Design Safety Factors Research Articles

지류론과 암주 강도의 추정에 의한 주방식 지하구조의 무지보 암주 설계

Room-and-pillar mining method is one of the most popular underground mining method in the world. If the room-and-pillar mining method is able to be adopted in civil works, it would be highly probable to reduce underground construction costs and to expand a underground structure in use. Therefore, this study aims to analyze the design procedure of unsupported rock pillars which are indispensable to ensure the stability of a room-and-pillar underground structure. Parametric studies on their key design parameters are also carried out for 125 different kinds of design conditions. From the study, the width of a rock pillar is found to show a linear relationship with its corresponding safety factor. The safety factor of a unsupported rock pillar decreased drastically like a negative exponential function as the ratio of room width to pillar width increases in the same rock strength condition. Based on the parametric studies, a design chart to simply evaluate the geometric design parameters of a unsupported rock pillar satisfying a design safety factor is also proposed in this study.

Research on Soil Nailing Method for Large Foundation Pit Treatment

Based on the introduction of the characteristics of the supporting project of a large building foundation pit, the paper analyzed the main reasons for side wall collapse and pile fracture, including leakage water in the backfill soil, complex geological environment and unreasonable soil pressure value, low safety factor and improper design parameters, and then respectively proposed three kinds of treatment schemes for side wall sliding and pile breaking, landslides and other dangers. Finally, tracking observation showed that the satisfactory effect can strongly verify the suitability and validity of the main control parameters for the deformation of deep foundation pit, which may provide reference for the design and construction of similar projects.

Research on the Optimization Design of Gas-Solid Coupling of Wind Turbine Blades

The effects of high-altitude wind shear of wind speed should be considered, the appropriate number of distribution group should be determined and the distribution of wind speed hybrid model parameters and solving should be loaded onto the blades. When the stress of rotor blade is analyzed, the elastic deformation of the blades and the effects of the vibration of oscillating flow field on reaction of the blades must be considered during the operation. Thus the laws of blade load distribution and the corresponding deformation can be accurately calculated to analyze the interplay between deformation vibration airflow and paddle, to determine the law of deformation of rotor blades, the surface pressure distribution, stress distribution and blade design safety factors. When wind load changes with the wind speed, the adaptive blades are powered by aeroelastic, the law of wielding and flapping of the blade will be studied. The torsion coupling effects of adaptive blade, blade bending ,the law of torsional deformation variation of the angle of attack and the impact on aeroelastic blades structural strength will be explored. The results of this study design will be optimized as the power factor, improving the stability of the design of wind turbine power output, reducing material consumption blades absorb vibration and reducing wind load maneuvering to avoid blade stall achieve pitch control. Thus it provides a theoretical basis for optimal design of strength and stiffness of the elongated paddles.

Application of Shape Optimization Method on Steel Wheel Rim

Nowadays, weight reduction is great opportunity for automotive components design due to meet the structural strength without reducing the components safety. A classical try-and-error approach to design the automotive components in industries is inadequate which mean new methods are needed to enhance the design process. An improvement of the component can be achieved by adopting suitable optimization techniques at the early design stage. In this paper, the problem of automotive component design; steel wheel rim design in a view of weight reduction was tackled by means of shape optimization method. Design methodology was proposed to the original design of selected wheel rim and was continued optimized them by various reduction targets. The optimized design together with the datum has been analyzed and the results were compared and discussed. Shape optimization with 15% of reduction target at steel wheel rim was the best optimal design and it was met the design criteria and safety factor. By applying shape optimization, weight of steel wheel also has been reduced by using proper reduction target. The methodology has been proven to be successful in finding innovative and efficient layouts for automotive components design.

Comparison and Sensitivity Investigations of a CALM and SALM Type Mooring System for Wave Energy Converters

A quasi-static analysis and sensitivity investigation of two different mooring configurations—a single anchor leg mooring (SALM) and a three-legged catenary anchor leg system (CALM)—is presented. The analysis aims to indicate what can be expected in terms of requirements for the mooring system size and stiffness. The two mooring systems were designed for the same reference load case, corresponding to a horizontal design load at the wave energy converter (WEC) of 2000 kN and a water depth of 30 m. This reference scenario seems to be representative for large WECs operating in intermediate water depths, such as Weptos, Wave Dragon and many others, including reasonable design safety factors. Around this reference scenario, the main influential parameters were modified in order to investigate their impact on the specifications of the mooring system, e.g. the water depth, the horizontal design load, and a mooring design parameter.

