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WSalaam.. ASD is design based on actual non-factored loads considering reduced material strength capacties. For example, for 100 kN load on a steel beam with yield of 350 MPa, you will design it using 100 Kn load and a strength of 2/3*350 MPa= 232 MPa. The reduction in material strength would provide safety, calculated as ratio of actual strength/ (reduction factor * actual strength). This ratio is called Factor of Safety (FOS). FOS would vary for foundations, steel or concrete design etc. ULS is based on factored loads (load factor as per the statistical probability of likely hood of occurance causing failure) with reduced material capacites. The reduction in material capacites is lower than what ASD has. Like for steel design, material reduction factor is 0.9 only which is a mere 10% reduction. ULS is improved and currently the way to go in most building codes worldwide. Some building codes even prohibit using ASD. The following link contains an excellent reply by @EngrUzair about cost Compairson b/w ASD and LRFD. WSD is same as ASD. Secant modulus represents elastic behavior between stress and strain values (as it is a straight line) whereas tangent modulus gives you relationship in any portion of stress strain curve (elastic, plastic etc). Thanks.