Prestressing concrete beams

I’m trying to model a tied arch bridge with a prestressed concrete deck as the tie. It is for hanger optimisation rather than detailed design so I’ve just simplified the deck into a concrete beam and a single steel beam acting as a tie by turning off bending and buckling if the modify element component. I have done hand calculations to determine the prestress required in the concrete and tie and converted them into an initial strain load. This should be enough to prevent the concrete going into compression but it doesn’t seem to work on Karamba. I keep getting loads of tension in the concrete tie rather than the steel taking it. Am I modelling this right?

I have attached my model below.
Network (140.6 KB)

Another quick question. I have modelled my hangers as beams and removed buckling and bending capacity. I still get compression in the hangers under some load cases. is there any way of making them only carry tension, or do i just have to apply an inital strain, enough so so they can’t go into compression?


Dear @Mg922,
I am not sure whether I fully understood your GH definition, since it is rather large.
The concrete deck is free to move horizontally on one side. Thus if one applies a pre-strain, it only gets longer or shorter but no stresses will develop.
If the hangers are very slender - like in your case - it is better to model them as trusses (set ‘Bending’ to false) in order to avoid numeric problems.
You could use the ‘Tension/Compression Eliminator’-component to remove tendons which are under compression.
– Clemens

Hi Clemens,
How do you suggest modelling a concrete deck tied arch? I set one support to a roller and then provided the horizontal restraint with a steel beam between the supports. The issue was that the concrete deck was taking all of the tension instead of the steel. This is why I tried pre-straining the concrete to prevent it going into tension, but the issue kept occuring. Is there a way of preventing the concrete taking the tension?


Hi @mg922,
you could integrate truss elements in your bridge deck and compress the concrete deck by applying a pre-tension load on the truss elements.
– Clemens