I am currently creating several slab models for my thesis using RFEM 6 version 6.13:0009. These are supposed to represent existing reinforced concrete (RC) flat slabs that were built in 1960. I have encountered several problems in the process. Since different materials were used at that time, I had to adapt the material models for the concrete and reinforcing steel to the standards of DIN 1045:1959. For the steel, I modified a reinforcing steel model. Since I want to model steel of group I, I entered a yield strength value of 220 and a tensile strength of 340 N/mm². In addition, I changed the material model to "Isotropic / Plastic". Now I wanted to model yielding starting at a stress of 340 N/mm², so that no higher stresses could be taken up. However, in my model, reinforcing steels experience stresses of over 500 N/mm². It should be mentioned that I manually assigned the reinforcing steels via the member type "reinforcing bar". I did this because, to my knowledge, the "normal" way does not allow evaluating reinforcing steel stresses.
Furthermore, these slabs are now to be subsequently strengthened with glued laminated timber (BSH) beams, which are to be glued to the underside. So I also modeled the BSH beams as members and connected them to the slab via a rigid coupling. Initially, I used a completely rigid connection, later a line hinge (which did not change much). Then I learned in a chat with the AI assistant that this function is actually not suitable for the present case. Instead, it was suggested to model the connection via surface contact. Accordingly, I then modeled all BSH beams as surfaces and inserted a result member. However, this solution was entirely misleading, as the deformations after attaching the beams under constant load even became larger (I assume due to the self-weight). In an older training video, I then saw that the somewhat older function of line release was used for a similar use case as the one here. Now I am not sure whether my chosen modeling technique actually represents the real load-bearing behavior or not.
I think surface contact is not suitable for the component here. I would definitely recommend rigid coupling with line hinges.
You can find more information on this at the following link.
Furthermore, it is important in your model that the ceiling has already deformed and the wooden beams are added to the model. This means the ceiling has already deformed and is under stress. Accounting for this is possible in many ways within the program. On the one hand, you can transfer initial strains under the structural modification in the program.
This can be fully considered best via the Bauzustände (construction stages) Add-On. You can find more information on this at the following link.
Thank you very much for your reply. I have now also tried continuing with the line hinges and find that this is the better method for this application case. Regarding the pre-deformation of the ceiling, it was planned to temporarily prop it up beforehand and thus bring it to the zero position (otherwise attaching the beams is also complicated). Therefore, at the moment of composite action, there would be no deformation of the ceiling. However, the shrinkage and creep processes from the previous load are completed. To consider this aspect, I have created a new ceiling and a new load case. Generally, I do not work with the combinations you suggested but have created my own with final determined surface loads (since today's safety concept differs significantly from that of 1960). I have then assigned the corresponding load duration to these so that the concrete is only loaded when its stiffness has already been reduced by creep and shrinkage. However, I can also try it with the construction stages and see what results from that.
Unfortunately, you did not answer my question regarding the reinforcement bars. Meanwhile, I am of the opinion that the bar type "reinforcement bar" is only suitable for the application of discontinuity areas. In my model, it only receives tensile forces; even determining normal stresses via RFEM is not possible. If I want to model the reinforcement separately, I will probably also have to introduce rigid couplings.
Sorry, I was not aware that this was a question.
Yes, in this case these rods do not make sense.
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Kein Problem, vielleicht war meine Nachricht auch etwas missverständlich formuliert. In der Tat würde mich interessieren, wie Sie meinen Anwendungsfall in RFEM am besten umsetzen würden. Also konkret: Wie würden Sie die Bewehrung modellieren?