Combined stress for moving load analysis does not match with Summation of Axial & Bending stress.

Question:

I want to get clarification on the combined moving load stress to composite section.

As show in attached image, I checked the max tension stress at element 1314 under moving load case "Cen MV". The combined stress at point 4(-y,-z) is 11.1MPa, however, the individual stresses are 2.99MP for Sax, 0.795MPa for Sby, 5.72MPa for Sbz. All this stress will give sum as 9.505MPa. It's different to the combined stress.


Answer:

Actually, this issue happened because of the unsymmetrical section. Let me explain this by the following example:- 

In the bending stress column, the program would show the maximum possible stress component of the section. For Mz in the above section maximum stress would occur at points 1 & 2. Therefore in bending stress column, it shows this stress result. 



Now for the combined stress calculation at points 3 & 4, we can not simply add this stress to the axial stress. The program internally calculates the stress at point 3 & 4 for the Mz component and then calculate the combined stress. I have attached a verification excel sheet for your reference. 

To further understand, this happens because for the moving load case the program gives us the envelope results. Let me explain this by the following example:- 
In combined stress, there are 3 components, A(axial stress), B (Bending -Y stress), and C (Bending - Z stress. Now to maximize the A+B+C, I have taken three static load case (created using MVL tracer):- (1) Amax & concurrent (2) Bmax & concurrent (3) Cmax & concurrent. 



But still, the stress results we got from the MIDAS are more from my calculated results. This happened because (A+B+C) max can occur at any other case in which all 3 (A, B, C) are not at its maximum individually but the submission is at the maximum. The program internally considers all the cases as per the influence line and gives us the result of the critical case. Unfortunately, the moving load tracer option is not available for the stresses (composite section) therefore exact static load cases can not be created for this. 
Creation date: 11/27/2020 6:18 AM      Updated: 5/21/2023 6:10 PM
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DataImage94.png
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Model SLS 002 - Collision E400Mpa - 04.mcb
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verification.xlsx
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DataImage85.png
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DataImage31.png
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Stress Verification.xlsx
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Model SLS 002 - Collision E400Mpa - 04.mcb
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