Several of our US calculations offer the option of either Allowable Stress Design (ASD) or Load and Resistance Factor Design (LRFD). At a high level, ASD checks service load stresses against an allowable stress whereas, LRFD uses factored load forces against a maximum strength LRFD. While LRFD is the newer and more ‘modern’ method, many engineers still prefer to design in ASD, particularly in wood. N.B. If you are coming from an older software package like StruCalc, you will want to use ASD calculators.