Design Philosophies

 

 

The structure should always be designed such that

 

Nominal Strength (Capacity) ≥ Required Strength (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.

 



AISC Specifications for Steel Buildings, Chapter B, Design Requirements, Section B3



 

 

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.

 

Allowable Strength ≥ Required Strength


where,

                     Ra        = Required strength calculated using ASD Load Combinations

                     Rn        = Nominal Strength

                                 Ω     = Safety Factor

                     Rn = 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)

 

Design Strength ≥ Required Strength

 

where,

                     Ru        = Required strength calculated using LRFD Load Combinations

                     Rn        = Nominal Strength

                                 ø      = Resistance Factor

                     ø.Rn = Design Strength