Set Of Partial Safety Factors Research Articles

Assessment of Railway Bridge Reliability Using Load Measurement Data

A methodology is introduced that allows to determine the reliability of railway bridges subject to freight trains and which comply with code based assessment according to the Swiss structural code SIA 269. The methodology uses probabilistic models for section forces from freight trains. These models are based on a large database of wheel load measurements. A case study demonstrates how the methodology is applied to determine the reliability of generic bridges and subsequently allows the assessment of the reliability for a portfolio of railway bridges. The results show that the target reliability is achieved according to SIA 269 considering the code format and current set of partial safety factors. The study identifies potential load bearing reserves for bridges with longer spans.

Closed-form expressions for partial safety factors used in the seismic assessment of existing RC buildings

A new set of partial safety factors is proposed for the seismic safety assessment of existing reinforced concrete buildings. These safety factors are derived using a framework that focuses on the determination of the uncertainty in the limit state capacities. Existing seismic safety assessment standards, such as EN1998-3 or ASCE 41-17, follow a component-based approach that consider limit-state acceptance criteria based on capacity-related parameters to verify the safety of ductile and brittle failure modes. Despite their similar limit-state philosophy, these standards consider different conceptual formats for introducing the effects of uncertainties: the current version of EN1998-3 adopts confidence factors (CF) that factorize the mean values of material strength, while ASCE 41-17 uses a knowledge factor (k) to reduce the capacity of the component. The CF-based approach of EN1998-3 has been seen to lead to unclear levels of safety that may not reflect the impact of all the uncertainties about the different parameters or the sensitivity of different capacity models to these parameters. To address these issues, the proposed framework establishes a set of partial safety factors derived using an approach similar to that of ASCE 41-17, and involves recent capacity models, their uncertainties and a clear uncertainty management strategy. Furthermore, these new partial safety factors can also be seen to be compatible with the safety format proposed by the upcoming second-generation EN1998-3.

A time-independent reliability based design approach for rockfall net fences: a comparative analysis within the Eurocode framework

The design of net fences as passive mitigation measures against rockfall events has represented a challenge since the last decades. The choice of the proper effect of the actions to consider in the design is still under debate. Recently, the Authors have proposed a novel time-independent reliability approach encompassing the large variability of the size and the kinematics of the possible impacting blocks. The entire statistics of all these quantities (size, velocity, and height) enters into the calculations, differently from other approaches that consider specific values of the parameters. In addition, the variability in time of the inputs is tackled, with particular reference to the size of the falling block. The recent approach is herein merged and compared with the current semi-probabilistic ultimate limit state design approach, suggested in the Eurocodes and implemented in the Italian recommendations UNI 11211-4:2018, with the purpose of finding the equivalent partial safety factors of kinetic energy and trajectory height of the impacting block. A sensitivity analysis with different synthetic profiles, representing possible real situations, is performed highlighting that if a set of partial safety factors is assigned to different sites, an intrinsic variability in the failure probability has to be accepted.

Reliability-Based Design of Protection Net Fences: Influence of Rockfall Uncertainties through a Statistical Analysis

Net fences are among the most widespread passive protective measures to mitigate the risk from rockfall events. Despite the current design approach being based on partial safety factors, a more efficient time-dependent reliability approach has been recently introduced by the authors. This method is influenced by various parameters related to the geometry and the kinematics of the block, i.e., the uncertainty related to the distribution of the size of the impacting block, its occurrence probability, and the shape of the right-tail of the distributions of its velocity and trajectory height at the location of the net fence. Furthermore, the block size distribution of the deposit greatly affects the results. The present work focuses on the possible range of such parameters to encompass the great majority of real events. The obtained results are compared with the current design approaches based on fixed partial safety factors. It emerges that the choice of the characteristic mass of the block and the failure probability greatly influence the results. Moreover, if a set of partial safety factors is assigned to different sites, an intrinsic variability in the failure probability has to be accepted. Suggestions for an accurate procedure and future developments are provided.

Reliability-based calibration of Brazilian structural design codes used in the design of concrete structures

Abstract This paper presents a reliability-based calibration of partial safety factors for Brazilian codes used in the design of concrete structures. The work is based on reliability theory, which allows an explicit representation of the uncertainties involved in terms of resistances and loads. Regarding the resistances, this study considers beams with concrete of five classes (C20, C30, C40, C50 and C60), three ratios between base and effective depth (0.25, 0.50 and 0.75), three longitudinal reinforcement ratios (ρmin, 0.5% and ρmax) and three transverse reinforcement ratios ( A s s m i n, 5 . A s s m i n and A s s m a x). In terms of loads, this work considers seven ratios between live loads and permanent loads (qn/gn), and seven ratios between wind loads and permanent loads (wn/gn). The study also adopts a single value for the target reliability index (βtarget = 3.0). Results show that the optimized set of partial safety factors leads to more uniform reliability for different design situations and load combinations.

