Section Property Calculator Torsion Constant for Composite Section
When using the Section Property Calculator for composite section, we are finding the torsion constant J for the composite section lower than the single section (part 2 deck concrete in this case).
Refer attached section property calculator file.
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The Ixx value shown in deck part is simply the Ixx value of rectangular section calculated by the SPC.
But when the Ixx of steel + deck is observed, the Ixx of deck is reduced by Ec/Es as the modulus of steel and concrete is different. Thus, the Ixx value shown in deck is reduced to Ixx*Ec/Es and then added to Ixx (girder).
Similar things are done for Iyy, area,Izz and other properties. We can see that area of deck is higher but the total are of deck + girder is less because the area of deck is reduced by Ec/Es and then added steel properties.