Steel Design Guide Series: Torsional Analysis of Structural Steel Members

How to Apply Steel Design Guide Series: Torsional Analysis of Structural Steel Members

Mastering the principles of torsional analysis is crucial for structural engineers seeking to design safe, efficient, and compliant steel structures. This guide will equip you with the practical knowledge to effectively apply the methodologies presented in the AISC Steel Design Guide Series on Torsional Analysis of Structural Steel Members, ensuring your designs can withstand complex loading scenarios and uphold the highest standards of structural integrity.

Before You Begin

Prerequisites:

• • Knowledge: A solid understanding of fundamental structural mechanics, statics, strength of materials, and general steel design principles. Familiarity with AISC specifications for Load and Resistance Factor Design (LRFD) and Allowable Stress Design (ASD) is beneficial.
• • Tools/Resources: Access to the AISC Steel Design Guide Series: Torsional Analysis of Structural Steel Members, relevant AISC design specifications (e.g., LRFD, ASD), and engineering calculation software or spreadsheets.
• • Time Required: Approximately 2-4 hours, depending on familiarity with the concepts and the complexity of the analysis.

Step-by-Step Implementation

Step 1: Identify and Quantify Torsional Loads

Begin by meticulously identifying all sources of torsional loads acting on the steel member. This includes eccentric loads, applied moments at disconnected points, and torsional effects arising from lateral loads on unsymmetrical framing. Accurately quantifying these moments (factored for LRFD or service for ASD) is the foundational step for any torsional analysis. Common mistakes include neglecting small eccentricities or assuming loads are applied at the shear center without verification.

Step 2: Determine the Shear Center Location

Locate the shear center for the cross-section of the member in question. For doubly symmetric shapes (e.g., W-shapes, HSS), the shear center coincides with the centroid. For singly symmetric (e.g., channels, tees) and unsymmetrical shapes (e.g., angles, Z-shapes), the shear center's location is critical and must be determined from tabulated data or calculated using established methods. Applying loads through the shear center will induce pure torsion without causing bending.

Step 3: Calculate Torsional Resistance Components

Understand that a cross-section resists torsion through two primary mechanisms: pure shear (T_s) and restrained warping (T_w). Calculate the torsional constant (J) for the cross-section, which is fundamental for determining the pure torsional shear stress. For open sections, also determine the warping constant (C_w) and the torsional functions (often denoted as $\phi$ and its derivatives), which are essential for assessing stresses due to warping.

Step 4: Compute Torsional Stresses

Calculate the shear stresses due to pure torsion using the torsional moment and the torsional constant (J). Subsequently, determine the shear and normal stresses arising from warping. For open sections, approximate methods for I-shapes are often used, while more complex calculations or design aids (found in the guide's appendices) may be necessary for other profiles. Remember to combine these stresses appropriately.

Step 5: Combine Torsional Stresses with Other Load Effects

Integrate the calculated torsional stresses (shear and normal) with stresses resulting from axial forces and bending. For open sections, this typically involves combining the maximum shear stresses and checking the combined stresses against allowable limits. For closed sections, the torsional moment is primarily resisted by a uniform shear stress, which is then combined with other stress components.

Step 6: Evaluate Against Specification Provisions and Serviceability Criteria

Compare the computed stresses and deformations against the relevant provisions of the AISC LRFD or ASD specifications. This includes checking the capacity of the member in torsion and ensuring that deflections and rotations under service loads meet established serviceability criteria. Pay close attention