Concrete Filled Steel Tubes (CFST)

[huffte]

For modeling CFST (concrete-filled steel tubes), it seems that there are two options (at least).

First, a composite circular section could be used. In this case, is the concrete considered confined by the tube internally for stress-strain behavior in a pushover analysis? Or are the two materials only made compatible at the interface with no confining effects of the tube incorporated?

Secondly, a jacketed circular section could be used. Again, is the concrete considered confined by the tube internally for stress-strain behavior? Or should I specify an equivalent area of transverse reinforcement to achieve the confining effect.

Thanks as always. - Tim

[seismosoft]

Hello Tim,

In the first case the concrete confinement is automatically calculated by the program, considering the effect of the steel tube. You can see the value of the confinement factors in the Section Characteristics tab of the Edit Section Parameters form (accessed from the Sections module). Even with minimal reinforcement the confinement factor assumes large values
In the second case though, if you employ an equivalent circular jacketed sections, you need to calculate the equivalent area of transverse reinforcement to achieve the confining effect.
I think that the first option is superior for your modelling.

Best Regards,
Seismosoft Support

[huffte]

Perfect. That is exactly what I needed to know! Thanks so much.

[Majeza]

Hi
I am modelling the cyclic behaviour of concrete filled steel tube (CFST)
column with RC beam. My numerical model fails to perform full analysis with
the full loading protocol whereas, in the experiment, the authors were able
to do that. I have given correct material properties input according to the
experiment and loading protocol is also right. To model composite
column-RC beam joint accurately, I have incorporated richard-abbott model
as link elements in the joint. As I cannot fully omit transverse
reinforcement from composite circular column, I assigned very less steel
strength to the transverse reinforcements with min spacing and dia. Also, I
have given right axial load as the permanent vertical loads in the nodes
like the experiment. Now my questions are:

1. What can be the reason behind error message of "analysis is terminated"?
In the experiment, the frame was able to take the load but why the loading
is too much in the numerical model? What to do to get rid of this error?
2. What can be the reason behind unmatched hysteresis curve? Why it doesn't match
with the experimental one despite having similar material properties and
loading history?

[huffte]

I have had great success modeling CFST in SeismoStruct using force-based inelastic frame elements without discretization. I have also had success using displacement basement inelastic elements with discretization. I am not sure what type element you are using, but try infrmFB or infrm DB without the link elements for joint behavior and that might help pinpoint where the problem is. Best of luck.

[Majeza]

I have tried using both the force-based and displacement-based inelastic elements. I have omitted the link element for joint behavior. Still it says "Unable to apply the entire permanent load. Analysis terminated" even though I have only given a cyclic load and no permanent load. What could be reason here? Please help.

[huffte]

Maybe try running a linear analysis (Project Settings --> Analysis --> Run with Linear Elastic Properties check box) to check that your support conditions do not result in an instability. If you have not applied external permanent loading, then it could be that the structure, for some reason, cannot support the self-weight as modelled.

[Majeza]

Thanks. I was able to solve the problem.
Another query: What could be influential factors behind the values of the negative side of global hysteresis curve of a structure? The positive side of my hysteresis curve matches very well with experimental result but the negative side (both force and displacement values) doesn't. I studied the curve parameters of the material models (for steel, I am using stl_mn and for concrete, con_ma) but changing values of them ends up altering the whole hysteresis curve, whereas I want to alter the negative side of the curve only.

[seismosoft]

In fiber-modelling this is not always easy to tell, because the member hysteretic curve is not directly controlled by the user. Apart from the material model parameters, have also a look at other, more 'global' parameters, e.g. the value of the axial loads applied on the member.
Seismosoft Support