• Title, Summary, Keyword: LRFD

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Analysis of Fatigue Truck Model for LRFD Code (LRFD 설계기준(안) 피로설계트럭 모형 분석)

  • Cho, Eun Young;Shin, Dong Ku
    • Journal of Korean Society of Steel Construction
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    • v.21 no.3
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    • pp.331-342
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    • 2009
  • A reliability analysis of the fatigue failure of highway steel bridges was performed by applying the Miner's fatigue damage rule for the fatigue design truck proposed for the LRFD code and for the current DB 24 truck. The limit state function for fatigue failure is expressed as a function of various random variables that affect fatigue damage. Among these variables, the statistical parameters for the equivalent moment, the impact factor, and the loadometer were obtained by analyzing recently measured domestic traffic data, and the parameters for the fatigue strength, the girder distribution factor, and the headway factor were obtained from the measured data reported in literature. Based on the reliability analysis, the fatigue truck model for the LRFD code was proposed. After applying the proposed fatigue truck to the LRFD code, 16 composite plate and box girder bridges were designed based on the LRFD method, and the LRFD design results for the fatigue limit state were compared with those by the current KHBDC.

A Comparative Study of LRFD Methods Using Linear Elastic and Nonlinear Inelastic Analysis (선형탄성해석 및 비선형비탄성해석을 이용한 LRFD 설계법의 비교 연구)

  • Jang, Eun Seok;Park, Jung Woong;Kim, Seung Eock
    • Journal of Korean Society of Steel Construction
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    • v.19 no.6
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    • pp.633-642
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    • 2007
  • Although the Load and Resistance Factor Design (LRFD) method is an advanced design approach, it does not accurately capture the interaction between individual members and structural system. A nonlinear inelastic analysis for the entire structure is required to solve this problem. According to many design codes of advanced countries, a nonlinear inelastic analysis can be applied to predict the structural behavior and strength reasonably. In this study, an LRFD design method using practical nonlinear inelastic analysis was proposed. Design examples using the proposed method waspresented, and the economical efficiency and adequacy of the proposed method was investigated by comparing the design results with that of the AISC-LRFD. It has been consequently demonstrated that the proposed method can reduce the construction cost through savings in steel.

Application of Load and Resistance Factor Design Format to Designing Flexible Pavements (LRFD 기법을 활용한 연성포장 설계방안에 관한 연구)

  • Kim, Hyung-Bae
    • International Journal of Highway Engineering
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    • v.5 no.1
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    • pp.1-10
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    • 2003
  • The objective of pavement design, just as with the design of other structures, is to obtain the most 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 U.S. and Europe 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 theories are used to illustrate the development of an LRFD format for Mechanistic-Empirical (M-E) design of flexible pavements as an alternative of its reliability module. It is shown in this paper that ten candidate pavement sections designed with a reliability level using the AASHTO design guide (1986) do not have uniform structural reliability in terms of pavement mechanistic distress such as fatigue cracking and the uniform reliability can be achieved by using the LRFD format.

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Simplified Load Distribution Factor Equation for the Design of Composite Steel Girder Bridges (강합성교 설계를 위한 하중분배계수 간략식)

  • Chung, Wonseok
    • Journal of Korean Society of Steel Construction
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    • v.17 no.2
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    • pp.131-138
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    • 2005
  • The AASHTO wheel load distribution factor (LDF) equation has been with us since 1931 and has undergone minor modifications. In 1994, an entirely new procedure was introduced in the AASHTO LRFD code based on parametric studies and finite element analyses. However, this LDF equation involves a longitudinal stiffness parameter, the design of which is not initially known. Thus, an iterative procedure is required to correctly determine the LDF value. The increased level of complexity puts undue burden on the designer resulting in a higher likelihood for misinterpretation and error. In this study, based on current AASHTO LRFD framework, a new simplified equation is developed that does not require an iterative procedure. A total of 43 representative composite steel girder bridges are selected and analyzed using a finite element model.The new simplified equation produces LDF values that are always conservative when compared to those obtained from the finite element analyses and are generally greater than the LDF obtained using AASHTO LRFD specification. Therefore, the proposed simplified equation is expected to streamline the determination of LDF for bridge design without sacrificing safety.

