How do you choose the best lateral torsional buckling (LTB) design method for your beam project?

1 Nominal Flexural Strength

The nominal flexural strength (Mn) is the maximum bending moment that a beam can resist before LTB occurs. It depends on the beam's cross-sectional shape, material properties, unbraced length, and loading pattern. The American Institute of Steel Construction (AISC) provides formulas and tables to calculate Mn for different beam types and load cases in its Specification for Structural Steel Buildings (AISC 360). You can use these formulas and tables to check if your beam meets the LTB requirements for your design load.

2 Lateral Bracing

Lateral bracing is a structural element that provides support and restraint to a beam's compression flange, preventing it from buckling out of plane. Lateral bracing can be attached to the beam itself, or to another beam, column, or wall that is connected to the beam. The spacing and stiffness of the lateral bracing affect the LTB behavior of the beam. The AISC Specification defines the unbraced length (Lb) as the distance between two points where the beam's compression flange is laterally braced. The smaller the Lb, the higher the Mn and the lower the LTB risk.

3 Torsional Bracing

Torsional bracing is a structural element that provides resistance and stiffness to a beam's torsion, preventing it from twisting out of plane. Torsional bracing can be applied to the beam's web, flanges, or both. The effectiveness of the torsional bracing depends on its location, orientation, and connection to the beam. The AISC Specification defines the torsional unbraced length (Ltb) as the distance between two points where the beam's torsion is braced. The smaller the Ltb, the higher the Mn and the lower the LTB risk.

4 Effective Length Factor

The effective length factor (K) is a dimensionless parameter that accounts for the influence of the lateral and torsional bracing on the LTB behavior of the beam. It modifies the Lb and Ltb values to reflect the actual buckling length of the beam. The K value depends on the beam's end conditions, bracing configuration, and loading pattern. The AISC Specification provides formulas and charts to determine K for different beam types and load cases. You can use K to adjust your Lb and Ltb values and calculate your Mn accordingly.

5 Design Aids

Design aids are tools that simplify the LTB design process by providing pre-calculated values of Mn, Lb, Ltb, and K for common beam types and load cases. They can be in the form of tables, charts, graphs, or software programs. Some examples of design aids are the AISC Design Examples, the AISC Steel Construction Manual, and the AISC Steel Tools website. You can use design aids to quickly compare different LTB design methods and select the best one for your beam project.