hi
i want to calculate polar moment of inertia of I beam. I know general equation of polar MI as J=Ix+Iy.
i calculated by using this but the vaue does not match with one that is calculated by staad software.
i got a reply from this community members that above equeation is used for circular sections only and for non circular sections it can not be used..
then i searched for it on google,com, there some places it is mentioned that above formula cant be used for non circular and at some places same formula as above is used to calculated Polar MI.
I got some formulaes for Polar MI for rectangular from google and its resulsts matchs with staad results for rectangular sections..
but when i used that formula to calculate POlar mI of I beam considering web and flanges as rectangular section and adding them,, the result does not match with results given by staad..
kindly help in this regards
ur views are very importants for me.......
Polar moment of inertia and torsional constant are two different things. You want torsional constant.
Please explain the difference between the two.
Polar moment of inertia is the sum of IY and IZ. It's only the same as the torsional constant for circular cross sections. For a wide flange the torsional constant can be approximated by the sum of 1/3bt^3 for each rectangle that comprises the cross section. You'll need to look it up in a structural book or google it for more complete info.
To illustrate just how different the two can be, for a W8x24 the polar moment of inertia is 101 in^4, the torsional constant is 0.35 in^4, so you see how important it is not to confuse the two.