45_HWKTTE 4004c - has been widely approved by the...

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35 ultimate loads to values less than the ultimate resistance divided by a factor of safety, (FS): i n Q FS R (5-2) where R n Nominal Resistance Σ Q i Load Effect (Dead, Live and Environmental Loads) FS Factor of Safety For the Service Limit State, the deformations (i.e. settlements) are calculated using the unfactored loads, and the values obtained are compared to the allowable deformation for that structure. Load Resistance Factor Design (LRFD) Method The LRFD specifications as approved by AASHTO in 1994 recommend the use of load(s) factors to account for uncertainty in the load(s) and a resistance(s) factor to account for the uncertainty in the material resistance(s). This safety criterion can be written as = i i n Q R γ η φ (5-3) where φ Statistically Based Resistance Factor
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36 R n Nominal Resistance η Load Modifier to Account for Effects of Ductility, Redundancy and Operational Importance γ i Statistically Based Load Factor Q i Load effect Even though the LRFD method differs from the accustomed ASD procedure, it
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Unformatted text preview: has been widely approved by the geotechnical engineers. Some of the advantages and disadvantages of the LRFD method over the ASD method are presented next (Withiam et al., 1997). Advantages of LRFD Over ASD 1. Account for variability in both resistance and load. 2. Achieves relatively uniform levels of safety based on the strength of soil and rock for different limit states, foundation types, and design methods. 3. Provide more consistent levels of safety in the superstructure and substructure when the same probabilities of failure are employed. 4. Using load and resistance factors provided in the code, no complex probability and statistical analysis is required. Limitation of LRFD 1. Implementation requires a change in design procedures for engineers accustomed to ASD. 2. Resistance factors vary with design methods and are not constants....
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45_HWKTTE 4004c - has been widely approved by the...

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