Calibration of the Resistance Factors for the LRFD of Shallow Foundations of Transmission Towers under Uplift Loading

Over the last few decades, reliability-based design methods have been widely implemented into the foundation design practice. However, several design codes for transmission tower foundations are still based on the allowable stress design (ASD) approach and are yet to adopt the reliability-based approach for design. This paper developed a load and resistance factor design (LRFD) method based on an existing ASD code to design spread foundations of transmission towers under uplift loading in cohesionless soils. In the LRFD development procedures, additional analysis was carried out to check the statistically significant dependency of the model factors (i.e., the ratio between the measured and calculated foundation capacities) on the foundation design parameters. The analysis results showed that there were statistical dependencies between the model factors and the foundation width, which are known as scale effects. By incorporating the identified dependencies, reliability analysis was carried out via Monte Carlo simulations (MCS) by considering the context of transmission tower foundations. From the reliability analysis outputs, it was found that the foundation width had notable effects on the foundation reliability and the calibrated resistance factors. The LRFD format of the existing code was proposed, along with the resistance factors for varying foundation widths and target reliability indices. Finally, a design example was illustrated to compare the design uplift capacities from the existing ASD format and the proposed LRFD format of the design code. Compared to the existing code, the developed LRFD method showed significant improvements in the consistency of the design capacities and the associated reliability levels for a given foundation design.