Committee for Specifications for the Design of Committee/Subcommittee Ballot: CS08-310C-Final

Cold-Formed Steel Structural Members Attachment A

Subcommittee 10, Element Behavior and Direct Strength Date: July 16, 2010

1.2.2 Beam Design

The nominal flexural strength [resistance], Mn, shall be the minimum of Mne, Mn, and Mnd as given in Sections 1.2.2.1 to 1.2.2.3. For beams meeting the geometric and material criteria of Section 1.1.1.2, Wb and fb shall be as follows:

Wb = 1.67 (ASD)

fb = 0.90 (LRFD)

= 0.85 (LSD)

For all other beams, W and f of the main Specification, Section A1.1(b), shall apply. The available strength [factored resistance] shall be determined in accordance with applicable method in Section A4, A5, or A6 of the main Specification.


1.2.2.1 Lateral-Torsional Buckling

The nominal flexural strength [resistance], Mne, for lateral-torsional buckling shall be calculated in accordance with Section 1.2.2.1.1. The strength [resistance] increase for inelastic reserve in lateral-torsional buckling shall be permitted in accordance with Section 1.2.2.1.2, as applicable.

1.2.2.1.1 Lateral-Torsional Buckling Strength

(a) For Mcre < 0.56My

Mne = Mcre (Eq. 1.2.2-1)

(b) For 2.78My Mcre 0.56My

Mne = (Eq. 1.2.2-2)

(c) For Mcre 2.78My

Mne = My (Eq. 1.2.2-3)

where

Mcre = Critical elastic lateral-torsional buckling moment, see determined by analysis in accordance with Section 1.1.2

My = SfFy (Eq. 1.2.2-4)

where

Sf = Gross section modulus referenced to the extreme fiber in first yield

1.2.2.1.2 Inelastic Reserve Lateral-Torsional Buckling Strength

For Mcre > 2.78 My

(Eq. 1.2.2-5)

where

Mcre = Critical elastic Lateral-torsional buckling moment, see Section 1.1.2 as defined in Section 1.2.2.1.1

My = Yield moment as defined in Eq. 1.2.2-4

Mp = ZfFy (Eq. 1.2.2-6)

where

Zf = Plastic section modulus


1.2.2.2 Local Buckling

The nominal flexural strength [resistance], Mn, for local buckling shall be calculated in accordance with Section 1.2.2.2.1. The strength [resistance] increase for inelastic reserve in local buckling shall be permitted in accordance with Section 1.2.2.2.2, as applicable.

1.2.2.2.1 Local Buckling Strength

(a) For l£0.776

Mn = Mne (Eq. 1.2.2-7)

(b) For l > 0.776

Mn = (Eq. 1.2.2-8)

where

l = (Eq. 1.2.2-9)

Mne = Nominal flexural strength [resistance] for lateral-torsional buckling as defined in Section 1.2.2.1.1

Mcr = Critical elastic local buckling moment, see determined by analysis in accordance with Section 1.1.2

1.2.2.2.2 Inelastic Reserve Local Buckling Strength

For l£0.776 and Mne My

Sections symmetric about the axis of bending or sections with first yield in compression:

(Eq. 1.2.2-10)

Sections with first yield in tension:

(Eq. 1.2.2-11)

where

l = (Eq. 1.2.2-12)

Mne = Nominal flexural strength [resistance] as defined in Section 1.2.2.1.2

(Eq. 1.2.213)

Mcr = Critical elastic lLocal buckling moment, see Section 1.1.2 as defined in Section 1.2.2.2.1

Mp = Plastic moment as defined in Eq. 1.2.2-6

My = Yield moment as defined in Eq. 1.2.2-4

Myc = Moment at which yielding initiates in compression (after yielding in tension). Myc = My may be used as a conservative approximation.

(Eq. 1.2.214)

Cyt = 3 (Maximum tension strain divided by the yield strain)

1.2.2.3 Distortional Buckling

The nominal flexural strength [resistance], Mnd, for distortional buckling shall be calculated in accordance with Section 1.2.2.3.1. The strength [resistance] increase for inelastic reserve in distortional buckling shall be permitted in accordance with Section 1.2.2.3.2, as applicable.

1.2.2.3.1 Distortional Buckling Strength

(a) For ld £ 0.673

Mnd = My (Eq. 1.2.2-15)

(b) For ld > 0.673

Mnd = (Eq. 1.2.2-16)

where

ld = (Eq. 1.2.2-17)

My = Yield moment as defined in Eq. 1.2.2-4

Mcrd = Critical elastic distortional buckling moment, see determined by analysis in accordance with Section 1.1.2

1.2.2.3.2 Inelastic Reserve Distortional Buckling Strength

For ld £ 0.673

Sections symmetric about the axis of bending or sections with first yield in compression:

(Eq. 1.2.2-18)

Sections with first yield in tension:

(Eq. 1.2.2-19)

where

ld = (Eq. 1.2.2-20)

(Eq. 1.2.2-21)

Mcrd = Critical elastic dDistortional buckling moment, see Section 1.1.2 as defined in Section 1.2.2.3.1

Mp = Plastic moment as defined in Eq. 1.2.2-6

My = Yield moment as defined in Eq. 1.2.2.-4

Myc = Moment for yield in compression as defined in Section 1.2.2.2.2

Myt3 = Maximum moment for yielding in tension as defined in Eq. 1.2.2-14