Steel Frame Design. Unbraced length ratio (K)

[darkjmf1]

Hello all,
I have been using SAP2000 Steel Frame Design (SFD) for a bunch of years now but there has always been something going inside my mind about the values of unbraced length ratio (K) calculated by SAP.
Depending in the way you model the frames of a column, the results are quite different.
I mean, it is not the same to model a single frame from foundation to roof (with automatic frame division between intermediate joints at floor levels, e.g.) than modelling several frames, divided at every floor joint.
Usually I tend to model frames attending to the "real member", meaning that I divide the frames at those nodes were in reality there shall be an actual bolted joint.
But that is just my interpretation...
I would really appreciate if somebody else could share his/her approach to this.

 

[Daex]

Hi,

1. About the term you mentioned: Unbraced length ratio (K) - I think your mean is Effective length - the unbraced length ratio in SAP2000 is (L).
2. I had tested a few models to determine how to SAP2000 calculate the K value (Major + Minor). Here is my conclusions about it:
- SAP2000 can not determine which frame's direction is braced or not and it assumed that all direction are sway frame. So if you want to use frames with 1 braced direction, you must manually input the K value for this direction.
- Columns must be divided by real joint (not by auto meshing) at every location which have the moment beam connected to - same as when using effective length alignment chart. And also the columns which be divided by real joint (not by auto meshing) at non rigid location shall take the wrong K value.
- To determining G value in the effective length equation (Hinge: 10 - Fix: 1), SAP2000 takes it based on the assigned restrains. With whatever directions of one point are released, SAP2000 will assume that support is hinge. So if you want to use the support with 1 fixed direction and 1 hinged direction, I suggest you should assign all the restraints full fixed then release the columns later.