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

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Selection of Spread Footings on Soils to Support Highway Bridge Structures

Selection of Spread Footings on Soils to Support Highway Bridge February 2010

Author(s)
Naresh C. Samtani, PE, PhD, Edward A Nowatzki, PE, PhD and
Dennis R. Mertz, PE, PhD

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; © 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

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