Truss Roof – Joint Force Imbalance Due to TRP Plate Modeling

[Miladm2]

Hi,
I'm designing a building in ETABS with a truss roof system, where TRP plates are placed on top of the trusses. To ensure the TRP plates do not carry any load from the truss structure, I set the membrane stiffness values (F11, F22, F12) to 0.001.


However, when designing the connections at the truss joints, I noticed that the member forces do not satisfy equilibrium conditions. After extensive checking, I found that the issue stems from the TRP plates: if I define an opening over the joint, the forces at that joint are in equilibrium. Without the opening, the TRP plates still show zero internal forces, but the equilibrium is not satisfied.


Has anyone encountered this issue before? Is there a recommended solution or modeling approach to resolve this? Any suggestions would be appreciated.

 

[Luceid]

How much of "not in equilibrium" are we talking? It depends on the relative stiffness of the elements, if your plates are super thick, maybe 0.001 isn't enough to make them completely negligible relative to the truss chord stiffness. If they have stiffness, they will attract some force.

I'm also not specifically familiar with what you mean by TRP plates - could you clarify? Maybe an image would help.

 

[Miladm2]

Hi, and thank you for your response.


I’ve attached an image of the TRP sheet I referred to. It’s a trapezoidal steel plate with a thickness of 1.5mm.


The internal forces in the TRP plate are very small—typically less than 0.1 kN. However, even under dead load conditions, I observe an imbalance in the equilibrium of member forces of up to 25 kN in X and 10 kN in Y direction (in Z direction equilibrium condition is satisfied), while the internal forces in the connected members are in the range of 20 to 50 kN.


This seems significant, and I would have expected better force balance, especially given the negligible internal forces in the plates. Do you think this could still be due to the relative stiffness, or is there something else I might be overlooking?

 

[sticksandtriangles]

Can you post some screen shots of what you are dealing with?

Maybe some quick math for the unbalance that you are seeing as well? Does the unbalance "balance" out with section cuts through both the truss and the TRP sheet?

In general, even though a "low" in plane stiffness modifier has been placed on the shell elements, I have noticed that shell elements still tend to mess up results when modelled in truss elements, sucking axial load out of the member as seems to be your case.

If the imbalance is really high as you have suggested, many people will make a truss only "gravity" model and use that to design for forces that would be more traditionally calculated and in line with hand calculations.