Seismosoft | Earthquake Engineering Solutions

Being concerned about the thickness of the FRP material in the con_frp model, this is what I found in the literature and assumed it might be helpful for seismostruct users:

"Chen et al. [a] concluded that the utilization of the actual (cured) thickness of the Fiber Reinforced Polymer (FRP) (instead of the nominal thickness provided by the manufacturer) is more appropriate in the Finite Element (FE) analysis."

[a] CHEN, J. F., LI, S. Q., BISBY, L. A., AI, J. “FRP rupture strains in the split-disk test.” Composites Part-B: Engineering, 42(4), 2011, pp. 962-972.

Thanks SeismoStruct for your already right answer to my question, believing the use of the Actual cured thickness of the fibers, impregnated into epoxy resin. [:)]

i want to simulate the CFRP wrapping on half length (height) of reinforced (longitudinal +Spiral)concrete column..HOw could i acheive this ???

As discussed in another thread, you can define two Force Based elements, which one has the FRP confined concrete assigned. However, under seismic transient loading sometimes numerical instability may occur for frp confined elements, if connected to a middle node. If the methods recommended by SSoft to improve numerical instability is not solving the problem, you could try to include the effect of the FRP confinement through Mander concrete model which is really numerically robust. But, you should make sure to calculate the right value of confining pressure.

As a starting point, confining pressure for spirals and jackets could be found from below equations:

f_spiral=(2*A_b*f_s)/(s*D')

f_jacket=(2*f_j*t_j)/(D)

where:
f_spiral=confining pressure from spirals
A_b=spiral cross sectional area
f_s=stress in spiral,
s = spiral pitch spacing,
D' = column core diameter.

f_jacket=confining pressure from jacket
f_j=stress in FRP jacket,
t_j=stress in FRP jacket
D=Column diameter

Sir Many many thanks for your valuable comments. Let me explain what i understand for defining the lower portion of column ( Reinforced with longitudinal and Spiral reinforcement ) and confined externally by CFRP wraps. This will require me to define
1. An additional Material type (Con_frp)
Question: it asks for 2 parameters i am not having with me
a)Strain at peak stress of Unconfined Concrete
b)FRP Jacket Ratio

2.Now in section tab i will have to define a new section for the lower portion.
Question : What section type i should choose.if reinfoced concrete circular section then do i have to select the con_frp material as cover for this ?? or something else

3. The next tab is Element Class.
Question : Is there need to define another Element class or the existing one is sufficient ??

4. Now the Nodes Tab
if i am not wrong then here i will define another node at mid center of column soo that in element connectivity i select different element types for the lower and upper half length of column

5. What is the importance/function of Link Element Type. In other finite element software ANSYS i found this Link as reinforcement but whats its purpose in seismostruct?


If any other changes to be made kindly mention. I would be very grateful if you share a link of file in which CFRP is modeled on a reinforced concrete column soo that i can have a look on it what things i am missing

Thanks in advance for your precious time.
Regards
Zain Saeed

Hi zsaeed.

You will need separate Element Classes for the FRP section and the SPiral-confined section.

The choice of section for the lower part of the column is up to you. You could model with con_frp. You could model with con_ma. Either way you will need to come up with a rational means of tweaking the input variables to get the combined effect of FRP and spirals. See the literature recommended by fakharifar.mostafa for this.

The default value of 0.0022 for strain at peak stress of unconfined concrete should be adequate.

Yes. You will indeed require a node along the column where you transition from spiral-confinement to FRP+Spiral confinement.

Link elements in SeismoStruct may be used to "model, for instance, pinned or flexible beam-column connections, structural gapping/pounding, energy dissipating devices, bridge bearings, inclined supports, base isolation, foundation flexibility, and so on."

The above quoted text is from the Help System, Element Classes --> Link Element, which gives an excellent introduction I highly recommend.

Happy modeling.

zsaeed, I would like to help you making your column model, but please bear in mind no one in this forum can provide you all the details for your work. Anyway, answer to your questions are:

1- Strain at peak stress around 0.002 to 0.0025 is appropriate, as long as using normal strength concrete.

You have to have this ratio. Actually this ratio corresponds to the volumetric ratio of FRP material used for racketing. This ratio could be simply defined according to number of FRP layers and member diameter.

