I형 프리스트레스트 콘크리트 거더교의 활하중 분배

Abstract

I-girder prestressed concrete bridge (PSC I-girder bridge hereinafter) is one of the most representative types applied to small and medium bridges of 25m～40m. In terms of the deck area, PSC I-girder bridges occupy approximately 30% of all bridges in Korea along express and national roads as of 2005. For this bridge type, there is a standard drawing to be easily utilized in practical design works and a number of constructions have been carried out previously. When determining member force in section to assess safety of a bridge, the general practice is to use the simplified practical equation or live load distribution factors proposed in design standards rather than precise interpretation to use finite element method, etc. Live load distribution factors currently used in practice are citing overseas research results or design standards without alternations. These live load distribution factors have been developed by reflecting design factors and girder cross-section used in foreign countries and have been applied for comparison to PSC I-girder bridges mainly used in Korea. The result of comparing live load distribution factors based on design standards and simplified practical equation indicated that there was the tendency of overestimating distribution of live load in internal girder and of insufficient safety rate by underestimating the range of practical work in terms of external girder. It has been analyzed that such live load distribution factors must be refrained from in practical design. Therefore, considering the economic values according to construction rate of PSC I-girder bridges, it became necessary to develop an equation of live load distribution factors to ensure appropriate level of safety. The course of developing equation of live load distribution factors consists with structure analysis model selection, decision on the range of parameters and parametric analysis. In the course of developing the equation of live load distribution factors, parametric analysis requires analysis on an enormously large number of values. Therefore, structural analysis model must be equipped with both appropriateness of the result and convenience of modeling by most successfully expressing movements. The structure analysis model was compared against the models used in previous studies and design practices. Characteristics of structure analysis model were compared and the impacts exerted on structure analysis model by stiffness of barrier and diaphragm were analyzed. As of the barrier, the difference in impacts exerted according to changes in stiffness was insignificant. This result showed that the barrier could be ignored as a structure. diaphragm exerted small impact at the stiffness of 25% or higher. When compared to the values of load test, satisfactory result in terms of safety was obtained. For structural analysis model for development of the equation of live load distribution factors, base surface and girder were connected with rigid elements by applying eccentricity. The structural analysis model without eccentricity to ignore barrier and to consider diaphragm to be valid in all cross-sections was found to be the most appropriate model. To develop equation of live load distribution factors, impacts exerted on live load distribution factors were analyzed and sensitivity analysis was carried out with span length, girder spacing, overhang length, width of bridge, width of traffic lane and traffic lane count as parameters. Parameters and range applied here were selected from design standards, research papers and the range of actual practices. Result of analysis on sensitivity of parameters indicated that the major variables to determine the size of distribution factors were girder spacing, overhang length and span length in case of external girders. For internal adjacent girders, the determinant factors were girder spacing, overhang length, span length and width of bridge. For internal girders, the factors were girder spacing, width of bridge and span length. By administering multiple linear regression analysis, equation of live load distribution factors was developed. There were 3 ~ 4 independent variables to determine live load distribution factors. The exponential model of multiple linear regression analysis was suitable to development of the equation of live load distribution factors. It was verified through the result of value analysis that the equation of live load distribution factors developed was in the standard capable of appropriately predicting live load distribution of a PSC I-girder bridge. The equation of live load distribution factors developed in this study satisfied economic perspectives together with the appropriate level of safety. Design engineers, by using the results of this study in actual practice, will be able to produce designs of appropriate standards. Also, using of the simplified and reliable equation of live load distribution factors in the repetitive pre-design processes will bring improvement in structural efficiency of PSC I-girder bridges.