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SUMMARY
LRFD Design and Construction of
Shallow Foundations for Highway
Bridge Structures
NCHRP Project 24-31, "LRFD Design Specifications for Shallow Foundations" was initi-
ated with the objective to "develop recommended changes to Section 10 of the AASHTO
LRFD Bridge Design Specifications for the strength limit state design of shallow foundations."
The AASHTO specifications are traditionally observed on all federally aided projects and are
generally viewed as the national code of U.S. highway practice; hence, they influence the con-
struction of all foundations of highway bridges throughout the United States. This report
represents the results of the studies and analyses conducted for NCHRP Project 24-31.
The current AASHTO specifications, as well as other existing codes employing reliability-
based design (RBD) principles, were calibrated using a combination of reliability theory, fit-
ting to allowable stress design (ASD) (also called working stress design [WSD]), and engi-
neering judgment. The main challenges of the project were, therefore, the compilation of
large, high-quality databases of tested foundations to failure and the development of a pro-
cedural and data management framework that would enable Load and Resistance Factor
Design (LRFD) parameter evaluation for the strength limit state of shallow foundations. The
presented research is the first to introduce large-scale, RBD calibration of shallow founda-
tions utilizing databases.
The state of the art was examined via a critical literature review of design methodologies and
RBD and LRFD principles. The state of the practice was established via a questionnaire, dis-
tributed to and gathered from state and federal transportation officials and supplemented by
telephone interviews. The use of shallow foundations for bridge construction across the United
States was found to be about 17%, and a comparison to previous questionnaires showed that
this percentage had not changed much. The use varies widely, however, in regions and states
across the country: from about two-thirds of all bridge foundations in Pennsylvania, Tennessee,
and Connecticut to six states that do not use shallow foundations at all. About three-quarters
of all shallow foundations were reported to be built on rock or Intermediate Geomaterial
(IGM), and the rest were predominantly built on granular materials. The presented research
focuses on the analysis and RBD calibration of foundations on granular soil and rock only.
Large databases were gathered containing 549 load test cases related to the performance
of shallow foundations in/on granular materials (of which 269 cases were utilized in the cali-
bration), and 122 cases for foundations in/on rock (of which 119 were utilized in the cali-
bration). The database for the performance of shallow foundations on soils includes the test-
ing of models and large foundations under vertical, eccentric, and inclined loading
conditions, as well as combinations of these conditions. The database for the performance
of shallow foundations on rock includes the performance of models and large shallow foun-
dations as well as the tip area of rock sockets for which the load-displacement relations could
have been distinctly obtained. Failure criteria were identified and examined for establishing
the ultimate limit state of the tested foundations. The application of methods to the cases
provided the measured resistance of each load test case.
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Bearing capacity methods were established for analyzing the ultimate limit state of shal-
low foundations. The general bearing capacity equation for soils was used with bearing
capacity parameters of Prandtl (Nc), Reissner (Nq), and Vesic´ (N); shape correction and
load inclination factors by Vesic ´ (1975); and depth correction factors by Brinch-Hansen
(1970). Methods from Goodman (1989) and Carter and Kulhawy (1988) were used for eval-
uating the bearing capacity of foundations in/on rock.
The performance of the bearing capacity methods was established via the bias defined as
the ratio of measured to calculated resistances. The statistics of the bias expressed via the mean
and coefficient of variation of the performance were utilized for calibrating the analyses under
a specific design application, developing the relevant resistance factor. The application of the
statistics to the calibration process was challenging because the factors controlling the accu-
racy of the design methods were not always easily identified. The performance of the general
bearing capacity equation for granular material is highly dependent on the bearing capacity
factor N, which in turn is sensitive to the magnitude of the soil's internal friction angle. The
bias of the design method was found to closely follow the bias of N, which increases with
the increase in the internal friction angle. Similarly, the bias of the Carter and Kulhawy
(1988) method was found to be dependent on the rock quality, increasing as the rock qual-
ity (measured by RMR) decreases. Both cases required, therefore, calibrations associated
with the level of the soil's friction angle and RMR, respectively.
The statistical parameters of lateral loads are not readily available or identified in the
AASHTO specifications. A separate study was undertaken to develop such parameters. Exam-
ination of lateral dead and live load statistics resulted in recommended lateral load distributions
used in the calibration. These parameters were utilized for developing the resistance factors
of footings' sliding analysis. The soil-structure interface mechanism was identified using basic
research findings and utilized to establish a framework. Data from foundation testing related
to two construction methods, i.e., concrete poured on the soil and prefabricated, allowed the
development and calibration of the resistances associated with these two prevailing conditions.
Based on the uncertainty established for the design methods and the loading, Monte Carlo
(MC) simulation was used to determine the resistance factor for a predetermined reliability
index. The resistance factors were also evaluated using the simplified closed-form solution
developed based on the First Order Second Moment (FOSM) principles. The findings sug-
gest that the simplified methodology provides conservative resistance factors similar to those
obtained by the MC simulations, hence adequate for local practice parameter development.
The recommended resistance factors are soundly based on the quantified performance of
the design methods and follow the parameters that control them. These parameters present
a radical change to the existing specifications, briefly summarized in the following way:
· The bearing capacity of shallow foundations on granular soils is calibrated according to the
soil placement (natural versus controlled) and the magnitude of the internal friction angle.
· All loading conditions--namely vertical-centric, eccentric, inclined-centric, and eccentric--
are calibrated.
· The reliability of frictional resistance to sliding is quantified and calibrated.
· Specific bearing capacity methods for shallow foundations on rock are identified, quan-
tified, and calibrated.
The implementation of the findings of this research is expected to provide a safe design of
shallow foundations with a consistent level of reliability between the different design meth-
ods and with the recommendations presented in NCHRP Report 507 for the design of deep
foundations. The application of the findings in the design of shallow foundations needs to
be implemented in the context of total design, i.e., the application of all limit states, of which
only the ultimate limit state is addressed in the presented study.