Safety variability assessment of reinforced concrete beams subjected to bending moment considering the NBR 6118:2014 safety partial factors

Abstract This paper presents a study about safety of reinforced concrete beams subjected to bending moment and designed with the partial safety factors proposed by ABNT NBR 6118:2014. The main goal was to assess the uniformity in the safety of the beams considering different values for the neutral axis position and the load ratio, by using the Reliability Theory. A simplified procedure to calibrate the partial safety factors was proposed taking into account the nature of each random variable and a target reliability index. From the analysis of the results, an alternative method for the design of reinforced concrete beams was also proposed, in which safety is guaranteed by the using of a probability of failure instead a set of partial safety factors. The results showed the lack of uniformity in the safety of the beams design with de constant set of partial safety factors. The procedures of design and calibration of the new safety factors were capable of to give uniformity to the safety of the beams and to achieve the proper structural configuration with the required safety level.

The 2016-update of the Dutch Design Guideline for Basal Reinforced Piled Embankments

The Dutch Design Guideline CUR226 for basal reinforced piled embankments was revised before publishing its second edition in Dutch and English in 2016. This paper reports about the main changes in comparison to its first edition, of 2010. One of the main changes was adopting the Concentric Arches method of Van Eekelen et al. (2012b, 2013, 2015) and van Eekelen (2015) for the design of the geosynthetic reinforcement (GR). An accompanying set of partial safety factors was derived with an extensive probabilistic study. The Eurocode guidelines for traffic load were adopted and converted into a uniformly distributed design load for piled embankments using the Boussinesq spreading method. This resulted in a practical set of tables. Finally, a design guideline for the pile caps on top of the piles was added. Extensive calculation examples support the use of the new guideline.

Safety factors for CFRP strengthening in bending of reinforced concrete bridges

Concerning the strengthening in bending of reinforced concrete bridge decks, the use of fiber reinforced polymers (FRP) has increased due to its easy application and excellent mechanical and chemical properties. However, due to the lack of codes and standards and the lack of experience in the long term behavior, uncertainties exist in the calculation bases along the dimensioning of this reinforcement and more precisely in the partial coefficients of safety to be adopted for the material properties. As a consequence, bridge engineers are reluctant to use composite materials in the strengthening of damaged reinforced concrete bridge decks. To try to overcome this problem, this paper describes the methodology for a reliability-based calibration of the partial safety factors to be used for the CFRP material in the design of strengthening to bending. The method requires the definition of a response model jointly with the statistical definition of the model error. This is discussed in the first part of the paper. The reliability-based procedure is developed based on the design equation and the corresponding model. A simple set of partial safety factors is finally proposed for a representative population of RC bridges. The conclusions highlight the importance of incorporating the model error in the calibration. In addition, the paper shows how in some cases it is not feasible to design a flexural strengthening by using CFRP.

Partial safety factors for CFRP-wrapped bridge piers: Model assessment and calibration

Concerning the strengthening or seismic retrofitting of bridge piers, the use of fiber reinforced polymers (FRP) has increased as an element of confinement of concrete due to its easy application and excellent mechanical and chemical properties. However, due to the lack of codes and standards and the lack of experience in the long term behavior, uncertainties exist in the calculation bases along the dimensioning of this reinforcement and more precisely in the partial coefficients of safety to be adopted for the material properties. As a consequence, bridge engineers are reluctant to use composite materials in the strengthening of damaged bridge piers. To try to overcome this problem, this paper describes the methodology for a reliability-based calibration of the partial safety factors to be used for the confined concrete in the design of strengthening to axial-bending forces using CFRP. The method requires the definition of a response model jointly with the statistical definition of the model error. This is discussed in the first part of the paper. The reliability-based procedure is developed based on the design equation and the corresponding model. A simple set of partial safety factors is finally proposed and compared with those proposed in existing guidelines.

A first attempt towards reliability-based calibration of Brazilian structural design codes

This paper addresses the reliability-based calibration of partial safety factors for Brazilian design codes NBR8681:2003 (Actions and Safety of Structures) and NBR8800:2008 (Design of Steel and Steel-Concrete Composite Structures). To the author’s knowledge, these codes have never been subject to reliability-based calibration of partial and load combination factors. This paper represents a first effort in reaching this goal. The present calibration effort is based on actual data for wind loads in south east of Brazil, but uses mainly international data for other problem parameters. So far, the investigation is limited to structural steel members. The investigation leads to a set of optimized partial safety factors, which are compared to partial factors currently used in Brazilian design codes. Results show that the optimized set of partial factors leads to more uniform reliability for different design situations and load combinations. The investigation includes an analysis of the economical impact of replacing the current set of partial factors by the calibrated factors found in this paper. It is shown that, with an optimized set of partial safety factors, it is possible to maintain the current level of reliability and still produce a 5% average reduction in expenditure with structural materials, nationwide. The paper also points to general similarities between Brazilian design codes, the new generation of EUROCODES and American ANSI/AISC codes. These codes are compared in terms of their ability to tuations. : structural reliability, design codes, code calibration

An empirical model for predicting the mechanical properties of FRP-confined concrete

The mechanism of confining concrete using FRP materials was studied experimentally in numerous works, whereas several models were proposed for the prediction of the mechanical properties of FRP-confined concrete. The first part of this paper is devoted to systematic assessment of the performance of some of the existing models. Since this assessment proves that there is room for more accurate prediction of both strength and ultimate strain of confined concrete, the second part of the paper deals with derivation of an empirical model valid for both circular and prismatic elements. The model adopted by Eurocode 2 for concrete confined by means of hoops or spirals was used as a basis. The validity of the proposed model is checked against experimental results provided by 84 experimental programs (including 785 cylindrical specimens and 289 prismatic specimens). The model proposed in this paper is in good agreement with the experimental results. A set of partial safety factors is proposed for the derivation of design values for strength and ultimate strain of FRP-confined concrete.

