Force Transfer in Column Splice?

[StructureMan44]

A design requires a column splice between a W6x25 and a W12x65. RISA was used to model the entire frame. For the same load case the axial forces in the members are significantly different where they join, the axial force in the W12x65 is 29kips while the axial force in W6x25 is 11kips. Is this to be expected? I'd expect the axial forces to be the same as the force would travel across the joint from one member into the other.

 

[CANPRO]

Anything else happening at that joint? Beam or brace framing in? There must be some source of additional load or you have an error in your model.

 

[StructureMan44]

Nothing else is framing into the joint. There must be a way that I have to find to tell RISA this is a column splice.

 

[Agent666]

Check how the self weight is accounted for, might be lumped at intermediate nodes, rather than being truly distributed along the member?

 

[jayrod12]

Maybe upload your model for review.

 

[StructureMan44]

Thanks jayrod12. The portion of the model with the spliced columns is attached.

 

[StructureMan44]

Is there some way to force RISA to transfer the forces across this joint?

 

[WARose]

Anyway you can post it in STAAD? Or a screen shot? What you have posted, I cannot open.

 

[JoshPlumSE]

Your model looks kind of strange to me. My thoughts:
1) Splicing between these two column sizes seems silly to me. Just stick with a lighter W12 for that upper column. It's only 7.5 ft long. The connection will be much easier.
2) The axial force at the splice location is the lower value. The axial force doesn't jump up until you hit the floor level and the brace force makes its way into the column.
3) You've got a cantilever portion of your structure (node N163) that is not properly supported. The program is spitting instability messages about you like crazy. You need to pay attention to these.
4) I wonder if you have let the program "optimize" member sizes too much. The program's member optimization is really pretty simplistic. You need to use some engineering judgment rather than just accept the sizes the program says are most efficient. Usually, I would expect a structure like this to have all 4 columns the same size.

 

[StructureMan44]

Thanks JoshPlumn
1. I agree, splicing the tanks is silly however the upper columns had to be reduced in depth to accommodate a tank w/insulation on the upper level. The location of the columns could not be changed.
2. RISA is showing the maximum axial force in the lower W12x65 column as ~30kip at the splice location while the upper W6x25 at the splice location as ~1kip. Why would these forces not be the same?
3. Good point; this model is taken from a much larger model and some of the forces are acting on members that were removed for posting a smaller model.
4. Agreed, these columns weren't sized using the optimize, instead they were sized based on their loading.

 

[JoshPlumSE]

2) You have applied load at those joints. See BLC's 25, 38 and 39. The difference in the column forces is because of this applied load.
a) It looks to me like the applied may be going in the wrong direction. (DL going upwards instead of downwards).
b) Is the DA Oper load merely the INCREASE over DA Empty and DA Hydro merely the increase over DA oper. If not, then it looks like you may be over counting these loads.