Safety factors for the structural design of glass

The safety verification of glass structures is usually made on the basis of a deterministic approach, without assessing the underlying probability of collapse. In this article, we propose to use the semi- probabilistic method in the limit-state design of glass structures by presenting properly-calibrated values of the partial safety factors of material strength, so as to obtain a probability of failure compatible with the target values indicated for each class of consequence by Eurocode 1 (EN 1990).Starting from a micromechanically-motivated model of fracture propagation, typically used for brittle materials, experimental results conducted in a previous campaign have been interpreted using the Weibull statistical distribution, taking into account that size-effect, type of stress (e.g., uniaxial vs. bi-axial) and the insidious phenomenon of subcritical crack growth (static fatigue due to fracture growth in time without increase of load) can affect the probability of failure. Actions like wind, snow and live (anthropic) loads have been modeled using the statistical distributions recommended in international structural codes. Then, the probabilistic method of level III has been applied for the verification of paradigmatic case studies, which have served to calibrate the partial safety factors to be used in the semi-probabilistic approach. A novelty, to our knowledge, is the proposal of a multiplication coefficient for the partial safety factor of material strength, instead that for the factors of loads, to distinguish in the verification the different classes of consequences, each one characterized by the probability threshold of collapse. The results of this study will furnish the basis for the design of glass structures according to the general performance requirements established by EN 1990.

Stress Analysis and Optimization Design of Pressure Vessel of Underwater Gamma Spectrometer

Pressure vessel makes the underwater gamma spectrometer can operation in underwater environment. In this paper, a kind of cylindrical pressure vessel has been simulated and analyzed using CAD software named Solidworks. Analysis results show the end covers are much thicker than the side wall to satisfy the same design safety factor and the centers of the end covers are the stress concentration areas. Further more, a 2× 2 LaBr3: Ce scintillation detector and a series of pressure vessels with various design safety factors and same inner space have been simulated by Monte Carlo code MCNP. Calculation indicates that the thicker the shell, the lower the detection efficiency. Further more, calculation shows the impact of the pressure vessel on detection efficiency of underwater gamma spectrometer varies with the photon energy. The law is that the higher the photon energy, the lower the influence on the detection efficiency.

Snow load models for probabilistic optimization of steel frames

Three probabilistic snow load models based on the Poisson spike process and the block maxima method are derived from the second moment probabilistic description of monthly extreme values of snow load. For comparison of the models, the upper tail asymptotic approximations of their cumulative probability distribution functions of yearly maxima are obtained. The performance of the models is checked by the probabilistic design and verification of representative portal frames of low-rise industrial buildings exposed to snow and wind loads using the first-order reliability method (FORM). The statistical parameters of monthly maxima of climatic loads related to measurements at six locations in Germany are employed. The considered snow load has a pattern of a seasonal occurrence of snowfall. The Poisson spike process model is then applied to the probabilistic optimization of safety factors for the standardized design of frames. A novel optimization of variable safety factors is proposed to differentiate design for frames with light to heavy weight roofs. It is shown that this differentiation significantly reduces scatter of the reliability level around the target value.

Pile heat exchangers: thermal behaviour and interactions

Thermal piles – that is structural foundation piles also used as heat exchangers as part of a ground energy system – are increasingly being adopted for their contribution to more sustainable energy strategies for new buildings. Despite over a quarter of a century having passed since the installation of the first thermal piles in northern Europe, uncertainties regarding their behaviour remain. This paper identifies the key factors which influence the heat transfer and thermal–mechanical interactions of such piles. In terms of heat output, pile aspect ratio is identified as an important parameter controlling the overall thermal performance. Temperature changes in the concrete and surrounding ground during thermal pile operation will lead to additional concrete stresses and displacements within the pile–soil system. Consequently designers must ensure that temperatures remain within acceptable limits, while the pile geotechnical analysis should demonstrate that any adverse thermal stresses are within design safety factors and that any additional displacements do not affect the serviceability of the structure.

