Composite (CFT) marine pile design to Eurocode4

[vtel]

i am currently working on the design of "concrete-filled steel circular marine piles". as i see most of the CFT research is on building columns but there is no design recommendation or implementation on marine piles.

in case of closed-ended pile driving, one can ensure that required bond strength could be achieved as interior surface of pile shaft is free from dust, oil, paint, organic materials and etc. (this is a similar case for also bored piles with pre-drilling or pre-augering). however, i sometimes encounter marine piles with open-ended because of need of socketing into bedrock and/or relatively larger piles in diameter (say, d/t>=60 and d>=60cm). i am also fine with the provisions in Eurocode4 to make use of them in composite marine pile design but i am not sure if any bond strength can be achieved in case of open-ended pile driving.

any input is appreciated.

 

[Focht3]

Are you concerned with axial (compressive) or lateral (flexural) loading of the piles? It makes a big difference...



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[vtel]

combined axial+flexural behaviour is of concern, indeed. you may take it as "beam-column". because piles are supporting a pier deck and also subject to berthing/mooring loads of vessels.

 

[Focht3]

There was some discussion on this topic in another thread within the last few months; I suggest you search [green]Eng-Tips[/green] for it. As I recall, the discussion had to do with closure plates.

From a lateral load standpoint, the concrete infill adds little or no EI to the section - usually less than five percent - so the presence of the concrete has more to do with corrosion protection and load transfer between the pile and cap than anything else. Axial capacity is different: the concrete infill can increase the effective bearing area of the pile, significantly increasing the pile capacity. However, special measures may need to be taken to assure the bearing capacity is achieved. This can include requiring that the soil plug be partially or fully removed before filling the pile completely with concrete.

Each case is unique; if you are designing a dock, loading platform or breasting platform then you should have a geotechnical engineer as a part of the design team. What does s/he say?



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[vtel]

i have reviewed a few past threads on this, however, those were not directly related to "natural bond strength". there was a similar case reported on "pre-drilled" pile shaft.

"Tests by Bridge on rectangular CFTs bent both uniaxially and biaxially showed that the concrete core only provides about 7.5% of the capacity in member under pure bending. Lu&Kennedy reported strength increases of order of 10-30% over rectangular hollow tubes, and they exhibited ductile failure modes" (Hajjar, 2000).

as to combined axial load and bending, behaviour of the beam-column is influenced by the ratios d/t, N/No (axial load ratio), and L/d. as axial compressive load is added to a member in pure bending, the contribution of concrete starts to increase, hence composite action of the section becomes significant and resulting in enhanced combined capacity.

i solved a numerical example to Eurocode4 and witnessed that member capacity (combined axial load and bending) of a composite column gets significantly higher compared to steel hollow section and relatively higher compared to R/C section with the same diameter and concrete class. this is the structural point of view. we are working on a "pile supported loading platform". in regard to geotechnical, we are advised to remove soil plug and employ "drill-and-drive" method to achieve a rock socket at the pile tip. piles will be completely in-filled with normal strength concrete. however, the question remains the same: (how) can a natural bond strength be achieved for an open-ended driven pile?

thanks for your help.

 

[Focht3]

Concrete infill can prevent/delay the onset of local buckling in square tubes; I haven't read Lu & Kennedy's paper, but suspect the capacity increase has to do with this failure mode. Could you kindly provide the full citation for the Lu & Kennedy paper?

If the pile is completely filled with concrete, why the interest in bond strength between the concrete and steel? I'm not sure that I fully understand your question -



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[vtel]

In order to have proper composite behaviour, bond strength should be achieved at the interface either by natural bond (friction) or by means of mechanical shear connectors. it can be provided for a building column or for a closed-end driven pile. on the other hand, after driving an open-ended pile, it seems it is quite suspicious to have it because of entry of soil, organic materials, etc. into the pile shaft. in addition, shear connectors do not seem practical to apply for an open-ended marine pile.

here are the following papers below, as per your request:

1)Concrete-filled steel tube columns under earthquake loads
J F Hajjar, Prog. Struct. Engng Mater. 2000; 2: 72–81

2)Bridge RQ. Concrete filled steel tubular columns. Report No R283. Sydney, Australia: School of Civil Engineering, University of Sydney. 1976.

3)Lu YQ & Kennedy DJL. The flexural behaviour of concrete-filled hollow structural sections. Canadian Journal of Civil Engineering 1994: 21(1):
111–130.

 

[Focht3]

I don't have any of those references - the CJCE may or may not be available locally, but the other two probably are not.

My gut says that the issue isn't as major as you think it is. Have you modeled the problem assuming the interior concrete is decoupled from the steel pile? What happens?



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