure mode rigorously, considering material and geometric nonlinearities, accounting for initial imperfections for pressure levels up to 80% of yield pressure. The accuracy of simplified axisymmetric models in modelling this instability mode is also examined. Finally, the numerical results are compared with the predictions from the relevant provisions of EN 1993-1-6 and EN 1998-4. Copyright ©: SDSS'2016

Seismic response of unanchored liquid storage tanks

Unanchored liquid storage tanks under strong earthquake loading tend to uplift. In the present study, the effects of base uplifting on the seismic response of unanchored tanks are presented with emphasis on elephant's foot buckling, base plate strength and shell-To-base connection capacity. Towards this purpose, base uplifting mechanics is analyzed through a detailed simulation of the tank using non-linear finite elements, and a static pushover analysis is conducted that considers the hydrodynamic pressure distribution due to seismic loading on the tank wall and the base plate. The uplifting provisions from EN 1998-4 and API 650 Appendix E standards are briefly described. Numerical results for a typical 27.8-meter-diameter steel tank are compared with the above design provisions. © 2013 ASME

Seismic response of unanchored liquid storage tanks

Unanchored liquid storage tanks under strong seismic loading may exhibit uplifting of their bottom plate, with significant effects on the dynamic behavior and the structural integrity of the tank. In the present paper, base uplifting mechanics is examined numerically through a two-step methodology: (a) a detailed finite element shell model of the tank for incremental static analysis, capable of describing the state of stress and deformation at different levels of loading and (b) a simplified modeling of the tank as a spring-mass system for dynamic analysis, enhanced by a nonlinear spring at its base to account for the effects of uplifting. Three cylindrical liquid storage tanks of different aspect ratios are modeled and examined both as anchored and unanchored. The results are aimed at possible revisions in the relevant seismic design provisions of EN 1998-4 and API 650. Copyright © 2013 by ASME

Performance criteria for liquid storage tanks and piping systems subjected to seismic loading

In this paper, performance criteria for the seismic design of industrial liquid storage tanks and piping systems are proposed, aimed at defining a performance-based design framework towards reliable development of fragility curves and assessment of seismic risk. Considering "loss of containment" as the ultimate damage state, the proposed performance limits are quantified in terms of local quantities obtained from a simple and efficient earthquake analysis. Liquid storage tanks and the corresponding principal failure modes (elephant's foot buckling, roof damage, base plate failure, anchorage failure and nozzle damage) are examined first. Subsequently, performance limits for piping systems are presented in terms of local strain at specific piping components (elbows, Tees and nozzles), against ultimate strain capacity (tensile and compressive) and low-cycle fatigue. Modeling issues for liquid storage tanks and piping systems are also discussed, for an efficient analysis that provides reliable estimates of local strain demand. These models are compared successfully with available experimental data. Using those reliable numerical models, the proposed performance limits are applied in two case studies: (a) a liquid storage tank and (b) a piping system, both located in areas of high seismicity. Copyright © 2015 by ASME

Performance Criteria for Liquid Storage Tanks and Piping Systems Subjected to Seismic Loading

In this paper, performance criteria for the seismic design of industrial liquid storage tanks and piping systems are proposed, aimed at introducing those industrial components into a performance-based design (PBD) framework. Considering "loss of containment" as the ultimate damage state, the proposed limit states are quantified in terms of local quantities obtained from a simple and efficient earthquake analysis. Liquid storage tanks and the corresponding principal failure modes (elephant's foot buckling, roof damage, base plate failure, anchorage failure, and nozzle damage) are examined first. Subsequently, limit states for piping systems are presented in terms of local strain at specific piping components (elbows, Tees, and nozzles) against ultimate strain capacity (tensile and compressive) and low-cycle fatigue. Modeling issues for liquid storage tanks and piping systems are also discussed, compared successfully with available experimental data, and simple and efficient analysis tools are proposed, toward reliable estimates of local strain demand. Using the above reliable numerical models, the proposed damage states are examined in two case studies: (a) a liquid storage tank and (b) a piping system, both located in areas of high seismicity. © 2017 by ASME

Association between domestic pig farm's characteristics in relation to the African swine fever occurrence in Peninsular Malaysia

As ASF outbreaks among domestic pig farms in Peninsular Malaysia involve farms of various scale with different numbers of pig populations, determination of the risk factors related to farm characteristics would be useful to develop and strategise risk-based preventive measures. The present study was conducted to determine the pig farms characteristics and related risk factors associated with the ASF outbreaks in domestic pig farms in Peninsular Malaysia. The association between the 441 domestic pig farm characteristics and the farms ASF status at the farm level was analysed using univariate and multivariate logistic regression. The farm characteristics included in the study are the season at the farm during the outbreak period, location of the farm based on the population of pigs, housing system, farm scale based on SPP, farm density, breed type, BOD, buffer zone and MyGap certification status. Meanwhile, ASF-positive farms are farms which had reported ASF cases confirmed through Real Time PCR (RT-PCR) method. Farms located in states with high pig populations and the season at the farm during the outbreak period were identified to be more likely exposed to ASF outbreaks. The finding of this study provides an important insight that farms located in states with high pig populations and the season during the outbreak period are significantly associated with the introduction of the ASFV into domestic pig farms