A Proposal for an Evaluation of Flexural Resistance of Longitudinally Stiffened Plate Girder with Slender Web (수평보강재로 보강된 세장 복부판을 갖는 플레이트 거더의 휨강도 평가 방법의 제안)

  • Park, Yong Myung;Lee, Kun Joon;Choi, Byung Ho;Back, Sung Yong
    • Journal of Korean Society of Steel Construction
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    • v.26 no.2
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    • pp.119-132
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    • 2014
  • In this paper, a series of numerical analyses were performed to evaluate the flexural resistance of steel plate girder with longitudinally stiffened and slender web. The SM490 steel was adopted for the study and the flexural resistances evaluated from the numerical analysis were compared with those suggested by the AASHTO LRFD and the Eurocode 3 codes, respectively. It was found that the AASHTO LRFD code could considerably underestimate the flexural resistance as the web slenderness becomes smaller. This comes from the fact that current AASHTO LRFD code does not consider a possible increase of slenderness limits for compact and noncompct web, and also an additional effect of web restraint on the rotation of compression flange in longitudinally stiffened web. Therefore, the slenderness limits of web and flange have been newly proposed for the plate girders with longitudinally stiffened web and it is analytically verified that the flexural resistance can be appropriately estimated by applying the proposed slenderness limits to the AASHTO LRFD code.

An Experimental Study on the Axial Stiffness of the Concrete Filled Rectangular Steel Tube Columns (콘크리트충전 각형강관 기둥의 축강성에 관한 실험연구)

  • Cha, Hee-Youn;Song, Hyung-Soo;Min, Chang-Shik
    • Proceedings of the Korea Concrete Institute Conference
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    • pp.145-148
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    • 2008
  • AISC-LRFD, ACI 318 and Korean design specifications for concrete filled rectangular steel tube columns do not consider the increasing of axial stiffness such as the elastic modulus and the yield strength due to the confinement effect. AISC-LRFD and ACI 318, however, shows different basic philosophy and equations for computing the elastic modulus and the strength of CFT columns. Through this experimental study, nine rectangular CFT column specimens were made by varying thickness steel tube and concrete strength, the axial stiffness were compared. The comparison between the design specifications and the test results shows different values on the yield strength of the CFT columns. Even though, yield strength of the CFT columns are very similar between AISC-LRFD and Korean design specifications.

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LRFD Design and Reliability Level Estimation of a Steel Closed-Box Girder Bridge (폐단면 강박스거더교의 LRFD 설계와 신뢰성수준 평가)

  • Huh, Jung-Won;Yun, Dong-Geon
    • Journal of the Computational Structural Engineering Institute of Korea
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    • v.23 no.2
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    • pp.217-225
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    • 2010
  • Most of the steel bridges in Korea are being currently designed by the allowable stress design method that uses the conventional deterministic factors of safety. However the limit state design based on the concept of probability, statistics and reliability engineering is becoming very popular as a global standard deign method, leading the rational and economic bridge design. As part of the fundamental research to establish the load and resistance factor design(LRFD) of steel bridges considering domestic environmental conditions and regional characteristics, an experimental design is conducted by applying AASHTO-LRFD specification especially to a steel closed-box girder, which occupies relatively a large portion of steel bridges in Korea. Throughout the experimental design according to various sectional changes, some of the issues to be considered in the LRFD design of a composite steel closed-box girder bridge are examined. In this process, an Excel-based design verification program is developed for easy computation and prevention of errors. Quantitative reliability levels of the bridge sections designed by LRFD are also estimated using a reliability analysis method, and compared with the target reliability indexes applied in the LRFD design to verify the validity of the procedure and methodology used in this study.

