Design Philosophies
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Required Strength of the material = The load that the structure will be carrying throughout its life span. (Also termed as Load Effect)
Nominal Strength of the material = The strength of the material depending upon the material size, shape and its behavior that is calculated per AISC Specification. (Also termed as Capacity)
The structure should always be designed such that
Nominal Strength (Capacity) ≥ Required Strength (Load Effect)
What happens if Capacity of the material is less than Load Effect?
When the load effect exceeds the load carrying capacity of the material, the failure of a component or the whole structures can occur. The failure can be either due to bending of the material, shear failure or buckling. That means, the material or structure has reached to its limit state. The goal of a structural designer should always be that the resulting load effect (axial force, shear force or bending moment) be well below each of the applicable limit states as specified by the code.
Two limit states are mainly considered during steel building member designs:
Ultimate Limit State
Ultimate limit state measures the safety of the structure based on factored loads.
Checks the strength failure that leads to structural collapse.
Example Includes, structural yielding, rupture, block shear, buckling…
Serviceability Limit State
Serviceability limit state measures the safety of the structure based on service loads.
Checks the failure to meet operational requirement and comfortability
Example Includes, deflection, vibration, cracking, durability…
Ref. Page 16.1-12 of AISC Steel Manual
AISC Specifications for Steel Buildings, Chapter B, Design Requirements, Section B3
AISC Specification provides two main design philosophies for steel structures:
Allowable Strength Design (ASD)
Load and Resistance Factor Design (LRFD)
Allowable Strength Design (ASD)
Allowable Strength Design (ASD) was first introduced in 2005 edition of the AISC Specification. Before 2005, it was referred to as Allowable Stress Design (ASD).
Allowable Stress Design was first introduced in 1923 and used until 1989 Specification for Structural Steel Buildings-Allowable Stress Design and Plastic Design. The Allowable Stress Design assumed that all the stresses in members remain elastic. A safety factor was established for each type of stresses so that the requirement of members remaining elastic would suffice.
Assumption:
Allowable Strength ≥ Required Strength
According to AISC Specification Section B3.2, Equation B3-2,
where,
Ra = Required strength calculated using ASD Load Combinations
Rn = Nominal Strength
Ω = Safety Factor
Rn/Ω = Allowable Strength
- Safety factor, Ω, is differs as per different conditions and is specified in AISC Specification from Chapters D through K.
2. Load and Resistance Factor Design (LRFD)
LRFD was first introduced in 1986. According to LRFD provision,
Design Strength ≥ Required Strength
According to AISC Specification Section B3.1, Equation B3-1,
where,
Ru = Required strength calculated using LRFD Load Combinations
Rn = Nominal Strength
ø = Resistance Factor
ø.Rn = Design Strength
LRFD provision takes both the uncertainties in capacity and demand into consideration. The load effect uncertainties are considered using load factor in load combinations and the uncertainties in resistance are taken into consideration using a resistance factor (ø).
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