Section data (Value tab) - Unexpected results

Question:
I want to analyze truss in MIDAS. Usually, sections for truss is built-up. MIDAS don't have pre-defined these sections.

Thus, I am defining box sections in section data (value) tab. After providing basic parameters, I click on "calculate properties tab". Now, I want to modify the moment of inertia and cross-sectional area of the member as per actual section properties.

Now, when I check beam results, beam stresses are not compatible to section properties defined.

Let's say, I change moment of inertia to 10times and bending moment in the section changes to twice of original one due to increased stiffness. Now, stresses should reduce to 1/5 times of earlier one. While, I don't observe the same in MIDAS. What could be the reason?


Answer:

Case1 when normal Iyy: Let's say for element no 133, which has Top chord-1 section property. Now, the Iyy moment of inertia is 0.00519 m4. The moment My for dead load case = 97.410 kNm at j-end for 133 element number. The stress Sbz obtained is 7.758 N/mm2.


Stress results are: 


Case2 when Iyy is 10 times: Now, when we increase the moment of inertia 10 times, such that Iyy = 0.0519 m4. The moment My for dead load case = 141.041kNm at jend for element 133. The stress Sbz obtained is -1.123N/mm2.


Stress obtained are: 


To verify the calculation, we know that stress at any location is dependent on M/I*y where y = 0.4138 m, 
So, ratio of moments, Mratio = 141.041 / 97.41 = 1.448
Ratio of I, Iratio = 10
So, net ratio of stress should be Mratio / Iratio = 1.448 /10 = 0.1448
Now, if stress in case 1 beam = 7.758, then case2 stress should be 7.758 * .1448 = 1.123 which is equal to stress obtained.
Creation date: 5/27/2021 11:35 AM (jatin2010)      Updated: 5/27/2021 12:07 PM (jatin2010)
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