ASCE has honored Bak Kong Low, Ph.D., P.E., F.ASCE, with the 2019 Thomas A. Middlebrooks Award for the paper “Insights from Reliability-Based Design to Complement Load and Resistance Factor Design Approach,” published in the November 2017 issue of the Journal of Geotechnical and Geoenvironmental Engineering.
The paper investigates the similarities and differences between the reliability-based design (RBD) approach and the load and resistance factor design (LRFD) approach so that researchers and practitioners can understand the complementary role that RBD can play toward LRFD under some circumstances.
Three geotechnical problems are investigated using RBD via the first-order reliability method (FORM) to demonstrate that much insight and information can be obtained from RBD-via-FORM to overcome some limitations and ambiguities in LRFD. The first example deals with RBD of a spread footing under vertical and horizontal loads, where it is revealed that the vertical load possesses a stabilizing-destabilizing duality when a horizontal load is also acting: the vertical load is unfavorable in increasing applied pressure, but favorable in reducing the adverse effect of load eccentricity and load inclination. RBD-via-FORM automatically resolves this duality issue and obtains the design values of loads and resistance which reflect the level of uncertainty, parametric sensitivities, correlations, and target reliability index.
The second example deals with a laterally loaded cantilever pile in a soil with depth-dependent undrained shear strength, and with soil lateral resistance modeled by the nonlinear and strain-softening Matlock p-y curves. The performance function is implicit, and both ultimate limit state and serviceability limit state are considered. The results of RBD-via-FORM demonstrate that, when the cantilever length is long, the pile performance is more sensitive to the uncertainty of the lateral load at pile head than to that of the undrained shear strength below the sea bed. The sensitivities of the lateral load at pile head and the soil resistance change again for a fully embedded pile with zero cantilever length.
The third example of RBD involves an anchored sheet pile wall in which the loads and resistance are nonlinear functions of soil shear strength, unit weight, geometry and parametric correlations, and in which loads and resistance are entangled and possess stabilizing-destabilizing duality due to common underlying parameters. It is demonstrated again that RBD can automatically reveal critical design scenarios and obtain design values of loads and resistance that reflect different parametric sensitivities across different scenarios. Although not required in RBD, load and resistance factors were back-calculated from the RBD examples of this study so that their nonintrinsic and case-dependent nature can be understood.
This paper focuses on insights from RBD-via-FORM, so that researchers and practitioners will:
- Understand the similarities and differences between the RBD approach and the LRFD approach
- Be alert of LRFD limitations and ambiguities under some circumstances, for example, when a load or resistance possesses stabilizing-destabilizing duality, or when parametric sensitivities can vary across different scenarios
- Appreciate that much information and insight can be gained from RBD to further improve the LRFD approach
The Thomas A. Middlebrooks Award is made to the author or authors of a paper published by the Society judged worthy of special commendation for its merit as a contribution to geotechnical engineering. Papers written by young engineers are given preference.
