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This allows you to choose between Allowable Stress Design (ASD) and Load and Resistance Factor Design (LRFD). Both methods are specified in the NDS, which applies a different set of modification factors to design strengths listed in the NDS Supplement.
Different load combinations and load factors are also used for ASD and LRFD.
LRFD is a design methodology that incorporates the variability in both loading and material resistance into design values and into separate safety factors for each, whereas ASD incorporates a factor of safety accounting for all sources of uncertainty, from both loads and resistance, into the allowable design stresses determined from the strength properties derived from the average of test samples.
It is possible to compare the design response of LRFD designs to those for ASD by the ratio of the load and resistance factors that are applied when using the methods. For typical 15 psf dead and 40 psf live floor loads, for bending moment design of joists and supporting beams, LRFD is advantageous by 16%, i.e., a member loaded to capacity for ASD could be loaded 1.16 times this much before failing for LRFD.
LRFD is found to be advantageous for situations with more than one transient load type e.g., wind, snow, and live load in combination, by as much as 30%.
An exception to this is combined axial and bending design for columns subject to wind and snow loads, where LRFD can be conservative with respect to ASD by as much as 50%.
Refer to https://www.awc.org/pdf/codes-standards/publications/archives/lrfd/AWC-ASAE984006-LRFDvsASD-9807.pdf for more details and comparisons.