Structural steel
design  General
Topics:
Topics in this page:
Steel sections are categorized in three category:
compact, noncompact, and slender sections based flange
width to thickness ratio, b/t, and web thickness to
thickness ratio, h/t_{w}.
Flange:
Compact section: when b/t £ l_{p}.
The section is a compact section.
Non compact section: when l_{p} £ b/t £ l_{r}.
The section is a noncompact section.
Slender section: when b/t > l_{r}.
The section is a slender section.
Web:
Compact section: when h/t_{w} £ l_{p}.
The section is a compact section
Non compact section: when l_{p} £ h/t_{w}£ l_{r}.
The section is a noncompact section.
Slender section: when h/t_{w} > l_{r }The
section is a slender section.
Limits of l_{p} and l_{r} AISC
13th edition Table B4.1
Flexure
in flanges of rolled I shapes and channels 
b/t 
0.38ÖE/F_{y} 
1.0ÖE/F_{y} 
Uniform
compression in flanges of rolled I shapes and
channels 
b/t 
N/A 
0.56ÖE/F_{y} 
Uniform
compression in legs of single angles 
b/t 
N/A 
0.45ÖE/F_{y} 
Flexure
in legs of single angles 
b/t 
0.54ÖE/F_{y} 
0.91ÖE/F_{y} 
Flexure
in web of I shape and channels 
h/t_{w} 
3.76ÖE/F_{y} 
5.70ÖE/F_{y} 
Uniform
compression in webs of rolled I shapes 
h/t_{w} 
N/A 
1.49ÖE/F_{y} 
Rectangular hollow structural section (HSS) 
b/t 
1.12ÖE/F_{y} 
1.40ÖE/F_{y} 
Rectangular hollow structural section (HSS) 
h/t 
2.42ÖE/F_{y} 
5.70ÖE/F_{y} 
Circle
Hollow structural section (HSS) in flexure 
D/t 
0.07ÖE/F_{y} 
0.31ÖE/F_{y} 
Circle
Hollow structural section (HSS) in uniform
compression 
D/t 
N/A 
0.11ÖE/F_{y} 
Note:
1. Most steel rolled sections are compact sections.
2. For other shapes, please see AISC manual.
M
Simply supported beams

For simply supported beam, the top flange is in
compression. If the beam is directly attached to roof
deck or floor slab, the compression flange is fully
supported. The
unsupported length L_{b} is
0.

When the beam supporting joists or other beams, and its
flange is directly attached to the supported joists or
other beams, the unsupported length is the spacing of
the joists or other beams.
Cantilever beams:

For cantilever beam, the compression flange is at the
bottom of the beam. If
the bottom flange is unbraced, the unsupported length is
the length of the cantilever beam.

If bracing is provided at the bottom flange, the
unsupported length is the spacing between bracings.
Continuous beams:

For the positive moment portion of the beam, the
compression flange is at the top of the beam. The
unsupported length is determined as a simply supported
beam.

For the negative moment portion of the beam, the
compression flange is at the bottom of the beam. The
unsupported length is determined as a cantilever beam.