Reliability Analysis of Single and Continuous Span Composite Plate and Box Girder Designed by LRFD Method under Flexure (LRFD법으로 설계된 단경간 및 연속경간 강합성 플레이트 거더 및 박스 거더의 휨에 대한 신뢰도해석)

  • Shin, Dong Ku;Roh, Joon Sik;Cho, Eun Young
    • Journal of Korean Society of Steel Construction
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    • v.20 no.1
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    • pp.183-193
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    • 2008
  • The reliability analysis of simply-supported and continuous composite plate girder and box girder bridges under flexure was performed to provide a basic data for the development of LRFD c ode. The bridges were designed based on LRFD specification with newly proposed design live load which was developed by analyzing traffic statistics from highways and local roads. A performance function for flexural failure was expressed as a function of the flexural resistance of composite section and the design moments due to permanent load and live load. For the flexural resistance, the statistical parameters obtained by analyzing over 16,000 domestic structural steel samples were used. Several different values of bias factors for the live load moment from 1.0 to 1.2 were used. Due to the lack of available domestic measured data on the moment by permanent loads, the same statistical properties used in the calibration of ASHTO-LRFD were ap plied. The reliability indices for the composite girder bridges with various span lengths, different live load factors, and bias fact or for the live load were obtained by applying the Rackwitz-Fiessler technique.

Design Comparison of Composite Girder Bridges Designed by ASD and LRFD Methods (허용응력설계법 및 하중저항계수설계법에 의한 강합성 거더교 설계결과 비교)

  • Cho, Eun-Young;Shin, Dong-Ku
    • Journal of The Korean Society of Civil Engineers
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    • v.29 no.5A
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    • pp.447-456
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    • 2009
  • The design comparison and flexural reliability analysis of continuous span composite plate girder bridges are performed. The girders are designed by the methods of allowable stress design (ASD) and load and resistance factor design (LRFD). For the LRFD design, the design specification under development mainly by KBRC, based on AASHTO-LRFD specification in case of steel structures, is applied with the newly proposed design live load which has been developed by analyzing domestic traffic statistics from highways and local roads. For the ASD based design, the current KHBDC code with DB-24 and DL-24 live loads is used. The longest span length for the 3-span continuous bridges with span arrangement ratio of 4:5:4 is assumed to be from 30 m to 80 m. The amount of steel, performance ratios, and governing design factors for the sections designed by the ASD and LRFD methods are compared. In the reliability analysis for the flexural failure of the sections designed by two methods, the statistical properties on flexural resistance based on the yield strength statistics for over 16,000 domestic structural steel samples are applied.

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The Study for the Evaluation of the Ship Collision Force to the Substructure of Bridges (교각에 작용하는 선박의 충돌력 산정에 대한 연구)

  • Hong, Kwan-Young;Lee, Gye-Hee;Chung, Hyun
    • Proceedings of the Computational Structural Engineering Institute Conference
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    • pp.738-741
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    • 2010
  • 최근 국내에서 해상교량 건설이 증가하면서 교량에 충돌하는 선박의 충돌력에 대한 관심도 증가하지만 선박충돌력에 대한 국내 기준은 AASHTO LRFD에 근거를 두고 있는 실정이다. AASHTO LRFD에 의한 선박충돌력은 Woisin의 평균충돌력 개념에 바탕을 두고 있으며, 충돌속도가 증가함에 따라 AASHTO LRFD에서 제시하는 충돌력의 변화곡선을 따르고 있다. 하지만 AASHTO에서 제시된 충돌력 변화곡선은 선박의 최대충돌력 변화곡선과 같이 선형적 변화를 보이는 반면, 본 선박 충돌해석 결과의 평균충돌력은 최대충돌력의 선형적 변화거동과 일치하지 않는 것으로 나타났다. 따라서 본 논문에서는 선박의 비선형 충돌해석을 통하여 AASHTO LRFD에 의해 산정되는 선박충돌력의 부적절성을 거론하였다.

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