2- Yes define a new cross section for lower portion and use your con_frp assigned to both cover and core concrete.

3- You need two elements, one for steel confined and one FRP confined.

4- Yes you need a middle node.

5- I am not sure what you mean, but LINK 8 element used in ANSYS to model reinforcement is completely different from what we refer to here in SStruct. Here, we deal with fiber based (spline) modelling approach, and link elements define hysterics, friction,... different behaviors at elements interconnection.

Your model seems straight forward to me. However, this modeling scheme might not work. You may have to define two cross sections, one for the FRP jacket and one for the existing column and try to connect them at equal heights (this approach is somehow difficult to calibrate the link elements between FRP tube and concrete column elements). More details on this approach is discussed below:

http://www.seismosoft.com/forum/topic.asp?TOPIC_ID=1748

Below paper have used CFRP jacketing. However, neither goes on modeling details.

Fakharifar, M., Sharbatdar, M. K., Lin, Z., Dalvand, A., Sivandi-Pour, A., & Chen, G. (2014). Seismic performance and global ductility of RC frames rehabilitated with retrofitted joints by CFRP laminates. Earthquake Engineering and Engineering Vibration, 13(1), 59-73.

Billah, A. H. M., & Alam, M. S. (2014). Seismic performance evaluation of multi-column bridge bents retrofitted with different alternatives using incremental dynamic analysis. Engineering Structures, 62, 105-117.

Sir i appreciate your help in this research of mine..
1. It means that for my model of Bridge column (fixed at bottom and at top subjected to gravity load and incremental load )there is no need of this element ..if i simply fix the base will i be able to compute hysteresis Curve for energy dissipation ??

2. I want to know that when i define the con_frp material it asks for FRP jacket ratio ..i searched this option in the user manual but couldn't found ..how to calculate this term ??

3. Sir a plotted hysteresis curve and want to know the area under the loop for calculation of energy ..how could i calculate that area ??

4. i also want to plot stiffness degradation and natural time period during free vibration test in this research for which i will need your assistance and guidance

5. Please find in following link a presentation what i am expecting from this software in this my current research work

https://drive.google.com/file/d/0B7rekW ... sp=sharing

1- Yes and No. It depends. If your column is lap spliced or it features a pinched behavior at base for any reason, then you have to use a link element at base. Go to Link elements in user manual and will give you great idea.

2- "FRP jacket ratio - This is the ratio between the frp and concrete areas, at any given cross-section. The default value is 0.01." from SStruct Help System.

3- You can either write a Matlab script or you can plot each cycle in excel and take them into Autocad. Use the area command in AutoCAD to find the area in each loop.

4- For stiffness degradation you just need to find stiffness for each cycle. You need to plot individual cycles and find stiffness as well.

Best of luck on your modelling.

quote:Originally posted by paky983

hi seismosoft support,
thanks for the answer. I have read the forum and i understood that there are two ways to modelling structures with frp.
1)the model con-frp to be using the existing sections for concrete cover and section core.
2)using a new cross-section to add to existing sections with
a material model featuring the properties of the retrofitting material.

Is it all correct?

Now,about this two ways i have any question:
1)In the first way,the thickness of frp is included in the "FRP jacket ratio"?
2)In the second way, what material should I choose in seismostruct between the existing ones, for the retrofitting material?maybe the trilinear FRP model?

Thanks for your precious support.




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Sir i cant get your second option ..can u please elaborate it for me . As i am also modelling CFRP on half length of Column and for that i used Con_frp and used it as cover and a reinforced concrete material as core of the section with appropriate confining factor..

I also want to know that what kind of anchorage is required for CFRP i read in previous post about it ..??

Yes, you've done right. con_frp for the cover concrete, and con_ma to your core if your existing spirals are significant and you can't neglect their contributory effect to the column confinement.

If your transverse reinforcement ratio is low, let's say less than 0.6, you can ignore it and just assign con_frp to both cover and core.

Anchorage of CFRP you'll need to do if you had longitudinal CFRP fibers (like laminates or anchors) embedded into foundation. That will provide one additional force transfer mechanism from oclumn to foundation, thus you'll need to include it. If your column is just confined with circumferential fibers, then no need for anchorage modelling.