Plate Girder Design Using LRFD

What differentiates a beam from a plate girder? This may seem to be a trivial question. However, it is a necessary and important part of plate girder design when applying the Load and Resistance Factor Design (LRFD) Specification.1 A beam can be a rolled or a welded shape, but it does not have intermediate stiffeners and its web width-thickness ratio hc/tw must not exceed yfF/970, where hc is twice the distance from the neutral axis to the compression flange (Fig. 1). Plate girders have stiffeners or hc/tw is greater than yfF/970, or both. Making this distinction and the compactness classification early in plate girder design is important when using the LRFD Specification. The significance of these items can be seen in the LRFD Manual flowcharts (Figs. 2 and 3) for the determination of flexural and shear design strength. With these flowcharts, this discussion will focus on the design of plate girders according to LRFD rules. An explanation of plate girder design in the LRFD Specification will include: flexural design strength, shear design strength, flexure-shear interaction, bearing strength under concentrated loads and stiffener design. Application of the LRFD method will show there is actually little difference between it and Allowable Stress Design (ASD)8 of plate girders. FLEXURAL STRENGTH The design flexural strength is calculated to provide an adequate section modulus. The first criterion necessary to separate a beam from a plate girder, yfF/970, relates to flexural design strength. Although there are preliminary steps, LRFD Specification Appendix G2 * gives the flexural capacity for a plate girder, whereas Appendix F1.7 is applicable to beams. Entering the flowchart (Fig. 2) with a trial web and flange plate size, the initial step is to determine whether plastic or elastic design is appropriate. Plastic design is permitted if the section is compact and adequate bracing is provided in accordance with Sect. F1.1, such that "the laterally unbraced length Lb of the compression flange at plastic hinge locations associated with the failure mechanism " does not excee