Abstract
Load and resistance factor design (LRFD) is a limit state design method that has been applied worldwide. Because the data for determining LRFD factors in Korea has been insufficient, the resistance factors suggested by American Association of State Highway and Transportation Officials (AASHTO) in the US have been used for design in Korea; however, these resistance factors were defined based on the characteristics of the predominant bedrock types in the U.S. As such, it remains necessary to determine resistance factors that reflect the bedrock conditions in Korea. Accordingly, in this study, LRFD resistance factors were determined using 13 sets of drilled shaft load test data. To obtain accurate resistance factors, calibration of the elastic modulus of the drilled shaft and the equivalent load–displacement curve considering the axial load and elastic settlement was conducted. After determining accurate resistance values, a reliability analysis was performed. The resistance factors were determined to be within 0.13–0.32 of the AASHTO factors for the shaft resistance, 0.19–0.29 for the base resistance, and 0.28–0.42 for the total resistance. This is equivalent to being 30–60% of the AASHTO-recommended values for the shaft resistance and 40–60% of the AASHTO-recommended values for the base resistance. These differences in resistance factors were entirely the result of discrepancies in the conditions of the rock in the US and Korea in which the shafts were founded.
Highlights
The load resistance factor design (LRFD) method can evaluate the resistance of a structure considering uncertainty based on the results of a reliability analysis
There were some researches to suggest the resistance factors for deep foundations using load test data in domestic region [14,15] and researches for suggestion of resistance factors and comparison according to design methods for shallow foundations [16,17]. Most of this previous research, has been limited to suggesting the total resistance factors, and the validity of the shaft and base resistance factors suggested by Jung are somewhat doubtful as they are derived from the results of a numerical analysis
To plot the equivalent load-displacement curves in this study, the axial load distributions were determined for each test drilled shaft and the load ratios quantifying the difference in load distribution between the top-down test and the bi-directional load test were calculated according to Kwon et al The shaft displacement can be calculated from the end bearing load as follows:
Summary
The load resistance factor design (LRFD) method can evaluate the resistance of a structure considering uncertainty based on the results of a reliability analysis. There were some researches to suggest the resistance factors for deep foundations using load test data in domestic region [14,15] and researches for suggestion of resistance factors and comparison according to design methods for shallow foundations [16,17] Most of this previous research, has been limited to suggesting the total resistance factors, and the validity of the shaft and base resistance factors suggested by Jung are somewhat doubtful as they are derived from the results of a numerical analysis. It remains necessary to determine accurate resistance factors for drilled shafts considering the discrepancy between the rock conditions in the U.S and Korea using reliable load test results. After determining accurate resistance values, a reliability analysis was performed to determine the target reliability index and resistance factors using the advanced first-order second-moment (AFOSM) reliability method
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