Applicability of Equivalent Constant Amplitude Loading for Assessing the Fatigue Life of Pipelines and Risers and the Influence of Compressive Stress Cycles

Fatigue life assessment of pipelines and risers is a complex process, involving various uncertainties. The selection of an appropriate fatigue model is important for establishing the inspection intervals and maintenance criteria. In offshore structures, the vortex-induced vibration (VIV) could cause severe fatigue damage in risers and pipelines, resulting in leakage or even catastrophic failure. The industry has customarily used simple fatigue models for fatigue life assessment of pipelines and risers (such as the Paris or Walker models); however, these models were developed based on constant amplitude loading scenarios. In contrast, VIV-induced stress-time history has a variable amplitude nature. The use of the simplified approach (which is inherently non conservative), has necessitated the implementation of large safety factors for fatigue design of pipelines and risers. Moreover, most of the experimental investigations conducted to date with the aim of characterizing the fatigue response of pipelines and risers have been done based on incorporation of constant amplitude loading (CAL) scenarios (which is unrealistic), or converting the variable amplitude loading (VAL) scenarios to an equivalent CAL. This study demonstrates that the use of such approaches would not be lead to accurate assessment of the fatigue response of risers subject to VIV-induced VAL. The experimental investigation performed in this study will also clarify the underlying reasons for the use of large safety factors by the industry when assessing the fatigue life of pipelines and risers. In addition, an experimental investigation was also conducted to highlight the influence of the compressive portion of VIV stress-time history on the fatigue life of such components. It is shown that the compressive stress cycles significantly influence the fatigue crack growth response of risers, and their presence should not be ignored.

Reliability-based partial safety factors for dual performance level design of prestressed inner containment shells in Indian nuclear power plants

Partial safety factors (PSFs) used in reliability-based design are intended to account for uncertainties in load, material and mathematical modeling while ensuring that the target reliability is satisfied for the relevant class of structural components in the given load combination and limit state. This paper describes the methodology in detail for developing a set of optimal reliability-based PSFs for the design of prestressed concrete inner containment shells in Indian NPPs under Main Steam Line Break (MSLB)/Loss of Coolant Accident (LOCA) conditions at two performance levels in flexure: cracking and collapse. The methodology follows current design practices in the country, accounts for uncertainties in loads and material properties and dependence among capacities and demands, develops the limit states from first principles, explicitly lays down the target reliabilities and criteria for PSF optimization. The optimization of the PSFs is based on reliability indices for each representative group of components obtained from importance sampling and a local linear response surface fit. A detailed numerical example on a typical 220MWe Indian PHWR demonstrating the methodology is provided.

Partial safety factor approach to the design of submarine pressure hulls using nonlinear finite element analysis

A framework for the design of submarine pressure hulls using nonlinear finite element (FE) analysis is presented in order to improve upon the conventional analytical–empirical design procedure. A numerical methodology is established that allows the collapse pressure of a hull to be predicted with controlled accuracy. The methodology is characterized by quasi-static incremental analysis, including material and geometric nonlinearities, of FE models constructed from shell elements. The numerical methodology is used with ANSYS to predict the results of 47 collapse experiments on small-scale ring-stiffened cylinders representative of submarine hulls. A probabilistic analysis is applied to the experimental–numerical comparisons in order to estimate the accuracy of the FE methodology and derive a partial safety factor (PSF) for design. It is demonstrated that a high level of accuracy, within 10% with 95% confidence, can be achieved if the prescribed FE methodology is followed. Furthermore, it is shown that the PSF for design does not need to be very large, even if a high degree of statistical confidence is built in. The designer can be 99.5% confident that the FE error has been accounted for by dividing the predicted collapse pressure by a PSF=1.134.