Ice loading on Danish wind turbines: Part 2. Analyses of dynamic model test results

The present paper deals with ice loads on wind turbine foundations from large floating ice floes. Some newly developed procedures to estimate the extreme ice load and combined loads from wind with loads from ice are described. The procedures have been developed as a part of the design basis for two of the first large offshore wind farms in Denmark, and they have been further developed in a research project including ice model tests with cones and vertical cylinders at the National Research Council (NRC), Canada. The principles are included in the new “Danish Recommendation to Technical Approval of Offshore Wind Turbines”. The design procedures are an integrated part of a selected design philosophy, which also includes a procedure for defining a consistent set of partial safety factors for the various extreme load cases. For a partial safety factor for wind loads of 1.4 to 1.5 (as defined in the Danish standards), the partial coefficients for ice load should be 2.0 to 2.5, depending on the determining ice load case.

Partial safety factors for designing and assessing flexible pavement performance

In designing and assessing pavement performance, the uncertainty in material properties and geometrical variables of pavement and in traffic and environmental actions should be considered. A single factor is employed to deal with these uncertainties in the current American Association of State Highway and Transportation Officials (AASHTO) guide for design of pavements. However, use of this single factor may not ensure reliability-consistent pavement design and assessment because different random variables that may have different degrees of uncertainty affect the safety and performance of pavement differently. Similar problems associated with structural design have been recognized by code writers and dealt with using partial safety factors or load resistance factors. The present study is focused on evaluating a set of partial safety factors to be used in conjunction with the flexible pavement deterioration model in the Ontario pavement analysis of cost and the model in the AASHTO guide for evaluating the flexible pavement performance or serviceability. Evaluation and probabilistic analyses are carried out using the first-order reliability method and simple simulation technique. The results of the analysis were used to suggest factors that could be used, in a partial safety factor format, for designing or assessing flexible pavement conditions to achieve a specified target safety level.Key words: deterioration, reliability, pavement, serviceability, stochastic process, performance, partial safety factor.

Optimization of a design code for wind-turbine rotor blades in fatigue

A reliability-based calibration of a design code for wind-turbine rotor blades is presented. Design against fatigue failure in flapwise bending is considered. The calibration is performed by means of a numerical optimization technique in conjunction with probabilistic and deterministic models for load and resistance. The probabilistic models allow for predictions of the reliability against fatigue failure. The deterministic models express design load and design resistance in terms of characteristic values and partial safety factors. The code calibration consists of determination of the partial safety factors such that — over a specified scope of code — designs with reliabilities with minimum deviations from some prescribed target reliability result when the deterministic models are applied with this particular set of partial safety factors. The scope of code consists of a number of representative design cases, each defined as a particular combination of wind turbine, location and blade material. Emphasis is placed on demonstration of the code calibration principles and a proper formulation of the code format.

Reliability-Based Optimization of Design Code for Tension Piles

A reliability-based calibration of a design code for offshore tension pile foundations in clay is presented by an example. The design against pullout in ultimate loading is considered. The calibration is performed by means of a numerical optimization technique in conjunction with probabilistic and deterministic models for load and capacity. The probabilistic models allow for predictions of the reliability against pullout. The deterministic models express design load and design capacity in terms of characteristic values and partial safety factors. The code calibration consists of determination of the partial safety factors such that, over a specified scope of code, designs with reliabilities with minimum deviations from some prescribed target reliability result when applying the deterministic models with this particular set of partial safety factors. For tutorial purposes, the scope of code is represented by a limited number of design cases, and emphasis is placed on the demonstration of the code calibration principles and a proper formulation of the code format.

Reliability-Based Partial Safety Factors in Cantilever Construction of Concrete Bridges

One of the most important aspects of the construction of concrete bridges using the balanced cantilever method is the limit state of the loss of stability by overturning. This problem is especially relevant because of the lack of guidance in most structural codes about the actions involved and the safety factors to be used. First, the main assumptions, actions, and the methodology involved in the construction of such bridges are presented. The result of the reliability-based calibration process is the definition of a set of partial safety factors to apply in the design of the elements used for ensuring stability during the construction of bridges using this method. The bridges covered in this study are those with main spans between 80 and 140 m. This range includes most of the concrete bridges constructed with the cantilever method. The derived partial safety factors are also valid for a wide range of estimated live loads and wind forces on the deck during consruction. Finally, an example is presented comparing the design values of the stabilizing elements for a bridge with a main span of 120 m, obtained using the proposed safety factors and those computed with a global safety factor of 1.5.