NCHRP 12-55 [Completed]
Load and Resistance Factors for Earth Pressures on Bridge Substructures and Retaining Walls
| Project Data
||James L. Withiam|
Background: The AASHTO LRFD Bridge Design Specifications were developed to implement a more rational approach for the design of highway structures. As opposed to allowable stress design (ASD), wherein all uncertainty is embedded within a factor of safety, the LRFD approach applies separate factors to account for uncertainty in load and material resistance. The load and resistance factors developed for the LRFD specifications were calibrated using a combination of reliability theory, fitting to ASD, and engineering judgment. Calibration using reliability theory is preferred because the approach permits selection of a target reliability or safety index that reflects the probability of failure of a structural component. However, reliability-based calibration requires access to sufficient data to statistically define the variation and distribution of load and resistance using mathematical relationships. Calibration by fitting and judgment was used in conjunction with reliability-based calibration, or in lieu of reliability-based calibration when sufficient data were not available, to ensure that designs were comparable with accepted engineering practice.
The load factors for soil loads were developed by fitting to ASD designs and engineering judgment. Reliability-based soil load and resistance factors based on available data are needed. In addition, no framework exists for adjusting soil load factors based on the quantity and quality of site data.
Objective: The objective of the research is to develop recommended load and resistance factors for foundations and retaining walls for possible inclusion in the AASHTO LRFD Bridge Design Specifications. Lateral, vertical, and surcharge earth loads are to be investigated.
Tasks: Accomplishment of the project objective will require at least the following tasks. (1.) Assemble, analyze, describe, and critique domestic and international research data for developing load and resistance factors for foundations and retaining walls. Information also shall be assembled from unpublished experiences of engineers, bridge owners, and others. Emphasis shall be placed on data from which the variation and distribution of loads for vertical and horizontal earth pressure, and earth surcharge can be determined. (2.) Review and evaluate the AASHTO LRFD Bridge Design Specifications applicable to the use of geotechnical load factors for foundations and retaining walls. Provide an assessment of the limitations of these specifications and recommend corrective actions. (3.) For each loading condition, soil type, and method of determining soil properties, assess the amount and quality of data needed to calibrate load factors. Based on the information obtained in Tasks 1 and 2, identify the calibration methods that are consistent with the quality and amount of data available. Comment on any additional calibration cases that should be addressed. (4.) Identify and discuss approaches for adjusting load factors for design that consider the quality and quantity of project-specific site data. (5.) Submit an interim report within 9 months of contract start date describing the findings of Tasks 1 through 4 for project panel review. Provide a detailed, updated work plan to complete the work on this project. NCHRP review and approval of the interim report will be required before proceeding with work on the remaining tasks. The contractor should anticipate meeting with the project panel to discuss the proposed work plan. (6.) Develop recommended load factors (for forces that increase and decrease stability) and resistance factors for a range of target reliabilities, for the cases approved by the NCHRP project panel. Provide guidance to adjust load factors for design to account for data quality using the approaches identified in Task 4 and information obtained in this research. (7.) Report on the results from Task 6 and recommend a target reliability to the project panel not later than 6 months after the approval of the Task 5 interim report. Include in the report an executive summary, pertinent backup information, and three case histories comparing designs using the recommended load and resistance factors to designs based on the current LRFD and Standard Specifications. The project panel will review this information and approve final values of load factors within 6 weeks of submission. (8.) Perform parametric studies demonstrating the effect of the approved load factors on the foundations and wall designs as compared with the current LRFD and Standard Specifications designs. Revise design examples B.3, B.5, and B.6 included in National Highway Institute course 13068 incorporating the approved load and resistance factors. (9.) Submit a final report that documents the entire research effort. In addition, provide a companion executive summary that outlines the research results.
Status: The contract is complete.
Product Availability: The contractors draft final report is available for distribution on CD.