Dear all,

I have modeled a 3D 7-floor structure using Opensees and SAP2000 too, because I am a new user of Opensees and i need to validate the results.My structure consists of RC frames and only one shear wall.The final goal is to run dynamic analysis using fibers in order to derive fragility curves for the building.I modeled the shear wall as a column using fibers and i used rigid elements(elastic BeamColumn with big values for stifness properties) at the level of each floor.I have already ran a modal analysis and I noticed that the value of Ubig affects the modes.When I chose a really big number (1.e10) the modes were too smaller than the results of SAP2000.In SAP2000 i didn't use any modification factor for the stifness of the structure..So, i decided to use the value 1.e4 for the Ubig (because in my structure G*J is near to this value) and the results of Opensees came very close to the results of SAP2000.When I tried to ran a static analysis the Opensees displayed an error about failing in convergence..I tried to ran the same analysis with a big value for Ubig and it ran but the reactions were very different from those of SAP2000.Can someone help me with the suitable value of Ubig??

## rotational stifness

**Moderators:** silvia, selimgunay, Moderators

### Re: rotational stifness

don't just pick a large number .. pick something in the order of the actual stiffness of the thing you are modeling. The SAP and OpenSees model should be the same for the same model.

as for the convergence problem of a linear model. what convergence criteria are you using? if unbalance it probably will not work if Ubig is too small. and as for Ubig, if using elastic elements don't give E and A and I all Ubig , as they get multiplied together to form the actual stiffness matix, which results in U stupidly big.

as for the convergence problem of a linear model. what convergence criteria are you using? if unbalance it probably will not work if Ubig is too small. and as for Ubig, if using elastic elements don't give E and A and I all Ubig , as they get multiplied together to form the actual stiffness matix, which results in U stupidly big.

### Re: rotational stifness

I tried two convergence criteria (energy and displacement) but it didn' t run for both criteria..

I need help about what exactly is the torsional stifness which is needed in a 3D model..Is the value of J of each section (which is a number near 1e-4 for the sections of my structure) or the product G*J??i used the value 1.e4 for Ubig..Is this value too small?

As for the elastic elements i didn' t give big values to all E, A, I but the model doesn' t run..

Thanks for helping me....

I need help about what exactly is the torsional stifness which is needed in a 3D model..Is the value of J of each section (which is a number near 1e-4 for the sections of my structure) or the product G*J??i used the value 1.e4 for Ubig..Is this value too small?

As for the elastic elements i didn' t give big values to all E, A, I but the model doesn' t run..

Thanks for helping me....