Ixx property of I-composite section

Question:

Hello,


While creating a model and verifying sectional properties I have noticed some difference in Ixx between the MIDAS value and the one I calculated by hand.


Please see below the screenshot of my calcs and the MIDAS output.



I have reviewed the clarification in Q&A you provided on similar issue: http://manual.midasuser.com/EN_Common/Civil/875/Questions_and_Answers.htm 

however I have the following queries in relation to the aspect of Ixx calculation:


  1. I noticed the ratio for slab component calculation used by MIDAS is based on G (shear) modulus not E (Young’s) modulus of the materials, could you please clarify.

  2. In composite I-section it appears the slab torsional stiffness is already reduced by 50% to account for double twist action at orthogonal directions in the grillage model, could you please confirm this is the case.

  3. Could you please clarify what does the Ixx value in Composite Section for Construction Stage represent as per the snapshot below:

     

    Thanks,




Answer:

Hello ,

To answer your questions:

1) This is because generally torsion is a shear effect, rather than normal stress, and so the equivalent section is calculated using the shear modulus.

2) Yes, for Composite Steel-I section generated in MIDAS Civil the torsional resistance of the slab is taken with a reduction of 50% as (bd^3)/6 rather than the value for a single thin rectangle (bd^3)/3.

3) The value you have highlighted represents the total composite section torsional stiffness. It accounts for part 1, stiffness scale factor for Ixx_part1, part 2, and stiffness scale factor for Ixx_part2. You can modify the stiffness scale factor for each part using the buttons in the Stiffness Scale column.

I believe you have already read this, but in case you haven't, all these items are descried in a bit more detail in our Online Manual, Q1 at the bottom of the page:

Kind regards,
MIDAS Support
Creation date: 4/8/2019 7:07 PM (MIDAS_UK_IM)      Updated: 4/9/2019 4:38 AM (MIDAS_UK_IM)
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