Equivalent Top Loading Curve Extrapolations and Their Impact on the Resistance Factor Calibration

摘要

The primary objective of the load and resistance factor design (LRFD) method used in the deep foundation geotechnical design industry is to differentiate uncertainties in loading from those existing in the resistance following a probability-based theory. The probability of occurrence associated with loads is reflected by a load factor, γ, and those uncertainties associated with the geotechnical resistance are reflected by the resistance factor, Φ. The calibration process for deep foundations is completed to determine resistance factors, Φ, using statistical data regarding loads and geotechnical resistances. One of the important factors used for the calibration process is the bias, λ, defined as the ratio of the nominal resistance determined from results of a full-scale load test, Qm, to the nominal resistance determined using an agreed predictive model, Q_p. The bi-directional static load test (BDSLT) is widely used to test the deep foundation's geotechnical resistance. After the load test is completed, the measured data are analyzed to determine required parameters for construction of the equivalent top-loaded (ETL) curve. The ETL curve is used to evaluate the geotechnical capacity using defined approaches. However, the ETL curve does not always reach 100% or more of the required values for a defined geotechnical resistance determination method. Therefore, extrapolation methods are sometimes required to extend the ETL curve to load and displacement values necessary for a defined method. The extrapolation process will introduce uncertainties which will further impact the calibration process. This paper presents a qualitative assessment of the American Association of State Highway and Transportation Officials (AASHTO), and the Federal Highway Administration (FHWA) predictive models by comparing the predicted nominal resistance to measured nominal resistance using the modified Davisson's criteria, 5% relative settlement, and the 10% relative settlement approach with primary focus on drilled shafts. From the comparison analysis, with a coefficient of determination of 80%, the modified Davisson's criterion was considered the method of analysis for this study. Furthermore, the extrapolation effect on resistance factors was explored by calibrating these factors using extrapolated and non-extrapolated ETL curves. The calibration process included 30 drilled shaft cases where ETL curves reached displacements equivalent to those determined using the Davisson's criteria.