Evaluation of Process Uncertainty in Activated Sludge Treatment by Probabilistic Modeling

AbstractSimulation-based design and analysis of biotreatment systems depends to a great extent on the accuracy of the model parameters used. In many cases, site-specific parameter estimates are not available and parameter values recommended in the literature are adopted along with a measure of judgment to account for bioprocess uncertainty. To better quantify process certitude associated with this approach and thus provide guidance to process designers lacking site-specific parameter values, stochastic simulations were conducted and the empirical frequency distributions derived from the simulation results were interpreted in terms of process certitude. In general, the simulation results confirmed field experience and current practice in biotreatment from a probabilistic standpoint and provided context in terms of process certitude. The certitude of achieving specific treatment levels increased with sludge age, validating the application of a design safety factor on the minimum solids retention time. The i...

Towards multiple hazard resilient bridges: a methodology for modeling frequent and infrequent time-varying loads Part II, Examples for live and earthquake load effects

The current AASHTO load and resistance factor design (LRFD) guidelines are formulated based on bridge reliability, which interprets traditional design safety factors into more rigorously deduced factors based on the theory of probability. This is a major advancement in bridge design specifications. However, LRFD is only calibrated for dead and live loads. In cases when extreme loads are significant, they need to be individually assessed. Combining regular loads with extreme loads has been a major challenge, mainly because the extreme loads are time variable and cannot be directly combined with time invariant loads to formulate the probability of structural failure. To overcome these difficulties, this paper suggests a methodology of comprehensive reliability, by introducing the concept of partial failure probability to separate the loads so that each individual load combination under a certain condition can be approximated as time invariant. Based on these conditions, the extreme loads (also referred to as multiple hazard or MH loads) can be broken down into single effects. In this paper, a further breakdown of these conditional occurrence probabilities into pure conditions is discussed by using a live truck and earthquake loads on a bridge as an example.

Towards multiple hazard resilient bridges: a methodology for modeling frequent and infrequent time-varying loads Part I, Comprehensive reliability and partial failure probabilities

The current AASHTO load and resistance factor design (LRFD) guidelines are formulated based on bridge reliability, which interprets traditional design safety factors into more rigorously deduced factors based on the theory of probability. This is a major advancement in bridge design specifications. However, LRFD is only calibrated for dead and live loads. In cases when extreme loads are significant, they need to be individually assessed. Combining regular loads with extreme loads has been a major challenge, mainly because the extreme loads are time variables and cannot be directly combined with time invariant loads to formulate the probability of structural failure. To overcome these difficulties, this paper suggests a methodology of comprehensive reliability, by introducing the concept of partial failure probability to separate the loads so that each individual load combination under a certain condition can be approximated as time invariant. Based on these conditions, the extreme loads (also referred to as multiple hazard or MH loads) can be broken down into single effects. In Part II of this paper, a further breakdown of these conditional occurrence probabilities into pure conditions is discussed by using a live truck and earthquake loads on a bridge as an example. There are three major steps in establishing load factors from MH load distributions: (1) formulate the failure probabilities; (2) normalize various load distributions; and (3) establish design limit state equations. This paper describes the formulation of the failure probabilities of single and combined loads.

Bored pile design in stiff clay II: mechanisms and uncertainty

The soil mechanics related to pile design in clay has been the subject of substantial engineering research. In a companion paper, various codes of practice were reviewed showing the effect on pile capacity of the different global factors of safety that emerge from the various partial factor combinations for the ultimate limit state. Factors of safety are generally specified based on the opinions of experts. In this paper an assessment will be made of various objective procedures that can be used to reduce uncertainty in the design process, especially regarding the adoption of a pile resistance model and the selection of a soil strength profile as part of a ultimate limit state check, and the estimation of pile head settlement in the context of a serviceability limit state check. It is shown that both total stress and effective stress calculation methods are applicable in London Clay. Estimates of settlement using a non-linear soil stress–strain relationship are made and compared with published data. It is shown that the compression of the concrete dominates the settlement of long piles. Given the low settlements observed, recommendations are made for a reduction in standard factors of safety for bored pile design in stiff clays.

