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6 4. Transformation of the codes into living documents that Today, the situation has changed somewhat, but not can be easily revised to include new information reflect- entirely. The present research team gathered robust data on ing statistical data on design factors. pile capacity from which a more objective calibration of resis- 5. The partial safety factor format used herein also pro- tance factors could be made. Nonetheless, there remain uncer- vides a framework for extrapolating existing design tainties associated with (1) site conditions, (2) soil behavior practice to new foundation concepts and materials where and the interpretation of soil parameters, and (3) construction experience is limited. methods and quality. These factors are difficult to understand from the pile databases alone. Such knowledge-based factors should be combined with the reliability-theory-based cali- 1.3.4 LRFD in Geotechnical Engineering bration of the database records to achieve a meaningful LRFD approach, requiring a major research effort. These difficul- Early use of LSD for geotechnical applications was exam- ties are addressed in the present research through the cali- ined by the Danish geotechnical institute (Hansen 1953, 1956) bration of specific combinations of design and parameter and later formulated into code (Hansen, 1966). Independent interpretation methods. load and resistance factors were used, with the resistance fac- tors applied directly to the soil properties rather than to the 1.3.5 LRFD for Deep Foundations nominal resistance. Considerable effort has been directed over the past decade Several efforts have been made to develop LRFD-based on the application of LRFD in geotechnical engineering. codes for deep foundation design. LRFD approaches have been developed in offshore engi- neering (e.g., Tang, 1993; Hamilton and Murff, 1992), gen- eral foundation design (e.g., Kulhawy et al., 1996), and pile 22.214.171.124 2001 AASHTO LRFD Bridge Design design for transportation structures (Barker et al., 1991; Specifications for Driven Piles O'Neill, 1995). In geotechnical practice, uncertainties concerning resis- LRFD Bridge Design Specifications (AASHTO, 2001) tance principally manifest themselves in design methodology, states that the ultimate resistance (Rn ) multiplied by a resis- site characterization, soil behavior, and construction quality. tance factor (), which thus becomes the factored resistance The uncertainties have to do with the formulation of the phys- (Rr), must be greater than or equal to the summation of loads ical problem, interpreting site conditions, understanding soil (Qi ) multiplied by corresponding load factors (i ), and a behavior (e.g., its representation in property values), and modifier (i ). For strength limit states: accounting for construction effects. Uncertainties in external loads are small compared with uncertainties in soil and water Rr = Rn i i Qi (4) loads and the strength-deformation behaviors of soils. The applied loads, however, are traditionally based on superstruc- where: ture analysis, whereas actual load transfer to substructures is poorly researched. The approach for selecting load and resis- i = D R I > 0.95 (5) tance factors developed in structural practice, though a useful starting point for geotechnical applications, is not sufficient. where i = factors to account for; D = effects of ductility; Work is needed to incorporate factors that are unique to geo- R = redundancy; and I = operational importance. technical design into the LRFD formulation. Philosophically, the selection of load and resistance factors The Specifications provide the following equations for does not have to be made probabilistically, although in current determining the factored bearing resistance of piles, QR, structural practice a calibration based on reliability theory is commonly used. This approach focuses more on load uncer- QR = Qn = q Qult = qp Qp + qs Qs (6) tainties than resistance uncertainties and does not include many subjective factors unique to geotechnical practice. An for which: expanded approach is needed if the full benefits of LRFD are to be achieved for foundation design. The National Research Qp = qp Ap (7) Council reports that the "subjective approach reflects the gen- eral lack of robust data sources from which a more objective Qs = qs As (8) set of factors can be derived" (National Research Council, 1995). The report continues, "realistically, because of the where q = resistance factor for the bearing resistance of a sin- tremendous range of property values and site conditions that gle pile specified for methods that do not distinguish between one may encounter, it is unlikely that completely objective total resistance and the individual contributions of tip resis- factors can be developed in the foreseeable future." tance and shaft resistance; Qult = bearing resistance of a single
