Modeling Stacked Members in RISA/STAAD/FEM

[ToadJones]

I am curious as to how others model members that a stacked or resting on each other in RISA/STAAD etc.

One example would be modeling a purlin resting atop a truss or beam.

I usually model an extremely stiff ("infinitely stiff")but very small cross-section area "dummy" member that runs from center line to center line of the stack members. I also put an additional node that is at the location of the interface of the two members.

I usually leave all the degrees of freedom of fixed on the dummy members where all members at the connection are continuous but I am now wondering if I should be releasing certain fixities depending on the stiffness of the connection and the members.

Let me be clear that I am not actually modeling a purlin on a roof truss, it is just an example.

Also, please spare me any rants about how modeling software has destroyed engineering and made engineers idiots.
This is not for a building at all in fact, but rather a pretty complicated tank structure.

 

[rowingengineer]

I release the fixities once I has established a model. I pick actions that should not be transferred between members. generally I release completely or replace with K stiffness restraints.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[JoshPlumSE]

The method you're using is reasonable. One comment that I would add to it is that I would not always choose to model both ends of the dummy member as fixed. Sometimes that's fine. But, feel free to use the end releases of the dummy member to dictate what sorts of forces you want to transfer between the two members.

A second comment would be that it's not always a good idea to model in this type of offset for the members. If all you want to do is transfer the vertical shear from your purlin into your supporting truss, then modeling in the vertical offset should not affect that load transfer at all.... And, would really just be a wasted modeling effort. No need to make your models more complicated than they have to be, right?

 

[ToadJones]

Josh-
Was hoping you'd chime in....

I 100% agree that this is not necessary always, but there are times I really feel that it is necessary.

Again, the purlin was just an example as the project is a shell structure with stiffeners , but now that we have brought it up, I do have another project where I need to model Z purlins on a sloped roof canopy. The system is not really a building per se but more of a support structure.

The client is asking for very detailed analysis of the purlins.
So, in this case I do feel it is goign to be necessary to model the offset in order to account for the purlins wanting to "roll over" due to gravity loads. The is no "sag rod" system in place and one cannot be used for various reasons.
In this case I have dummy members extending from Center Line to Center Line of the purlin and the supporting member but have no releases on the dummy member.
My thinking is that the stiffness of the connection (bolts) relative to the members themselves is so much greater that the joint is essentially fixed. I am not sure if this is the correct approach. Basically, the Z purlin material itself will bend/deflect with no rotation at the connection. Problem I am having is that the dummy member is connected from Center Line to Center Line of the members and in reality, they are connected at the interface. Not sure how to get around this.

 

[JoshPlumSE]

Your description sounds valid to me. Though it is always tough to say for sure without reviewing the model... (insert standard caveats and disclaimers here).

In cases like this I look at the top end of the dummy member (where it connects to the purlin) as the main connection. If its purely a shear connection, then I release the moment at that end of the dummy member. Similarly, I could individually use the end releases to limit force transfer for slotted bolt holes or such.

Then I model the other end of the dummy link member as fully fixed to the supporting member. Sometimes this will result in some odd forces or torsional moments being imparted into that member. But, it's not hard to track those forces/ moments through this link member to see how they relate to the purlin reactions.

Ideally for something like this, I wouldn't want to model the WHOLE STRUCTURE with these links everywhere. That's just so much modeling work that I wouldn't do it unless it becomes clear that it will significantly affect the main force resisting system. Hopefully modeling it for only a couple of members will adequately demonstrate that it has a local effect on the purlins, but not on the main structure.