The 2005 American steel structures design code

ABSTRACT: Load and resistance factor design (LRFD) is now recommended in North American design codes for reinforced-soil structures, including internal stability limit states. The selection of load and resistance factors that appear in limit state equations is best carried out using reliability-based analysis. In this paper the conventional approach to compute the limit state for geosynthetic reinforcement tensile rupture is reviewed, and is then recast in a reliability-based analysis framework suitable for LRFD calibration using bias statistics. The paper describes how to compute bias statistics from product-specific laboratory creep tests for the reinforcement rupture limit state. A database of results from creep tests on 94 different geosynthetic products was collected from 21 different sources. A total of 1086 in-air tensile test results and 540 creep-rupture data points were examined. This database is used to compute virgin and creep-reduced strength bias statistics for three different geosynthetic product categories. The results of analysis show that variability in the prediction of creep-reduced strength is very low, and is probably captured by the magnitude of variance in the original tensile strength of the test specimens. This greatly simplifies future LRFD calibration for the geosynthetic rupture limit state. An important implication of this study for LRFD design is that creep strength reduction factors can be taken as deterministic. The paper also provides a summary of computed creep-reduction factors that is a useful reference for future estimates of this factor from laboratory creep testing, and for preliminary design purposes using allowable stress design or LRFD approaches.

API RP2A Load and Resistance Factor Design is a limit state design practice which is intended to replace the existing Working Stress Design practice for designing fixed offshore structures. Being in partial safety factor format, LRFD offers plenty of scope for influencing and improving design by calibrating the partial factors to suit regional environmental conditions and design practices. This paper presents the results from a joint industry project on the calibration of LRFD for Mediterranean Sea platforms. Advanced structural reliability methods formed the basis of the code calibration procedure which is briefly outlined. Probabilistic models for environmental and gravity loads and modelling uncertainties for tubular member and joint strength adopted in the reliability analysis are described. Comparisons of reliability indices for components designed under LRFD and WSD 18th Edition indicate that for the storm condition LRFDwould introduce a very small weight penalty whilst for the operating condition there would be a weight reduction. The application of LRFD would lead to significantly more uniform component reliability compared to WSD designs. Based on the proposed partial load factors for the MediterraneanSea the impact of LRFD to the design of jacket structures is assessed through verification studies of two typical platforms in the region. INTRODUCTION Regional variations in wave climate and design practice can be accounted for in LRFD-type design codes for offshore structures. The reliability based API RnA LRFD (Load andResistance Factor Design) [1], which has been proposed as an alternative to the Working Stress Design practice [2], takes into account the uncertainties associated with applied loads and structural component resistances and factors both loads and resistances. In contrast, in WSD only resistance factors are applied to derive allowable stresses. By explicitly defining partial factors for gravity and environmental loads it is possible to account for the variabilities in environmental conditions and design practices that exist worldwide. The partial load and resistance factors in the LRFD draft issue have been calibrated primarily for Gulf of Mexico structures [3]. Partial load factors for North Sea structures have beensubsequently proposed, based on the results of a joint industry project where advanced structural reliability methods were used for code calibration [4,5]. Following the conclusion of the North Sea adaptation the project was extended to adapt LRFD to the Mediterranean Sea [6]. Details of the underlying philosophy and procedures adopted for the calibration are given in [4]. This paper focuses on the work undertaken in the context of the Mediterranean Sea and presents the background information. Partial load factors for the in-place extreme and operating conditions have been derived using the same reliability based procedures to those adopted for the North Sea calibration. Based on the proposed load factors verification studies havebeen carried out for two typical jacket structures. The results are reported through comparisons of utilisation ratios for tubular members, legs and joints designed using LRFD and RP2A-18th Edition. They demonstrate potential effects of implementing LRFD to the Mediterranean Sea environment and highlight problem areas. Reference to the proposed LRFD version incorporatingrecommended Mediterranean Sea adjustments will be to LRFDMed.

