Torsion Check for Steel Beam-CISC

The torsion check for a steel beam using the correlation factor for the modified flexure analogy is a method used to determine the torsional capacity of a steel beam under combined bending and torsional loads. The modified flexure analogy is an approximate method that simplifies the analysis of torsional stresses in steel beams by comparing it to a more familiar problem, such as bending.

Here is a step-by-step description of the torsion check process:

1. Determine the beam's properties: First, obtain the geometric properties of the steel beam, such as its cross-sectional area, moment of inertia, and polar moment of inertia. Also, obtain the material properties, including the yield strength and modulus of elasticity.

2. Calculate the applied loads: Determine the applied loads on the beam, including the bending moments, shear forces, and torsional moments.

3. Calculate the torsional stresses: Using the modified flexure analogy, calculate the torsional stresses in the steel beam. The analogy involves multiplying the torsional moment by a correlation factor (k) and then treating the resulting moment as if it were a bending moment. The correlation factor accounts for the difference in the behavior of the beam under torsion compared to bending.

4. Calculate the combined stresses: Determine the combined stresses in the steel beam due to bending and torsion. The bending stress can be calculated using the bending moment and the moment of inertia. The torsional stress is calculated in step 3. Add the bending and torsional stresses to find the combined stresses.

5. Check the stress limits: Compare the combined stresses to the allowable stress limits for the steel beam. The allowable stress limits are typically based on the yield strength of the material, reduced by a safety factor. If the combined stresses exceed the allowable stress limits, the beam may fail under the applied loads.

6. Evaluate the beam's performance: If the combined stresses are within the allowable limits, the beam is considered safe for the applied loads. If not, you may need to revise the beam's design or select a different steel section to ensure the structure's safety.

The modified flexure analogy with the correlation factor simplifies the torsion analysis for steel beams and makes it more accessible for engineers to perform the torsion check. However, it is important to remember that it is an approximate method and may not be as accurate as more advanced torsional analysis techniques.