# Development of Rock Embedded Drilled Shaft Resistance Factors in Korea based on Field Tests
## 3 Determination of Resistance Factors
### Determination of Resistance Factors  
Considering three different target reliability indices (\(\beta_{\text{T}}\) = 2.5, 3.0, and 3.5), the resistance factors were calibrated using the AFOSM method. The calibration results were expressed as the relationship between the reliability index and the resistance factor. The results are shown in Figure 10, and the resistance factors for each bearing capacity equation and target reliability index are summarized in Table 11.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{3}{c}{**(a) Shift resistance.**} \\ \cline{2-5}  & \multicolumn{3}{c}{**Resistance factor**} & \multicolumn{2}{c}{**Note**} \\ \cline{2-5}
**Bearing capacity equation** & \(\beta_{\text{T}}\) = 2.5 & \(\beta_{\text{T}}\) = 3.0 & \(\beta_{\text{T}}\) = 3.5 & **AASHTO (2015)** \\ \hline [PERSON] \& [PERSON] (1988) & 0.45 & 0.32 & 0.24 & 0.5 \\ \hline [PERSON] \& [PERSON] (1979) & 0.42 & 0.30 & 0.22 & 0.55 \\ \hline FHWA (1999) & 0.44 & 0.32 & 0.24 & 0.55 \\ \hline [PERSON] \& [PERSON] (1987) & 0.19 & 0.13 & 0.11 & - \\ \hline \hline \multicolumn{5}{c}{**(b) Base resistance.**} \\ \cline{2-5}
**Bearing capacity equation** & \(\beta_{\text{T}}\) = 2.5 & \(\beta_{\text{T}}\) = 3.0 & \(\beta_{\text{T}}\) = 3.5 & **AASHTO (2015)** \\ \hline [PERSON] \& [PERSON] (1988) & 0.32 & 0.24 & 0.21 & 0.5 \\ \hline FHWA (1999) & 0.25 & 0.19 & 0.17 & 0.5 \\ \hline [PERSON] \& [PERSON] (1998) & 0.37 & 0.29 & 0.24 & - \\ \hline \hline \multicolumn{5}{c}{**(c) Total resistance.**} \\ \cline{2-5}
**Bearing capacity equation** & \multicolumn{3}{c}{**Resistance factor**} & \multicolumn{2}{c}{**Note**} \\ \cline{2-5}
**Bearing capacity equation** & \(\beta_{\text{T}}\) = 2.5 & \(\beta_{\text{T}}\) = 3.0 & \(\beta_{\text{T}}\) = 3.5 & **AASHTO (2015)** \\ \hline [PERSON] \& [PERSON] (1988) & 0.57 & 0.42 & 0.30 & - \\ \hline FHWA (1999) & 0.40 & 0.28 & 0.22 & - \\ \hline \hline \end{tabular}
\end{table}
Table 11: Results for the resistance factors.  
Figure 10: Relationship between the reliability index and the resistance factors for each component.  
The resistance factors were determined for the target reliability indices of 2.5, 3.0, and 3.5 and compared with the resistance factors suggested by AASHTO. For the AASHTO-recommended reliability index of 3.0, the shaft resistance factors are within 0.13-0.32 of the AASHTO-based values, and the base resistance factors are within 0.19-0.29 of the AASHTO-based values each bearing capacity equation. These resistance factors are 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 are the result of the discrepancy in the conditions of the rocks in which the drilled shafts are founded. The resistance factors suggested by AASHTO are determined based on sedimentary and intact rock conditions, while according to geologic maps from Korea Structure and Foundation Structure Design Specifications [30], the representative types of bedrock in Korea are granite and gneiss, classified as weathered or soft rock by the weathering degree criterion. Clearly, this difference in rock condition has a clear and detrimental effect on drilled shaft performance that must be appropriately accounted for in the LRFD design resistance factors used in Korea. Also, suggested resistance factors were based on only 13 load test cases considering regional characteristics in Korea. But, if additional load test data will be collected, LRFD methods can be updated and calibrated the resistance factors based on reliability analysis. So, results in this study are expected to be helpful for future reference.