scope:

Except where otherwise noted, the recommendations in this Design Guide apply to I-section members satisfying the following scope:

(1) Specified minimum yield strength within the limits permitted by the AISC Specification (AISC, 2016b).

(2) Homogeneous and hybrid members. The extension to hybrid members is addressed in an abbreviated manner; the AASHTO LRFD Bridge Design Specifications, 9th Ed. (AASHTO, 2020), provides a detailed summary of the corresponding flexural resistance calculations.

(3) Any single or multiple web taper with a taper angle (the angle between the longitudinal axes of the flanges) less than or equal to 15°. In addition, the procedures provided in this document accommodate members with steps in the cross-section dimensions, which can occur at field or shop splices.

For taper angles larger than the limit specified in Item (3), beam theory starts to deviate significantly from the physical behavior (Lee, 1959). In addition, the tension field action (TFA) resistances for stiffened webs discussed in this Design Guide are not generally valid for web taper angles greater than 15°.

Item (4) is basically a limit of the supporting research studies. Furthermore, the elastic flange local buckling (FLB) resistance of slender-web I-section members having a compression flange slenderness equal to this value is only 28 ksi, although there is clear evidence that slender flanges of this nature have substantial post-local buckling strength (Tog˘ay and White, 2017, 2018). However, there is also evidence of the potential for significant local global buckling interaction for I-section members with slender rectangular compression flanges (Cherry, 1960), which the AISC Specification provisions do not directly address. Furthermore, considering post-buckling strength under member axial compression, the effective area of the flanges can be as low as approximately 60% of the flange gross area when their slenderness is at the limit specified in Item (4). Lastly, flange distortion during fabrication can be more difficult to control for relatively thin flanges.

Items (5) and (6) are also largely limits of the supporting research. The ratio of test strengths to the predictions by the AISC Specification flexural resistance equations tends to be lower and the dispersion in the predictions larger for embers with flanges narrower than h/6 (White and Jung, 2008; White and Kim, 2008; White and Barker, 2008; White et al., 2008). In addition, for ratios of the average flange area to the web area, (b_f1t_f1 + b_f2t_f2)/(2ht_w), larger than 2.5, the AISC Specification defines a smaller tension field action shear resistance for stiffened webs. This is based on the fact that these types of sections have difficulty achieving the traditional complete TFA shear resistance given in prior Specifications (White et al., 2008). I-sections with b_f values smaller than h/7 generally require very close brace spacing to develop substantial flange flexural stresses in uniform or near uniform bending, and they can be more susceptible to reductions in shear strength due to the lack of ability of the flanges to provide sufficient lateral restraint at the top and bottom of the web. In addition, a number of researchers have shown a correlation between the ratio of the flange and web thicknesses and the shear buckling strength of I-section members, with thinner flanges resulting in smaller shear buckling strength (White and Barker, 2008).

The limit in Item (7) is a simplified form of an equation originally developed by Basler and Thurlimann (1961) and adopted by the AISC Specification to guard against vertical flange buckling, a theoretical limit state in which the flange in flexural compression can buckle into the web. This limit also tends to protect against the use of I-section members that have relatively low web crippling strength and potential concomitant interaction between web crippling and other strength limit states. The maximum limit of 260 in Item (7) was first adopted in the 1986 AISC LRFD Specification (AISC, 1986) and appears to be an upper limit on the term 0.40E/F_y to guard against particularly large values of h/t_w in cases of unstiffened webs where the yield strength is less than 45 ksi.

Item (8) is a more liberal AISC Specification limit on h/t_w than Item (7), found to be permissible as a relaxed precaution against vertical flange buckling for members with closely spaced transverse web stiffeners. This limit is based on the evaluation of a number of hybrid I-girder tests (ASCE, 1968). The modification to the AISC Specification equation is the specific use of the yield strength of the compression flange, F_yc.

Item (9) ensures that the ratio of the flange lateral bending moments of inertia is not such that the behavior is more like that of a tee-section member, which can have a more limited flexural-torsional, lateral-torsional, and shear buckling capacity.

All the detailed recommendations in this Design Guide focus on the design of members and frames subjected to axial force and bending within the plane of the framing. This is the predominant application of web-tapered and general nonprismatic members, and frames composed of these types of members. Unless noted otherwise, the framing is assumed to be braced in the out-of-plane direction. The members and rames may be braced or unbraced within the plane of bending. Although the primary focus is as explained previously, the resistance calculations discussed in this Design Guide are also applicable as a portion of the overall calculations required for evaluation of general three-dimensional spatially loaded members and frames.