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7 pile; Qp = pile tip resistance; Qs = pile shaft resistance (F); 0.50 to 0.85 when utilizing dynamic measurements with sig- qp = unit tip resistance of pile; qs = unit shaft resistance of nal matching analysis. Selection of the appropriate resis- pile; As = surface area of pile shaft; Ap = area of pile tip; and tance factor depends on driving conditions, geotechnical qp, qs = resistance factor for tip and shaft resistance, respec- factors (e.g., extent of site investigation), and extent of test- tively, for those methods that separate the resistance of a pile ing (e.g., low range for <3% of the pile tested and high range into contributions from tip resistance and shaft resistance. for >15%). In traditional structural design specifications, a The resistance factors for use in the above equations are pre- nominal value is given and the value used is based primar- sented in Table 10.5.5-2 of the Specifications for different ily on engineering judgment and cannot exceed the nominal design methods based on soil type and area of resistance (tip value. The Australian Standard is therefore unique by pro- and side). The resistance factors for compression vary between viding a guide for choosing the appropriate resistance fac- 0.45 and 0.70. The table also incorporates a factor, v, for dif- tor. Interestingly, no distinction is made regarding either soil ferent methods and level of field capacity verification. As an type or time of driving (i.e. EOD, BOR) when referring to example, if, in analysis, an method is used to determine the the signal matching based on dynamic measurements. The pile's friction resistance in clay, a resistance factor of 0.70 is method by which the resistance factors were generated is not recommended. If, in verification of the pile capacity, a pile provided in the code. driving formula, e.g., an ENR (Engineering News-Record) The AUSTROADS Bridge Design Code (1992) provides equation, is used without stress wave measurements during resistance factors for the construction stage alone including driving, a v factor of 0.80 is recommended. The actual resis- static load test (to failure = 0.9, proof test = 0.8), and four tance factor to be used in the above analysis verification sequence is, therefore, 0.56 (i.e., 0.70 × 0.80). categories of dynamic methods. The range of resistance fac- tors is quite large and there is no explanation as to how the resistance factors were obtained. Goble (1999) postulates that 126.96.36.199 2001 AASHTO LRFD Bridge Design the resistance factors were calibrated via the working stress Specifications for Drilled Shafts design method. The Ontario Bridge Code (1992) recommends relatively LRFD Bridge Design Specifications (AASHTO, 2001) low resistance factors with no differentiation between the provides detailed resistance factors for a large number of individual static or dynamic analyses. For example, the resis- design methods for drilled shafts. Differentiation is made tance factors for static analyses and static load tests in com- between base and side resistance, as for driven piles, with pression and tension are 0.4, 0.3, 0.6 and 0.4 respectively. No resistance factors varying between 0.45 and 0.65. Static test- information is provided on how the resistance factors were ing is included with the same resistance factor as for driven obtained. piles (0.8). Resistance factors are not provided for drilled The Bridge Code (1992) is brief in its design requirements shafts in sand. The v factor, used for field verification for for deep foundations. Resistance factors are based on pile driven piles, is not used for drilled shafts, and no distinction type, = 0.4 for all timber and concrete piles (precast, filled is made on the basis of construction method. pipe, and cast in place) and 0.5 for steel piles. For dynamic load testing, resistance factors of 0.4 and 0.5 are recom- mended for routine testing and analyses based on dynamic 188.8.131.52 Worldwide LRFD Codes for Deep measurements, respectively. Foundations and Drilled Shafts Eurocode 7 (1997) deals with driven piles and drilled A review of foundation design standards in the world was shafts at a single section. Factors for static load testing conducted by the Japanese Geotechnical Society (1998). A depend on the number of tested piles (irrelevant to the num- review of the development of LRFD applications for Geo- ber of piles at the specific site). Range of values from 0.67 to technical Engineering is presented by Goble (1999). A review 0.91 is provided for one to three tests, related to the mean or of LRFD parameters for dynamic analyses of piles is pre- lowest value of the test results. The code is quite complex sented by Paikowsky and Stenerson in Appendix B. The with quantitative descriptions and limiting conditions. The present section provides a short review of non-US LRFD code is presented with multiple component factors, and for codes for deep foundations. comparison with the form used by U.S. codes, Goble (1999) The Australian Standard for Piling-Design and Installa- inverted and combined the factors resulting in values rang- tion (1995) provides ranges of resistance factors for static ing from 0.63 to 0.77 for base, skin, and total resistance of load tests (0.7 to 0.9) and static pile analyses (0.400.65) driven, bored, and CFA piles. DiMaggio et al. (1998) pre- related to the source of soil parameters and soil type (e.g., sented a summary report of a geotechnical engineering study SPT in cohesionless soils). Detailed recommendations are tour, stating "The team found Eurocode 7 to be a difficult doc- provided for resistance factors to be used with the dynamic ument to read and understand, which may explain the various methods ranging between 0.45 to 0.65 for methods without interpretations that were expressed in the countries visited." dynamic measurements (including WEAP), and between Improvements in that direction were achieved through a text