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Structural Steel Design

Structural steel design - General

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Compact, non-compact, and slender sections

Steel sections are categorized in three category: compact, non-compact, and slender sections based flange width to thickness ratio, b/t, and web thickness to thickness ratio, h/tw.

Flange: 

Compact section: when b/t £ lp. The section is a compact section.

Non compact section: when lp £ b/t £ lr. The section is a non-compact section.

Slender section: when b/t > lr. The section is a slender section.

Web: 

Compact section: when h/tw £ lp. The section is a compact section

Non compact section: when lp £ h/tw£ lr. The section is a non-compact section.

Slender section: when h/tw > lr The section is a slender section.

Limits of  lp and lr AISC 13th edition Table B4.1

Flexure in flanges of rolled I shapes and channels b/t 0.38ÖE/Fy 1.0ÖE/Fy
Uniform compression in flanges of rolled I shapes and channels b/t N/A 0.56ÖE/Fy
Uniform compression in legs of single angles b/t N/A 0.45ÖE/Fy
Flexure in legs of single angles b/t 0.54ÖE/Fy 0.91ÖE/Fy
Flexure in web of I shape and channels h/tw 3.76ÖE/Fy 5.70ÖE/Fy
Uniform compression in webs of rolled I shapes h/tw N/A 1.49ÖE/Fy
Rectangular hollow structural section (HSS) b/t 1.12ÖE/Fy 1.40ÖE/Fy
Rectangular hollow structural section (HSS) h/t 2.42ÖE/Fy 5.70ÖE/Fy
Circle Hollow structural section (HSS) in flexure D/t 0.07ÖE/Fy 0.31ÖE/Fy
Circle Hollow structural section (HSS) in uniform compression D/t N/A 0.11ÖE/Fy

Note: 

1. Most steel rolled sections are compact sections.  

2. For other shapes, please see AISC manual. 

 

Determine unsupported length for beams

 

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Simply supported beams

  1. 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 Lb is 0.

  2. 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:

  1. 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. 

  2. If bracing is provided at the bottom flange, the unsupported length is the spacing between bracings.

Continuous beams:

  1. 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.

  2. 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.