Design of Steel Members for Compression per AISC 36016 §E
How does ideCAD calculate steel members' compression strength according to AISC 36016?
The compression strength of steel elements is calculated automatically according to AISC 36016.
The nominal compressive strength limit states of flexural buckling, torsional buckling, and flexuraltorsional buckling are controlled automatically according to AISC 36016.
Symbols
F_{y} = Specified minimum yield stress
E = Modulus of elasticity of steel = 29,000 ksi (200 000 MPa)
λ_{r }= Limiting widthtothickness parameter for noncompact element
λ_{p} = Limiting widthtothickness parameter for compact element
Compression Strength of the Steel Elements
Compression steel members are structural elements subjected only to axial compressive forces; the loads are applied along a longitudinal axis through the centroid of the member crosssection.
Since some eccentricity of the load is inevitable, the ideal state cannot be reached, and therefore, a bending situation called buckling occurs.
There are two main types of buckling, local and global buckling, in the elements under compressive force.
Local Buckling
Local buckling occurs when some part or parts of the crosssection of a column are so slender that they buckle locally in compression.
Limit values for the classification of crosssections for the local buckling boundary condition are given in AISC 36016 B.4 and Table B4.1a and b for crosssections under compressive force.
TABLE B4.1a  

Case  Element Description  WidthtoThickness Ratio  Limiting WidthtoThickness Ratio λ_{r} (nonslender/slender)  Examples 
1  Flanges of rolled Ishaped sections, plates projecting from rolled Ishaped sections, outstanding legs of pairs of angles connected with continuous contact, flanges of channels, and flanges of tees  b/t 
 
2  Flanges of builtup Ishaped sections and plates or angle legs projecting from builtup Ishaped sections  b/t 
 
3  Legs of single angles, legs of double angles with separators, and all other unstiffened elements  b/t 
 
4  Stems of tees  d/t 
 
5  Webs of doubly symmetric rolled and builtup Ishaped sections and channels.  h/t_{w} 
 
6  Walls of rectangular HSS  b/t 
 
7  Flange cover plates and diaphragm plates between lines of fasteners or welds  b/t 
 
8  All other stiffened elements  b/t 
 
9  Round HSS  D/t 
 
*Case 1, 2, 3, 4 are Unstiffened Elements. Cases 5, 6, 7, 8, 9 are Stiffened Elements. 
TABLE B4.1b  

Case  Element Description  Widthto Thickness Ratio  Limiting WidthtoThickness Ratio  Examples  
λ_{p}  λ_{r}  
10  Flanges of rolled Ishaped sections, channels, and tees  b/t 

 
11  Flanges of doubly and singly symmetric Ishaped builtup sections  b/t 

 
12  Legs of single angles  b/t 

 
13  Flanges of all Ishaped sections and channels in flexure about the minor axis  b/t 

 
14  Stems of tees  d/t 


Global Buckling
Elements under the effect of compression may not function as an ideal compression element for various reasons, although the flexure moment does not occur due to loads. The most important reasons are:
Initial Curvature
Load eccentricity
Residual Stresses
Global buckling is divided into three categories: Flexural buckling, Torsional buckling, and FlexuralTorsional buckling. These types of buckling are also the limit states of members under compression.
Flexural Buckling Limit State
The buckling deformations all lie in one of the principal planes of the column crosssection. No twisting of the crosssection occurs for flexural buckling.
The limit state of flexural buckling is applicable for axially loaded columns with doubly symmetric sections, such as bars, HSS, and round HSS, and Ishapes and singly symmetric sections, such as T and Ushapes. Flexural buckling is the simplest type of buckling.
Flexural Buckling Design with AISC 36016
The compressive strength of the elements is determined according to the axial force acting from the section center of gravity. According to the regulation, the flexural buckling limit state is considered in all compression elements, regardless of crosssection properties.
First of all, local buckling control should be done. The calculation is made to determine whether the elements are compact or noncompact.
Torsional Buckling Limit State
Buckling occurs when the element rotates around its longitudinal axis. The limit state of torsional buckling applies to axially loaded columns with doubly symmetric open sections with very slender crosssectional elements consisting of 4 corners placed back to back.
Torsional Buckling Design with AISC 36016
The compressive strength of the elements is determined according to the axial force acting from the section center of gravity.
In the torsional buckling boundary case where buckling occurs by the rotation of the element around its longitudinal axis (+ shaped crosssection or open crosssection elements consisting of 4 corners placed back to back), the elastic buckling stress F_{e} is calculated for doubly symmetric members by equation E4.2.
Flexural  Torsional Buckling Limit State
The buckling deformations consist of a combination of twisting and bending about two flexural axes of the member.
The symmetry axis is the yaxis, where the buckling around the yaxis is caused by the tilting and rotation of the element around its longitudinal axis. The limit state of flexuraltorsional buckling applies to columns with singly symmetric shapes, such as double angle, T and Ushapes, and asymmetric crosssections.
Flexural  Torsional Buckling Design with AISC 36016
The compressive strength of the elements is determined according to the axial force acting from the section center of gravity.
With the symmetry axis being the yaxis, the elastic buckling stress F_{e} in the flexuraltorsional buckling limit state where buckling around the yaxis occurs by tilting and rotating around the longitudinal axis, F_{e} Equation E43 is calculated.