Trouble to understand Rotational Stiffness knM/rad to kN/m in etabs?

[sky1379]

Let's say i already analyzed my steel connection rotational stiffness in idea statica and i got the result 0.3kNm/rad, and i assigned this to the beam in etabs like in the picture (like you assigned when you released the beam moment). And when i right clicked the beam i the value 0.3kNm/rad turn into 300000kN/m. How can this happen? kNm/rad to kN/m .

 

[centondollar]

I don't know ETABS, but the units in boxes "Stiffness I" and "Stiffness J" do not correspond to rotational stiffness. Maybe read the software manual or ask customer support for an explanation? You also refer to different moments in your figures (major in left figure, minor in the figure on the right) - why?

PS. That stiffness sounds quite low - 57.3 degrees to generate 0,3 kNm of bending moment, which in practice means zero rotational fixity in any reasonable building industry-scale connection detail.

 

[human909]

Yes. Idea Statica outputs in kNm/mrad.

Watch your units and check you output how sensible it is. If you enter 0.3kNm/rad into as a rotational stiffness the moment diagram would likely look like almost a perfect pin.

 

[Luceid]

Can always test it using a simple stick model - use a super stiff frame element with a pin + rotational spring at one end and apply a point load at the tip. Calculate how much rotation you would expect based on a kN-m/rad unit assumption and see how it works out. Not sure why the units are so goofy in the menu though

 

[JoshPlumSE]

Sounds like a simple units conversion issue. A lot of programs will store a value using internal units and then display it to the user with different units depending on what unit settings they have.

Hard to tell if this is a display issue or a real issue though. I'd save the model. Exit out and then re-open it and look at both places again to see if the units match.

If not, I'd do what Luceid suggested and run a couple of tests to see how it's actually behaving.