RISA Floor / RISA 3D - Integration (column loading)

[jsc43]

I have been working on a small one story building, using RisaFloor for the gravity load analysis, and then transferring the model from floor to 3D to evaluate the lateral loads (wind/ seismic).

Problems:
-On most columns in the 3D model, the axial load experienced is not equal to that in RisaFloor (generally the loads in 3D are much less than in Floor)

Possible Helpful Information:
-I have made all beams and girders "gravity members" so they do not act on the lateral resistance system in the 3D model. This causes a large number of vertical point loads to be applied throughout the model.
-The roof deck is being supported by a system of beams. The beams are being supported by a stud wall. At locations of large openings, that stud wall is being supported by a header, which is then being supported by the columns. On Risa3D, a point load is shown acting downward at the node located at the top of the column (where the header is framing into the column on RisaFloor)with the correct load determined from RisaFloor, but no load is being applied to the column when evaluated in 3D.

Questions:
-Why is the load not being applied to the column?
-Are there any tips for converting the floor model to the 3d model efficiently?

 

[JoshPlumSE]

JSC43 -

This sort of thing should be a piece of cake for the program. There are differences between the 3D and the Floor solution, of course. But, unless you have columns coming down onto transfer girders the results should be pretty darned close.

You might send your model into the RISA tech support for a more thorough review (support@risatech.com). Because, I'm guessing that there is a simple modeling error that is causing the confustion.

 

[EdArr]

I do not know if this is of significance to your modeling efforts, but as I recall, RisaFloor does not account for any fixity between the top of a column supports and the beams resting on them -- pinned supports are always assumed. On the other hand, Risa3D can have a variety of fixity options between the beam and the columns. This could cause a different distribution of loading between the software.

 

[Stazz]

EdArr-

I just found this out, if your beams are modeled as fixed but aren't carrying moment at their ends as if the columns are acting as noodles, then...

You have the "Use Column Stiffness" box under Global Parameters>>Solution unchecked. Thus, you are inherently pinning the columns at the top. Where you have two continuous beams framing together, it is appearing that you are getting a moment connection, when really you could think about this as a continuous beam framing over the column. Here is the help file explanation:

If the Use Column Stiffness box is not checked, columns will be viewed as simple supports with no rotational stiffness of their own. If this box is checked the column’s rotational stiffness will be calculated based upon the column material, moment of inertia, orientation and story height.

Thus, if you have a moment frame, you would always want to have this box checked. I believe by default we have this box checked on.

 

[EdArr]

Thanks Stazz. I had been asking Risa about this. I did not know they had implemented the feature.