ENGINEERING DESIGN FROM A SAFETY PERSPECTIVE

Engineering design is an iterative decision- making process involving interactions between three elements: geometry, materials and loads. The objective is to provide an optimum combination of these design parameters. Unfortunately, the absolute optimum can rarely be achieved because the design criteria typically place counter opposing demands and uncertainties must be accommodated. To this end, the integration of both deterministic and stochastic methods into the product development process is encouraged. The deterministic method allows designers to calculate a design safety factor based on the uncertainties of a loss-of-function parameter and a maximum allowable parameter. Stochastic methods are based on the statistical nature of the design parameters and focus on the reliability of the design. Links between these elements will thus be emphasized and supported with examples from the recreational product industry.

A Study on the Holding Capacity Safety Factors for Torpedo Anchors

The use of powerful numerical tools based on the finite‐element method has been improving the prediction of the holding capacity of fixed anchors employed by the offshore oil industry. One of the main achievements of these tools is the reduction of the uncertainty related to the holding capacity calculation of these anchors. Therefore, it is also possible to reduce the values of the associated design safety factors, which have been calibrated relying on models with higher uncertainty, without impairing the original level of structural safety. This paper presents a study on the calibration of reliability‐based safety factors for the design of torpedo anchors considering the statistical model uncertainty evaluated using results from experimental tests and their correspondent finite‐element‐based numerical predictions. Both working stress design (WSD) and load and resistance factors design (LRFD) design methodologies are investigated. Considering the WSD design methodology, the single safety is considerably lower than the value typically employed in the design of torpedo anchors. Moreover, a LRFD design code format for torpedo anchors is more appropriate since it leads to designs having less‐scattered safety levels around the target value.

Verification of Recommended Load and Resistance Factor Design and Construction of Piles in Cohesive Soils

To enhance regional design and construction practices for driven piles, FHWA permitted the development of regional resistance factors for the design of foundation piles. By fitting allowable stress design safety factors to the load and resistance factor design (LRFD) framework, several state departments of transportation (DOTs), including the Iowa DOT, have adopted interim procedures. Subsequently, an LRFD procedure that incorporates setup was developed for piles in cohesive soils through comprehensive research in Iowa. The proposed LRFD procedure used an Iowa DOT in-house static analysis method and the wave equation analysis program for construction control. To verify the adequacy of the proposed procedure and investigate its economic implications, differences in pile design between the interim and the proposed LRFD procedures were evaluated on the basis of independent data collected from more than 600 production steel H-piles driven in cohesive soils. This study concluded that the proposed LRFD procedure would not significantly increase the design and construction costs. The incorporation of pile setup into the LRFD procedure was found to provide additional economic benefits. Although the current Iowa DOT policy is to drive piles to the contract length, if a suitable pile termination procedure were used once the desired resistance was achieved, a general saving of 20% in pile length would be anticipated for both procedures. Although the research and findings presented in this paper are specific to a local area, these methods could be adopted nationally to increase the efficiency of bridge foundations for all states.

Assessment of MARS Code for Predicting Critical Heat Flux under Heaving Conditions

A major safety factor in marine reactor design, critical heat flux (CHF), is assessed using the MARS system analysis code under heaving conditions. As gravity acceleration changes, the CHF is affected by the thermal hydraulics in the reactor through inlet flow fluctuations. Performing the analysis with the MARS code, which uses the properties of water for the working fluid, requires applying the CHF experimental data using fluid-to-fluid (FTF) scaling because most CHF experiments are conducted with Freon (R-113) as the working fluid. The FTF scaling methods suggested by Ahmad, Katto, and Coffield are adopted and compared. Otsuji et al.’s experiment, which was conducted using mass flow rate oscillation, is applied to evaluate the capability of MARS for heaving conditions. According to the calculations the FTF methods of Ahmad, Katto, and Coffield show good agreement (within an error of ±10.73% for Otsuji et al.’s experiment) for inlet flow rate oscillation corresponding to gravity acceleration in a vertical direction. In addition, variation of the acceleration affects the flow conditions, such as the mass flow rate and the void fraction. Thus, MARS has a noteworthy ability to predict the CHF for heaving conditions by simulating inlet flow rate oscillation.