Selection of Spread Footings on Soils to Support Highway Bridge Structures

Document Info

Title: Selection of Spread Footings on Soils to Support Highway Bridge Structures

Authored By: Naresh C. Samtani, PE, PhD, Edward A. Nowatzki, PE, PhD, Dennis R. Mertz, PE, PhD

Sponsoring Agency: Federal Highway Administration (FHWA)

Abstract

The FHWA believes that spread footings on soils are underutilized because designers encounter one or more of the following obstacles: (a) limited knowledge of AASHTO/FHWA technical references that pertain to spread footings on soils to support bridges; (b) limited knowledge of adequate performance data for spread footings; (c) unrealistic tolerable settlement criteria; (d) overestimation of loads used to calculate settlement; and (e) the use of conservative settlement prediction methods. These obstacles have resulted in institutional biases and overly conservative and excessively costly institutional processes that lead to the unnecessary use of costlier deep foundation systems.

The primary goal of this report is to promote the use of spread footings bearing on competent natural soils, improved soils, and engineered fill materials as a routine alternative to deep foundations for support of bridges by addressing the factors identified above. Documented performance data is used to make this case. The report also presents powerful concepts such as construction-point analysis and angular distortions to demonstrate the efficacy of using spread footings. Implementation of these concepts requires only that conventional computations be taken one step further without any requirement for advanced computational skills. The report presents sources of information that agencies and designers can use as references in their project applications. The report contains comprehensive appendices that treat in detail many of the topics discussed in the report. For example, one such appendix provides an introduction to Load and Resistance Factor Design (LRFD) that permits a rational approach to the consideration of spread footings on soils as a feasible alternative to deep foundations.

This report is referenced in Articles C10.5.2.2.2 and C10.6.2.4.1 of the 9th Edition (2020) of AASHTO LRFD Bridge Design Specifications as a key reference for the Construction-Point Approach for evaluating total relevant settlements and the S-0 approach for evaluating differential settlements..