Probabilistic Methods in Geotechnical Engineering (1995) / Chapter Skim
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3. The Role of Probability in Codes and Regulations
3. The Role of Probability in Codes and Regulations
Pages 36-44
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From page 36... ...
is applied to the elements of the design that are most uncertain. The determination of the partial safety factors may be done using probabilistic methods, but these are transparent to the designer, because the designer may not necessarily be aware of the probabilistic basis underlying those partial safety factors that are recommenciec!
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From page 37... ...
Structures such as offshore platforms, transmission towers, bridges, and buildings in active seismic zones are examples in which extreme or transient loads infrequently applied can control choice of structural section. The ERFD may not be as useful in those geotechnical engineering problems where design is largely controlled by uncertainty in material properties, such as in stability analysis of embankments and excavations where loading is chiefly from soil and water, and live loads are small compared with the earth loading.
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From page 38... ...
in section 2.2.5 ofthe code, "Material Properties," a design value is either assessed directly or is derived from the characteristic value by dividing the value by a partial safety factor, described as follows: Characteristic values shall be selected with the intention that the probability of a more unfavorable value governing the occurrence of a limit state is not greater than 5 percent. For parameters for which the values governing the field behavior are well established with little uncertainty, the characteristic value may be taken as the best estimate of the value in the field.
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From page 39... ...
The following example of resistance factors for pile design may be illuminating. Section 7.6.3.2 of the Eurocode describes a procedure for using the measured value of the ultimate bearing resistance from a pile load test, Rm' to determine a design value of the ultimate bearing resistance, Rig.
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From page 40... ...
This process weighs both the mean value and the minimum value measured and offers incentives flower safety factors) for doing more tests.
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From page 41... ...
On the other hand, this approach neglects the larger issues in traditional geotechnical practice, which emphasize the importance of interpreting site conditions based on a sound knowledge of local geology. It does not fully account for the vagaries of soil behavior, such as the significance of progressive failure, nor for the strong interrelationship between design and construction.
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From page 42... ...
at a site, which is then compared with the compliance standards given in the regulations. The approach for ascertaining reasonable maximum exposure has been essentially deterministic in nature, assuming the definite occurrence of many conditional factors, even though the likelihood of each of these factors occurring is considerably less than I.0.
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From page 43... ...
In the case of foundation engineering, the impetus comes from structural engineers, and the impact is not greatly different from that of systematized common sense. The initial versions of the probability-based foundation design codes represent a step toward a more systematic and rational approach.
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From page 44... ...
1992. Selection of LRFD Resistance Factors for Pile Foundation Design.
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