Nab 24 hour number

CE 470: Design of Steel Structures –Prof. Varma1CHAPTER 4. COMPRESSION MEMBER DESIGN4.1 INTRODUCTORY CONCEPTSCompression Members: Structural elements that are subjected to axial compressive forces only are called columns. Columns are subjected to axial loads throughthe centroid. Stress:The stress in the column cross-section can be calculated as APf(2.1)wherefis assumed to be uniform over the entire cross-section. This ideal state is never reached. The stress-state will be non-uniform due to:-Accidental eccentricity of loading with respect to the centroid-Member out-of –straightness (crookedness), or-Residual stresses in the member cross-section due to fabrication processes.Accidental eccentricity and member out-of-straightness can cause bending moments in the member. However, these are secondary and are usually ignored.Bending moments cannot be neglected if they are acting on the member. Members with axial compression and bending moment are called beam-columns.4.2 COLUMN BUCKLINGConsider a long slender compression member. If an axial load P is applied and increased slowly, it will ultimately reach a value Pcrthat will cause buckling of the column(Figure 1). Pcris called the critical buckling load of the column.
CE 470: Design of Steel Structures –Prof. Varma2What is buckling?Buckling occurs when a straight column subjected to axial compression suddenly undergoes bending as shown in the Figure 1(b). Buckling is identified as a failure limit-state for columns. Figure 1.Buckling of axially loaded compression membersThe critical buckling load Pcrfor columns is theoreticallygiven by Equation (4.1)Pcr= 22LKIE(4.1)where, I = moment of inertia about axis of bucklingK = effective length factor based on end boundary conditionsEffective length factors are given on page 16.1-511(Table C-A-7.1) of the AISC manual.PcrPcrPP(a)(b)PcrPcrPPPP(a)(b)