LRFD refers to Load and Resistance Factor Design which is also a Limit States Design methodology. This method uses a load factor to ‘factor up or down’ service level loads and also reduce member strength based on reliability and statistical data. When using LRFD the strength shall be based on the LRFD load combinations, and factors however deflection should be based on service level loads, so you must keep track of your loads.
LRFD is a more reliable and statistical based method for predicting loads and material strengths. Whereas the allowable stress safety factors where based on engineering judgement and past experiences. It is debated which will give you a more efficient design however it seems in most situations LRFD will produce a smaller sized beam based on strength but not always. Also serviceability and deflection control many designs, in which case both methods will yield the same result as the design is not base on strength at that point.
Allowable Stress Design (1989 9th Edition AISC Manual) or Allowable Strength Design (2005 13th Edition AISC Manual). Both use service level loads and a safety factor to member strength.
Working Stress Design (not used in design anymore). Uses services level loads and a safety factor to member strength.
Load and Resistance Factor Design. Uses factored loads and applies a reduction factor to member strength.
Limit States Design. A design methodology where different failure mechanisms or states are checked and allowable strengths for each failure mechanism or state are determined. The controlling limit state is normally the one that results in the least available strength. This is more of a general term and includes ASD ’89, ASD 2005 and LRFD.
Strength Design = Generally refers to LRFD however the most new manuals which include ASD could be considered strength design methods as well. Meaning stresses are typically not calculated anymore…well they are but the end result is usually in terms of a members strength. In concrete you may also hear the term Ultimate Strength Design (where the old ’63 code used Working Stress Design) which is referring to LRFD.
Ultimate or Strength Level = Generally strength or ultimate level loads refers to Factored Loads in LRFD design. Ultimate capacity is generally the Factored Resistance or Capacity of the member being designed with LRFD.
Service Level = Generally service level loads are used with ASD methods. They are also used when checking deflection for serviceability.
Nominal Strength = This is the strength of the member for a given limit state before any safety factor or reduction factor is applied to the member. This is used with ASD or LRFD and is normal given in manuals that present a “Unified Approach” aka they give you a nominal capacity then the user applies a safety factor or resistance factor.
Available Strength = This is the strength of the member based on the nominal strength reduced by the applicable safety factor or reduction factor. In LRFD it is common to refer to this as the Ultimate Strength. In ASD it is commonly referred to as the Allowable Strength.
Required Strength = This is the strength required based on the applicable ASD or LRFD combination. The required strength should always be less than the available strength.
Resistance Factor = The reduction factor applied to the nominal strength as used in LRFD.
Safety Factor = This is the factor which reduces the nominal strength as used in ASD.