In this paper, the detail of Load and Resistance Factor Design (LRFD) approach which is based on reliability concept for the design of Reinforced Soil (RS) walls is presented. For conventional methods i.e Allowable Stress Design (ASD) Method, the factor of safety is applied only to resistance and loads are considered without variations. For LRFD method, factor of safeties are applied for both load and resistance. Due to availability of large statistical data and economy, this method is preferred. An attempt is made to solve one numerical example of geosynthetic RS walls due to soil selfweight plus permanent uniform surcharge using LRFD as well as other conventional methods (ASD) viz. FHWA, Modified Rankine, NCMA and B.S Code Methods and the results of the LRFD methods are compared with conventional design methods and concluding remarks are presented. The various equations are obtained based on various curves plotted by using ASD and LRFD approaches. From these equations it is clear that if FOS against tensile rupture is known for any RS wall having 7m height and same properties and environmental conditions as mentioned in current study then FOS against pullout failure and pullout capacity can be computed for these walls.

Load combinations in Section 2 of ASCE Standard 7 are provided for both Load and Resistance Factor Design (LRFD) and for Allowable Stress Design (ASD). Included in Section 2 are combinations addressing counteracting load effects, where the permanent gravity (dead) load stabilizes the structural system. Counteracting effects occur most commonly when destabilizing forces occur due to wind uplift or lateral wind or earthquake effects. In the 1998 and subsequent editions of ASCE Standard 7, the load factor on nominal dead load in the corresponding ASD combinations has been 0.6 rather than 1.0 to achieve some measure of consistency between ASD and LRFD. This reduction has been questioned by some engineers. This paper examines the treatment of counteracting loads in ASD from a structural reliability viewpoint similar to that used to develop the ASCE 7-05 load combinations for LRFD, and explains why the reduction in the dead load factor was necessary to achieve consistent reliability between ASD and LRFD.

The load and resistance factor design (LRFD) specification developed by the American Institute of Steel Construction allows the designer to consider the partial restraint in a steel connection. A generated nonlinear equation is fitted to available experimental data on partially restrained steel connections for six different types of connections. The connections analyzed are single web angle, double web angle, top and seat angle, end plate with column stiffeners, end plate without column stiffeners, and header plate. The statistics of parameters governing the moment‐rotation curves of the connection are determined. Design examples from the American Institute of Steel Construction Load and Resistance Factor Design Manual are used as an application of the use of statistical data in finding the reliability of a uniformly loaded, partially restrained beam. The reliability is evaluated by imposing end rotations and calculating the reliability index for each imposed deformation. The connection reliability index ...

A lot of moment resisting steel frames were damaged, after Northridge earthquake. In these building, the usual rigid weld connection didn’t work properly and fielded. In the rigid weld connections, available Shear force in the flanges and lateral torsional buckling are the main reason for weld fracture.Slotted web connections, by improving many weaknesses present in moment connections were introduced by Seismic Structural Design Associates (SSDA), and has been classified as a special moment connection in the American Institute of Steel Construction (AISC), managed to qualify for the Federal Emergency Management Agency (FEMA) regulations.The Slotted Web connection design rationale that sizes the beam slot length, shear plate, and connection weldments, is based upon successful FEMA/AISC ATC-24 protocol test results and extensive inelastic finite element analyses of the beam and column stress and strain distributions. Incorporated in this rationale, which has been accepted by numerous departments of building and safety, are the UBC and AISC Load and Resistance Factor Design (LRFD) Specifications and the 1997 AISC Seismic Design Provisions for Steel Buildings and its supplements. In this connection, the separation of beam flanges removes the shear force from beam flange and it causes to transfer it on beam web. This connection is using moment resisting steel frame structure with proper behaviour and good seismic performance One of disadvantage of this connection is fast buckling of beam flange, to revise this problem in Slotted web connection we add a couple of vertical stiffness up and down the beam flange. Using of these stiffness, not only prevent fast buckling but also increase ductility and seismic performance and when we use these stiffness we will see, the absorbing of energy in hysteric loop will be more than the usual Slotted web connection.

