AISC Design Guide 9: Torsional Analysis of Structural Steel Members

How to Apply AISC Design Guide 9: Torsional Analysis of Structural Steel Members

Mastering the principles of torsional analysis for structural steel members is a hallmark of advanced engineering practice. By effectively applying AISC Design Guide 9, you can enhance the safety, serviceability, and economic efficiency of your designs, preventing potentially catastrophic failures and solidifying your reputation as a competent and thorough structural engineer. This guide will equip you with a practical framework to navigate the complexities of torsional loading and mitigation.

Before You Begin

Prerequisites:

• • Knowledge: A solid understanding of structural mechanics, including statics, strength of materials, and fundamental steel design principles as outlined in the AISC Steel Construction Manual. Familiarity with concepts like shear center and torsional rigidity is essential.
• • Tools/Resources: Access to AISC Design Guide 9: Torsional Analysis of Structural Steel Members, a calculator or computational software capable of performing engineering calculations, and relevant project drawings.
• • Time Required: Approximately 1-2 hours for initial review and comprehension, with subsequent application time varying based on project complexity.

Step-by-Step Implementation

Step 1: Identify Potential Torsional Load Sources

Begin by meticulously reviewing your project plans to pinpoint any elements or load conditions that could induce torsion. Common culprits include eccentric connections, lateral loads applied at a distance from the member's shear center (e.g., facade elements, crane rails), and unbalanced loads on beams supporting non-symmetrical configurations. Understanding the origin of torsion is the critical first step in addressing it.

Step 2: Determine Member Shear Center Location

For any member identified as potentially experiencing torsion, accurately locate its shear center. The shear center is the point in a cross-section through which a shear force must pass to produce no rotation. For standard rolled shapes, these locations are often tabulated in steel design resources, but for built-up sections or unusual geometries, you may need to calculate it. Incorrectly identifying the shear center is a primary cause of unintended torsion.

Step 3: Evaluate the Feasibility of Avoiding Torsion

Prioritize designing load connections and reactions to coincide with the member's shear center whenever feasible. This is the most effective strategy for preventing torsion altogether. Examine detailing options, such as adjusting connection points or utilizing supporting framing to transfer eccentric loads to diaphragms, thereby bypassing direct torsional loading on the member.

Step 4: Quantify Torsional Loads When Avoidance is Not Possible

When torsion cannot be eliminated, meticulously calculate the magnitude and distribution of the torsional moments. This involves considering the eccentricities of applied loads and reactions relative to the shear center. Refer to the methods and examples provided in Design Guide 9 for calculating these torques accurately.

Step 5: Assess the Member's Torsional Resistance

Once torsional loads are quantified, evaluate the member's inherent capacity to resist torsion. Design Guide 9 provides guidance on calculating torsional stiffness ($k$) using the formula $T = k\theta$, where $T$ is the torque and $\theta$ is the angle of twist. This assessment will inform whether the member's torsional resistance is adequate or if additional measures are required.

Step 6: Implement Torsional Mitigation Strategies

If the member's torsional resistance is insufficient, implement appropriate mitigation strategies. This can include providing intermediate torsional bracing at points of torsional load application, utilizing secondary framing to offer torsional support, or selecting member cross-sections with improved torsional characteristics. Consider the rotational stiffness of restraining elements in your analysis.

Step 7: Verify Design with Load and Stability Checks

Finally, perform comprehensive load and stability checks on the member, incorporating the effects of torsion. Ensure that the combined stresses from bending, shear, and torsion, along with any induced secondary effects, do not exceed the allowable