A Beginner's Guide to the Steel Construction Manual, 14^{th} ed. Chapter 8  Bending Members © 2006, 2007, 2008, 2011 T. Bartlett Quimby 

Section 8.2.5 Compression Flange Local Buckling Limit State Last Revised: 07/30/2011 Local buckling of the compression flange (FLB) occurs when the width/thickness ratio of the plate elements is high. The general concept was discussed in some detail in the notes on plate buckling. The specific application to flexural members is found in SCM Chapter F. Using SCM Table User Note F1.1 as a reference, one or the other of FLB appears in SCM F3, F4, F5, F6, F7, and F9. Note that there are a limited number of rolled sections which have problems with FLB and WLB. The SCM user notes in F2, F3, and F6 point out the sections that are likely to have problems with WLB and FLB for a given range of F_{y}. As F_{y} increases, more sections become noncompact and slender, thus invoking WLB and FLB limits on strength. The Limit State The basic limit state follows the standard form. The statement of the limit states and the associated reduction factor and factor of safety are given here:
The values of M_{u} and M_{a} are the LRFD and ASD factored loads, respectively, applied to the flexural member. In this case M_{n} is the nominal FLB limited flexural strength of the member. Since this is a buckling phenomenon, limits need to be found for the three strength regions: plastic, inelastic buckling, and elastic buckling as shown in Figure 8.2.1.5. The General Form The general form of the FLB limit state follows the typical buckling curve. The slenderness parameter used is width/thickness ratio (b/t) as specified in SCM B4. The limits of the buckling regions are specified by the terms l_{p} (the limit of the plastic region) and l_{r} (the limit of the inelastic buckling region). Hence:
Figure 8.2.5.1 illustrates the concept. Figure 8.2.5.1 Both l_{p} and l_{r} are computed using the appropriate cases and equations from SCM Table B4.1. M_{p}
M_{r} The moment at the interface of the elastic and inelastic ranges, M_{r}, is found embedded in the linear interpolation function found in several of the specification sections used to compute the strength in the inelastic buckling range.
Plastic Range As noted above, when l < l_{p} FLB does not happen. Consequently in the plastic range, M_{n} equals M_{p}. Inelastic Buckling Range
M_{n} = min [(M_{p}  (M_{p}  M_{r})*(l  l_{p})/(l_{r}  l_{p})), M_{p}]
M_{n} = F_{cr}R_{pg}S_{xc} = min [(F_{y}  0.3F_{y }(l  l_{p})/(l_{r}  l_{p})), F_{y}] R_{pg}S_{xc}
M_{n} = min [(M_{p}  (M_{p}  M_{r})*(l  l_{p})/(l_{r}  l_{p})), 1.6M_{y}] Elastic Buckling Range The nominal moment capacity, M_{n}, in the elastic range is found by computing the elastic moment that creates the critical buckling stress, F_{cr}, in the compression flange. M_{n} = min[F_{cr}S_{xc}, M_{p}] A modified Euler type function is used to the compute the critical buckling stress, F_{cr}.
M_{n} = F_{y}S_{e}
Summary Download a summary of FLB equations here. Sample Spreadsheet CalculationThe following spreadsheet example computes the major axis flexural capacity, M_{nx}, including FLB effects, for a typical W section. The input values are in the grey shaded cells and the result in the yellow highlighted cell.
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