In the past, allowable stress design (ASD) was used to design steel structures. The allowable stresses used were determined from previous practice, with limited understanding of the reliability and risk performance provided by the structure. Engineering methods based on Load and Resistance Factor Design (LRFD) provide more accurate lifetime models of structures by providing risk-based load factors. Besides improved safety, cost savings can be provided through improved performance and, in some cases, by delaying rehabilitation. This research project develops LRFD-based engineering procedures for the evaluation and design of hydraulic steel structures (HSS). Hydraulic loads are a key element to the LRFD analysis. This report identifies the primary hydraulic loads and describes procedures that can be used to determine these hydraulic loads. Existing design guidance for HSS is described and presented in the individual chapters. The appendixes to the report provide examples of the procedures used to compute the hydrostatic, wave, and hydrodynamic loads. A new approach for determining wind-induced wave loads was developed. Design guidance for computing the hydrodynamic load was limited for many of the HSS. Additional research is recommended to improve capabilities for computing hydraulic loads. Details on these recommendations can be found in this report.

Achieving uniform reliability is a desired intent of design standards based on Load and Resistance Factor Design (LRFD) methodology. The design standard for offshore wind turbines, the IEC 61400-3 (IEC, 2009), also follows the LRFD. This standard is based on European experience and it may not necessarily represent offshore environment in the US waters, where several offshore wind farms are being planned. For these reasons, it is of interest to investigate factors that may influence the reliability of offshore wind turbines under various levels of wind and wave loads. We evaluate the reliability index, which is a measure of the probability of failure, for the ultimate limit state associated with the fore-aft tower bending moment at the mudline for offshore wind turbines with fixed support structures. We compare reliability index for various values of characteristic levels and coefficients of variation of wind and wave loads, as well as for various ratios of hydrodynamic to aerodynamic loads. These parameters represent different sites and turbine designs. Using the combined wind and wave load effect model for offshore wind turbines proposed by Tarp-Johansen (2005), we show that the range of reliability levels are reasonably uniform under various combinations of wind and wave loads. This meets the intent of codes based on the LRFD. Since large diameter monopile support structures tend to be dominated by inertia forces, while jacket support structures with smaller diameter members are often dominated by drag forces, we also employ a more general wave load formulation that models both drag and inertia forces. We include this wave load model in the combined wind and wave load effect model for offshore wind turbines, and estimate resulting reliability levels. Results show that dragonly case results in smaller estimates of reliability index than the inertia-only case. Moreover, for inertia-dominated cases, reliability levels are found to be even more uniform than those for drag-dominated case. Introduction Several design codes, including the IEC 61400-3 (IEC, 2009) standard for offshore wind turbines, are based on the Load and Resistance Factor Design (LRFD). Designs of an entire class of structures based on the LRFD methodology tend to have a uniform reliability. On the other hand, different designs in a class of structures may have quite different reliability levels if working stress design methodology is followed. LRFD is particularly useful for loads that exhibit considerable variability. Offshore wind turbines are influenced by wind as well as wave loads, both of which can exhibit significant variability. Therefore, it is of interest to understand influence of various factors on the reliability of offshore wind turbines under various levels of wind and wave loads. Understanding reliability levels is even more important for relatively new industries like offshore wind energy for which the design practice is still evolving. The first edition of IEC 61400-3, the Design Requirements for Offshore Wind Turbines (IEC, 2009), was issued only recently in 2009. This standard is based more on the European experience, where the offshore wind industry is about a decade old. Lately, there has been considerable interest in developing offshore wind farms in the US waters. Care has to be used in applying IEC standards in the US where offshore environment is quite different than that in Europe. The offshore oil and gas industry is quite mature in the US, and American standards for design of fixed-bottom offshore structures exist, such as API RP 2A-LRFD (API, 2003) and API RP 2A-WSD (API, 2005). However, these standards were not intended for wind turbines, which are active machines instead of passive structures, and for which loads on support structures are mainly dominated by aerodynamic loads.

Design of beam-columns using allowable stress design and load and resistance factor design

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.

