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Square And Rectangular HSS and Box Sections Flexural Design per AISC 360-16 with ideCAD

How does ideCAD calculate the flexural strength of square and rectangular HSS and box sections according to AISC 360-16?


  • The flexural strength of steel elements is calculated automatically according to AISC 360-16.

  • The nominal flexural strength limit states yielding, lateral-torsional buckling, flange local buckling, and web local buckling are controlled automatically according to AISC 360-16.

  • For design members for flexure, sections are automatically classified as compact, noncompact, or slender-element sections, according to AISC 360-16.


Symbols

b : Width of compression flange as defined in AISC 3610-16 Table B4.1b
Cb : The lateral-torsional buckling modification factor
E : Modulus of elasticity of steel = 29,000 ksi (200 000 MPa)
Fcr : Lateral-torsional buckling stress for the section as determined by analysis, ksi (MPa
Fy : Specified minimum yield stress of the type of steel being used, ksi
Its : Effective radius of inertia
J : Torsional constant
h : depth of web, as defined in AISC 3610-16 Table B4.1b
ho : Distance between the flange centroids, in. (mm)
Lb : Length between points that are either braced against lateral displacement of the compression flange or braced against twist of the cross-section, in. (mm)
Lp : The limiting laterally unbraced length for the limit state of yielding, in. (mm)
Lr : The limiting unbraced length for the limit state of inelastic lateral-torsional buckling, in. (mm),
Mn : The nominal flexural strength
Mp : Plastic bending moment
ry : Radius of gyration about the y-axis
S : elastic section modulus about the axis of bending, in.3 (mm3)
Sx : Elastic section modulus taken about the x-axis, in.3 (mm3)
Z = plastic section modulus about the axis of bending, in.3 (mm3)
Zx = Plastic section modulus about the x-axis, in.3 (mm3)


The nominal flexural strength, Mn, should be the lower value obtained according to the limit states of yielding (plastic moment), flange local buckling, web local buckling, and lateral-torsional buckling.

Yielding Limit State

Flange Local Buckling Limit State

  • The limit state of flange local buckling does not apply to compact sections.

  • The nominal flexural strength, Mn, is calculated below for sections with noncompact flanges.

  • The nominal flexural strength, Mn, for sections with slender flanges;

Se is the effective section modulus determined with the effective width, be, of the compression flange taken as given below.

  • For HSS

  • For Box sections

Web Local Buckling Limit State

  • The limit state of web local buckling does not apply to compact sections.

  • The nominal flexural strength, Mn, is calculated below for sections with noncompact webs.

  • The nominal flexural strength, Mn, is calculated below for sections with noncompact webs.

    • Compression flange yielding

    • Compression flange local buckling

Lateral Torsional Buckling Limit State

  • When Lb ≤ Lp, the limit state of lateral-torsional buckling does not apply

  • When Lp < Lb ≤ Lr

  • When LbLr

Laterally unbraced length for the limit state of yielding, Lp, is calculated below.

Laterally unbraced length for the limit state of inelastic lateral torsional buckling, Lr, is calculated below.


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