Rotational Spring (with 84000 kip-in. stiffness for example)

[husayn]

Hello everyone,

I have problems with understanding the modeling of a rotational spring. As far as I know, the dimension for the stiffness of a rotational spring is FL (i.e. moment/rotation_angle; kip-in./rad).
How can we model a rotational spring using a zeroLength element and a uniaxial material by giving only the elastic modulus (elastic material for example), whereas, for modelling a rotational spring with 84000 kip-in/rad stiffness, we have to give it a length and a moment of inertia to be able to calculate EI/L whose dimenion is FL (not only E whose dimension is FL^-2).

Is there something missing in my understanding of this matter? Or maybe opensees assumes that I=1 and L=1, and if I set E=84000 ksi, the stiffness of the zeroLength element would be actually k_rot=84000 kip-in./rad! Am I correct about it?

Could somebody enlighten me please?

[mhscott]

There's no assumed units with materials in OpenSees. There's no assumption of L=1 or I=1. The interpretation of the material response is the responsibility of the element that calls the material. So if you use a zero length element and elastic material as moment-rotation, the interpretation is as such. It's simple, not complicated!