Instability type 3 and supports issue.

[Anonymous]

Hello, I have about two years of experience with Robot Structural analysis. I have successfully used the program several times to determine the best sections for the job. 

This time however I am getting frustrated with error messages everywhere and am not able to figure out how to stiffen the building with steel shell, nor why the roof truss does not want to cooperate.

This is a building that I have been tasked with expanding the existing area to make room for concrete hollow slab storage area. And determine the load that is to be transferred to the foundation. 

I have been struggling with this for several days now, alone. And I just want some input from more experienced users that could point me in a logical direction. 

First I tried to make panels in building design to add to the roof and walls, to act stabilising. I used the given model to determine the shape of the trapezoidal plate with 1mm thickness. Then I added them to the roof and walls, and set them to be calculated as shells. This gave me error instability type 3 in different nodes each time. Then I researched meshing, got it perfect, still instability. I tried to calculate them differently, as walls, shells, with concrete reinforcement as walls and floors still got several error codes. 

Then I ignored the errors and tried to calculate nevertheless, I found that the steel roof trusses were unstable and gave me a ratio of 200000 or more. I then gave up entirely and decided to model the original building alone, with only claddings and fixed supports, as the concrete foundation more or less gives it approximately a fixed situation. Still the roof is unstable. I am expecting to be able to add 4kN/m^2 snow load, but it is unstable without anything. 

I am at a loss here, if anyone could help and give the model a look I'd be very much appreciative. 

Mind you the building is actually supposed to handle this as it is modeled, it is already buildt and standing proud. Also the bar force is a moving crane of 10kN

Using windows 10, robot 2019.

[Krzysztof_Wasik · Accepted solution]

Hi @Anonymous 

I have opened your both files and have the following remarks.

In both cases instability type 3 can be ignored, please refer to 

Instability type 3

This does not seem to be the main reason of all trouble.

Before you start steel check of your model please verify stress and displacement of your model.

Actually in model of original building you have maximal bar deflection about 2000 mm. You have to update model to get reasonable results, before steel check.

I have opened few checked bars with high efficiency ratio and for instance:

For bar 21, buckling length in both direction is 20 m. This element length is 20 m,  but it is supported along length in both directions. So buckling length should be reduced. For many other bars this is the reason for big efficiency ratio. 

For bar 111 (truss element) very big nodal bending moment is calculated. Its value is 41 kN*m while section capacity reported by steel check is 4 kN*m. Maybe you should consider release definition to avoid bending moment in truss element.

Generally code check is not good way of checking if model is build and calculated correctly. I would suggest starting from displacements and internal forces verification. Stress analysis can be useful as well. Then if model works correctly and gives reasonable static analysis results, steel check could be the next step.