Torsional Stiffness for closed composite section

Creation date: 3/22/2022 12:01 PM Updated: 3/24/2022 6:11 AM

Question:

Hi

I have two identical models of simply supported composite beams. The first model (Test - CS) includes construction sequence and the second one ( Test - no CS) is a normal linear model without construction sequence. All the section properties and geometry are the same. But the moving load analysis on the two models give very different results, while other static load cases (Load case: Test) give the same results. As the model is a static determinate structure, my understanding is the construction sequence will not cause any difference in the moving load analysis. Please help to explain what's the reason cause the difference, and which model gives the correct results.

Regards

Answer:

Hello,

Actually, Test Load is applied directly on the Beam section. So, My moment is governed by Iyy of the beam.

However, Moving Load acts on the cross beams via which the load is transferred to the main girders. Thus the Ixx of the girder comes into play as well.

Now, the results of the two cases (with CS and without CS) varied because of one reason. That is the use of "Composite Section for Construction Stage".

Check out the Ixx value of "After Composite" Section for the two cases.

Note that the images below are sample images and the values may not exactly match with your model. This is because I had made some random changes to the existing model. However, the concept is the same.

We need to understand why that is.

Actually, currently it is limitation of the software.

So, workaround for this is giving the section stiffness scale factor as shown below. Enter the appropriate section stiffness scale factor for Ixx only.