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Determination of Load-resistance Factors
of Cable and Girder for Reliability-based Design of Cable-supported Bridges Ho Hyun Lee
ABSTRACT This paper figures out a reliability based code calibration of designs for cables and stiffening girders in cable-supported bridges under gravitational vehicular load combination. Since a cable is a nonlinear element, it is necessary to calibrate appropriate load and resistance factors. With this purpose, in this paper, two code calibration processes are proposed. First method is based on nonlinear reliability analysis of the cable element. Reliability analyses of cable members in existing cable-supported bridges are performed, and then variations of reliability indices to resistance factors are evaluate while load factors are fixed. Resistance factors which satisfy target reliability indices are evaluated utilizing a best-fit line which is derived from results of the reliability analyses, and this process is adopted to develop Korean Highway Bridge Design Codes (Limit State Design)-Cable-supported bridges. Since the method which is mentioned before is based on reliability analyses of real cable members, there exist some limits. At first, because of high computational cost, the method needs long computational time. In addition, since the method is based on reliability analyses of existing bridges, it is hard to get generalized results. Thus more generalized approach adopting optimization based code calibration is proposed. In order to calibrate load and resistance factors, a limit state function of cable member is linearized, and then optimization scheme, which is developed for code calibrations of linear ordinary members, is utilized. The limit state function is parameterized by three load ratios, and thus the optimization based code calibration can derive generalized load and resistance factors regardless of structural types of cable-supported bridges. Simultaneously, since the load and resistance factors are evaluated adopting optimization, a strength derived from the factors satisfies uniform reliability indices. Because of nonlinearity, initial equilibrium state analyses is necessary for designs of cable-supported bridges. In general, initial equilibrium state of cable-supported bridge is determined to minimize bending moment of girders, and thus directions of bending moments induced by DC and DW load become opposite. With this reason, DC ratios of stiffening girders are not within range between 1 and 0. Furthermore, values of the DC ratios become very huge. In case of the stiffening girders, it is hard to determine proper integration ranges of DC ratios for optimizations, instead, reliability analyses adopting proposed values of load and resistance factors are performed with the purpose of verification.
Key Word Cable-supported bridge; Limit state design; Gravitational vehicular load combination; Code calibration; Load-resistance factors
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