The AASHTO LRFD Highway Bridge Design Specifications (LRFD Specifications) were developed with the intent of achieving a more consistent level of reliability in the design of structure components compared to past practices using Allowable Stress Design (ASD) and Load Factor Design (LFD). This objective has been accomplished for strength design where the load and resistance factors could be calibrated using reliability theory.
Serviceability design of foundations is accomplished in a manner analogous to that for strength, with the estimated displacement of the foundation (analogous to load) being compared with the tolerable movement of the structure (resistance). Because the serviceability design procedure is uncalibrated, the level of reliability against serviceability failure is unknown. This inconsistency in the design processes for strength and serviceability may result in larger and more expensive foundations than are required to safely support the applied loads.
The objective of this project is to develop and calibrate procedures and specifications for serviceability design of bridge foundations. These procedures and specifications shall be suitable for recommendation for inclusion in the AASHTO LRFD Bridge Design Specifications.
TASKS (1.) Assemble, analyze, describe, and critique domestic and international data for calibrating the LRFD Specifications for serviceability in the design of bridge foundations. Information also shall be assembled from experiences of engineers, bridge owners, and others. Emphasis shall be placed on data from which the uncertainty in predicting vertical (uplift and settlement) and horizontal displacements of spread footings, piles, and drilled shafts can be determined. Information on the tolerable movement of bridge foundations is also needed. (2.) Assess the amount and quality of data available to calibrate displacement prediction methods for the following variables: foundation type, displacement component, soil type, and method of determining soil properties. Identify calibration methods that are consistent with the quality and amount of data available. Propose methods of calibration suitable for use when the quality and quantity of data are limited. Comment on any additional calibration variables that should be addressed. Displacement prediction methods considered should include, but not be limited to, all methods outlined in the LRFD Specifications. (3.) Develop tolerable movement criteria consistent with the performance of foundations supporting existing bridges. (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 6 months of the contract start date, to document Tasks 1 through 4 for review by the NCHRP. The interim report shall contain specific recommendations with supporting documentation for tolerable movement criteria and displacement-prediction methods for review and approval by NCHRP. An expanded work plan for Tasks 6 through 10 shall be included. The contractor will be expected to meet with the NCHRP approximately 1 month later. Work may not proceed without NCHRP approval of the displacement-prediction methods and the expanded work plan. (6.) Quantify the uncertainty associated with each approved displacement-prediction method for relevant variables listed in Task 2. (7.) Develop recommended load factors for a range of target reliabilities for each displacement prediction method. Provide guidance to adjust load factors for design to account for data quality using the approaches identified in Task 4 and other information obtained in this research. (8.) Report on the results from Tasks 6 and 7 and recommended target reliability not later than 8 months after the approval of the Task 5 Interim Report. Include in the report examples comparing designs using the recommended load factors with designs based on current specifications. Examples shall be provided for each foundation type. Within 6 weeks of submission, the project panel will review this information and approve final values of load factors for recommendation to AASHTO. (9.) Develop recommended serviceability design procedures and specifications. The anticipated limits of applicability of the procedures must be clearly specified. (10.) Perform regression testing to compare outcomes of applying the recommended procedures and specifications and those from the LRFD Specifications. A complete list of all input and output parameters, including proposed values for upper and lower bounds and increments for input shall be submitted for NCHRP review. Regression testing shall not begin without NCHRP approval of the input values. (11.) Submit a final report that documents the entire research effort. The complete data base used in the calibration shall be provided on a CD-ROM. In addition, a companion executive summary that outlines the research results and recommended serviceability design procedures and specifications shall be provided.
STATUS: The contract for this project was terminated. The work described here was completed through Task 8. The last deliverable received is the Task 8 report that comprises the results from Tasks 6 and 7 and makes recommendations on target reliability. This report is available on request.