Development of load and resistance factor design format for flexible pavements

The objective of pavement design, just as with the design of other structures, is to provide economical designs at specified levels of reliability. Methods that yield designs with different levels of reliability are undesirable, and over the course of time design approaches in the United States have converged toward the load and resistance factor design (LRFD) format in order to assure uniform reliability. At present the LRFD format has been implemented in concrete, steel, wood, and bridge design specifications. In this paper, reliability concepts are used to illustrate the development of an LRFD format for fatigue design of flexible pavements. It is shown that 10 candidate pavement sections designed against premature fatigue failure according to standard practice using the DNPS86 software do not have uniform reliability. It is demonstrated that uniform reliability can be achieved by using the LRFD format. The work reported is based on assumed variations of pavement layer properties and on analytical formulation; field verification was not attempted.Key words: LRFD, reliability index, fatigue, partial safety factors, flexible pavement design.

Download Full-text

Load and Resistance Factor Design (LRFD) of Nuclear Straight Pipes for Loads That Cause Primary Stress

Journal of Pressure Vessel Technology ◽

10.1115/1.4000976 ◽

2010 ◽

Vol 132 (2) ◽

Cited By ~ 6

Author(s):

Kleio Avrithi ◽

Bilal M. Ayyub

Keyword(s):

Resistance Factor ◽

Safety Factors ◽

Primary Stress ◽

Safety Margins ◽

Division 1 ◽

Partial Safety Factors ◽

Factor Design ◽

Load And Resistance Factor ◽

Different Levels ◽

Allowable Stress Design

Class 2 and 3 nuclear piping is designed according to the allowable stress design (ASD) method used in the ASME Boiler and Pressure Vessel (B&PV) code, Sec. III, Division 1, NC and ND-3600 according to which safety factors applied to the strength of steel (resistance) provide acceptable safety margins for the piping design. This paper describes the development of design equations according to the load and resistance factor design (LRFD) method for loads that cause primary stress such as sustained weight, internal pressure, and earthquake for different levels of piping operation. The LRFD method differs from the ASD since multiple factors, applied separately to each load and the strength of steel, provide safety margins that correspond to a known and acceptable probability of failure for the piping. Load combinations are provided, statistical properties for the variables under consideration are presented and the partial safety factors are moreover illustrated for different values of the target reliability index.

Download Full-text

Risk-Informed Load and Resistance Factor Design (LRFD) Methods for Piping

Recent Advances in Solids and Structures ◽

10.1115/imece2005-80592 ◽

2005 ◽

Cited By ~ 3

Author(s):

Bilal M. Ayyub ◽

Ibrahim A. Assakkaf ◽

Klieo Avrithi ◽

Abinav Gupta ◽

Nitin Shah ◽

...

Keyword(s):

Resistance Factor ◽

Future Research ◽

Performance Requirements ◽

Reliability Method ◽

Load Effects ◽

Partial Safety Factors ◽

Factor Design ◽

And Performance ◽

Load And Resistance Factor ◽

Technical Basis

The main objective of structural design is to insure safety, functional, and performance requirements of a structural system for selected target reliability levels, for specified period of time and for a specified environment. As this must be accomplished under conditions of uncertainty, risk and reliability analyses are deemed necessary in the development of such methods as risk-informed load and resistance factor design for piping. This paper provides a summary of the methodology and technical basis for reliability-based, load and resistance factor design suitable for the ASME Section III, Class 2/3 piping for primary loading, i.e., pressure, deadweight and seismic. The methodology includes analytical procedures, such as the First-Order Reliability Method (FORM) for calculating the LRFD-based partial safety factors for piping. These factors were developed in this paper for demonstration purposes, and they can be used ultimately in LRFD design formats to account for the uncertainties in strength and in the load effects. The technical basis provided in the paper is suitable for a proof-of-concept in that LRFD can be used in the design of piping with consistent reliability levels. Also, the results from additional projects in this area, including future research for piping secondary loads, will form the basis for future code cases.

Download Full-text

Target Reliability Index for Load and Resistance Factor Design of Class 2 Carbon Steel Pipes

Volume 3A: Design and Analysis ◽

10.1115/pvp2017-65797 ◽

2017 ◽

Cited By ~ 1

Author(s):

Kleio Avrithi ◽

Ramiro Mendoza

Keyword(s):

Carbon Steel ◽

Internal Pressure ◽

Reliability Index ◽

Safety Margin ◽

Resistance Factor ◽

Safety Factors ◽

Target Reliability Index ◽

Partial Safety Factors ◽

Factor Design ◽

Load And Resistance Factor

The use of the Load and Resistance Factor Design (LRFD) for Class 2 nuclear piping can be an alternative of the traditional Allowable Stress Design (ASD) method currently used in the ASME Boiler Pressure Vessel Code, Section III, Div. 1 providing the benefit of a known and consistent reliability for the designed piping. The design uncertainties and the necessary safety margin are evaluated for each equation for all service levels by considering the applied loads (e.g., earthquake, deadweight, internal pressure, etc.) and the resistance of steel, in the form of either the yield or ultimate strength, as separate variables described by their mean value, distribution, and coefficient of variation. The procedure yields different partial safety factors for each load and the resistance in opposition to the one safety factor used in each of the ASD equations of the Code. Although LRFD equations have been developed in the past, a range of possible partial safety factors were assigned to the variables, corresponding to different levels of reliability. This paper discusses the method used, namely calibration, for achieving same reliability as in the Code equations, and the progress made to assess a minimum target reliability index or else acceptable probability of failure for the LRFD equations that consider the earthquake load for pressurized pipes as well as the design for internal pressure for Class 2 nuclear pipes made of carbon steel.

Download Full-text

Load and Resistance Factor Design of Driven Piles

Transportation Research Record Journal of the Transportation Research Board ◽

10.1177/0361198196154600110 ◽

1996 ◽

Vol 1546 (1) ◽

pp. 88-93 ◽

Cited By ~ 2

Author(s):

George G. Goble

Keyword(s):

Quality Control ◽

Highway Bridges ◽

The United States ◽

Resistance Factor ◽

Limit States ◽

Resistance Factors ◽

Nominal Strength ◽

Factor Design ◽

Load And Resistance Factor ◽

Driven Pile

A load and resistance factor design (LRFD) bridge specification has been accepted by the AASHTO Bridge Committee. This design approach is now being implemented for highway bridges in the United States, including the design of driven pile foundations. To test the new specification's practicality and usefulness, an example problem has been solved using it. In the example, a pipe pile was designed to be driven into a granular soil to support a bridge column subjected to a factored axial compression load of 10 MN. The nominal strength selected for the pile was 1.58 MN with an estimated length of 25 m. Since the resistance factors are defined by the specified quality control procedures, the number of piles required in the foundation also depends on the quality control. In this example, the number of piles required varied from 15 to 8 with improved quality control, for a savings of almost half of the piles. This example indicated that the new AASHTO LRFD specification for driven pile design can be used effectively to produce a more rationally designed foundation. Some modifications should be made to include additional serviceability limit states, and additional research may indicate that changes should be made in some of the resistance factors.

Download Full-text