Load and Resistance Factor Design (LRFD) shows promise as a viable alternative to the present working stress design (WSD) approach to foundation design. The key improvements of LRFD over the traditional Working Stress Design (WSD) are the ability to provide a more consistent level of reliability and the possibility of accounting for load and resistance uncertainties separately. In order for foundation design to be consistent with current structural design practice, the use of the same loads, load factors and load combinations would be required. In this study, we review the load factors presented in various LRFD Codes from the US, Canada and Europe. A simple firstorder second moment (FOSM) reliability analysis is presented to determine appropriate ranges for the values of the load factors. These values are compared with those proposed in the Codes. The comparisons between the analysis and the Codes show that the values of load factors given in the Codes generally fall within ranges consistent with the results of the FOSM analysis. For LRFD to gain acceptance in geotechnical engineering, a framework for the objective assessment of resistance factors is needed. Such a framework, based on reliability analysis is proposed in this study. Probability Density Functions (PDFs), representing design variable uncertainties, are required for analysis. A systematic approach to the selection of PDFs is presented. Such a procedure is a critical prerequisite to a rational probabilistic analysis in the development of LRFD methods in geotechnical engineering. Additionally, in order for LRFD to fulfill its promise for designs with more consistent reliability, the methods used to execute a design must be consistent with the methods assumed in the development of the LRFD factors. In this study, a methodology for the estimation of soil parameters for use in design equations is proposed that should allow for more statistical consistency in design inputs than is possible in traditional methods. Resistance factor values are dependent upon the values of load factors used. Thus, a method to adjust the resistance factors to account for code-specified load factors is also presented. Resistance factors for ultimate bearing capacity are computed using reliability analysis for shallow and deep foundations both in sand and in clay, for use with both ASCE-7 (1996) and AASHTO (1998) load factors. The various considered methods obtain their input parameters from the CPT, the SPT, or laboratory testing. Designers may wish to use design methods that are not considered in this

What differentiates a beam from a plate girder? This may seem to be a trivial question. However, it is a necessary and important part of plate girder design when applying the Load and Resistance Factor Design (LRFD) Specification. Making this distinction and the compactness classification early in plate girder design is important when using the LRFD Specification. The significance of these items can be seen in the LRFD Manual flowcharts for the determination of flexural and shear design strength. With these flowcharts, this discussion will focus on the design of plate girders according to LRFD rules. An explanation of plate girder design in the LRFD Specification will include: flexural design strength, shear design strength, flexure-shear interaction, bearing strength under concentrated loads and stiffener design. Application of the LRFD method will show there is actually little difference between it and Allowable Stress Design (ASD) of plate girders.

Facing the escalating demands in both technical competence and financial accountability, practicing structural engineers are being forced into an ever-intensified fast-pace working environment. With more and more structural design codes becoming lengthy and cumbersome and, meanwhile, the time allocated for structural engineering design being drastically reduced, the quality assurance of the structural engineering design has been becoming more critical than ever for practicing structural engineers to succeed. Structural engineering design errors and failures have been observed in a variety of design phases and categories. Among them, there are several typical problems needed to be discussed and a systematical procedure of quality assurance of the structural engineering design shall be studied and developed. As a part of our collective professional effort to assure the quality of the structural engineering design, this paper will discuss following eight typical aspects related to quality control and assurance of structural engineering design and a number of real world practical examples will be presented to assist in illustrating the importance of the quality control and assurance for real world structural engineering design projects.

This paper addresses the procedures established for the derivation of design properties for use in load and resistance factor design (LRFD) of structural wood products. It covers derivation of reference strength (stress) and reference resistance (force or moment) values using either a format conversion or a reliability-based conversion process. Reference design properties will be used with the first edition of the \ILRFD Specification for Engineered Wood Construction.\N The majority of the reference design properties were derived through format conversion. Derivation examples are provided for glued laminated timber piles, structural panels, dimension lumber, and timber piles. Development of end-use condition adjustment factors is reviewed. Impacts of the transition from allowable stress design to LRFD are discussed.

Derivation of stainless steel material factors for European and U.S. design standards