Bilinear steel model for ductile elasto-plastic materials

[fakharifar_mostafa]

I have a question on modeling the flexural behavior of concrete filled tubular columns, where the outside tube consists of plastic pipe, like PVC (polyvinyl chloride) material. This system is similar to conventional steel tube filled concrete, where the outside tube serves both as the formwork and confining device, in addition to flexural and shear reinforcement.
The PVC material exhibits a similar ductile behavior to steel, though it is more ductile with less strength. The PVC stress-strain diagram is almost a bilinear elasto-perfectly plastic response. The yield strength and modulus of elasticity for PVC pipe is approximately 50 MPa and 4 GPa, respectively. I have adopted the bilinear steel model (stl_bl) in SStruct (with zero strain hardening) to model the confining PVC tube, and could simulate the experimental results within good level of accuracy. I need to mention that, using SStruct I have noticed pretty much every type of confining device could be considered (such as FRP, steel, welded spirals, etc) as long as its corresponding confining pressure is appropriately incorporated in the numerical model.
My question is adopting the stl_bl model for simulating isotopic ductile materials, such as PVC, appropriate? Broadly speaking, my question is how one may use bilinear steel model in SStruct, or similar packages such as ZeusNL, to simulate the behavior of similar plastic materials with ductile elasto-plastic behavior (often assumed for steel material)? Reason for this question is that I want to explicitly simulate the PVC tube, and not only considering its confining effect in the numerical model. I want to obtain the state of stress-strain in the PVC tube, thus, to do so I need to have it in the model.

Moreover, in addition to limiting fracture/buckling strain, how can I ensure that appropriate strength deterioration is attained, if no link element is used at the column base (e.g. such as Sivaselvan and Reinhorn link element at the base of the column). Do I have to use link element at column base, if I use stl_bl model?

Many thanks in advance to all of you for your great inputs.

[ruipinho]

Hi fakharifar_mostafa,

Judging from some PVC uniaxial stress-strain monotonic test results that I quickly found on the internet, it does seem indeed feasible for one to use 'stl_bl' to model explicity the PVC material in pushover analyses. For what concerns cyclic response, instead, I could not readily find material test results, and hence cannot really express an opinion on the matter.

With regards to the possibility of modelling strength deterioration (I suppose as a consequence of cyclic loading), my suggestion would be to use the 'stl_mp' model with a negative value for the a3 parameter (assuming of course that the uniaxial cyclic response of PVC is somehwat similar to that of steel, which, I repeat, is something I have not necessarily seen so far).

Best,

Rui

[fakharifar_mostafa]

Thank you really so much for your great input. I had the same expectation when comparing the similar stress-strain behavior of PVC to steel. THANK YOU!

[devina9]

It is great opinion.